Human embryology.Prof. Malkanthi Chandrasekara
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Human embryology.Prof. Malkanthi Chandrasekara

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1st edition of Prof. Malkanthi S. Chandrasekera’s (Professor of Anatomy,Faculty of Medicine,University of Peradeniya, Sri Lanka) famous embryology book “Human Embyology” 2006 for medical ...

1st edition of Prof. Malkanthi S. Chandrasekera’s (Professor of Anatomy,Faculty of Medicine,University of Peradeniya, Sri Lanka) famous embryology book “Human Embyology” 2006 for medical undergraduates.

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    Human embryology.Prof. Malkanthi Chandrasekara Human embryology.Prof. Malkanthi Chandrasekara Presentation Transcript

    • HUMAN EMBRYOLOGY FIRST EDITION MALKANTHI S. CHANDRASEKERA Professor of Anatomy Faculty of Medicine U n iversity of Peradeniya Sri Lanka 2006
    • GENERAL EMBRYOLOGY E m b r y o l o g y is t h e s t u d y o f t h e d e v e l o p m e n t a n c i g r o w t h T h e i m p o r t a n c e o f s t u d y i n g e m b r y o l o g y is t h a t it h e l p s in ; - th e in te r p r e ta tio n - e x p la in in g - th e s tu d y o r g a n is m . o f A n a to m y d e v e lo p m e n ta l d e fe c ts o f th e o f a n p h y lo g e n y o f m a n o n a n a n a t o m ic a l b a s is ( a n c e s tr a l h is to r y /e v o lu tio n ) H u m a n d e v e lo p m e n t s ta r ts a t th e tim e o f fe r tilis a tio n w h e r e th e o v u m , th e f e m a l e g e r m c e ll is p e n e t r a t e d b y t h e s p e r m a t o z o a t h e m a l e g e r m c e ll. G e r m c e l l s a r e f o r m e d i n t h e g o n a d s , “T h e m a l e g o n a d i s t h e t e s t i s a n d t h e f e m a l e g o n a d is t h e o v a r y . T h e p r o c e s s b y w h ic h t h e g e r m c e lt s ( g a m e t e s ) a re p ro d u c e d b y th e g o n a d s is te rm e d g a m e to g e n e s is . D u r in g th e p r o d u c tio n o f g a m e te s th e c h r o m o s o m e n u m b e r is r e d u c e d to h a lf o r b e c o m e s h a p lo id . In t h e m a le t h i s p r o c e s s is c a lle d s p e r m a t o g e n e s i s , w h e re s p e r m a tid s ( h a p lo id ) a re p ro d u c e d fro m th e s p e r m a to g o n ia ( d ip lo id ) , a n d s p e r m io g e n e s is , w h e r e s p e r m a t id s a r e tr a n s f o r m e d in to h i g h l y m o t i l e s p e r m a t o z o a o r s p e r m ( h a p l o i d ) . In t h e f e m a l e t h i s p r o c e s s is c a lle d o o g e n e s is w h e r e n o n m o tile o v a ( h a p lo id ) a r e p r o d u c e d fr o m th e p r i m o r d i a l g e r m c e l l s ( d i p l o i d ) in t h e o v a r y . F e rn a I e O v a ry ( D ip lo id ) Oogenesis O v u m (Haploid) Male T e s tis Spermatogenesis (Diploid j HUM AN EM BRYO LO G Y S p e r m a tid (H a p lo id ;• Sperm iogenesis S p e rm (Haploid > 01 -
    • MALE REPRODUCTIVE SYSTEM Functions of the male reproductive system include the following ■ Production of sperms (spermatogenesis and spermiogenesis) ■ Collection, storage and transport of sperms ■ Production of seminal fluid ■ Secretion of male sex hormones ■ Deposition of sperms in the female genital tract The component parts of the male reproductive system include the testes, a paired duct system which includes the rete testis, efferent ducts (ductuli efferentes), epididymis, ductus deferens (vas deferens), ejaculatory duct, urethra (prostatic, membranous and penile), seminal vesicles, prostate gland and bulbourethral glands. The anatomical parts of the male reproductive system are shown diagrammatically in figure 1. Urinary bladder Prostatic urethra Bulbourethral gland Penile urethra Penis Vas deferens Scrotum Epididymis Testis Fig 01 HUMAN EMBRYOLOGY -02-
    • The testes develop in the posterior abdominal wall and descend down to the scrotal sac and become extraperitoneal. The testis has a thick connective tissue capsule known as the tunica albugenea. It consists of lobules separated by connective tissue septa within which contain several highly convoluted seminiferous tubules. The germ cells from the wall of the yolk sac migrate to the developing testes at 6thweek intra uterine life. These germ cells give rise to spermatogonia that lines the walls of the seminiferous tubules. Spermatids are produced within these tubules. The process by which spermatogonia (diploid) produce spermatids (haploid) is called spermatogenesis. The spermatids undergo transition and become highly motile spermatozoa in the epididymis. This is called spermiogenesis. The changes that take place during spermiogenesis include the following. ■ Formation of acrosome around two thirds of the head of the sperm ■ Condensation of nuclear material ■ Formation of head, neck, middle piece and tail of the sperm ■ Shedding of most of the cytoplasm These changes are diagrammatically shown in figure 2. Acrosome Head Nucleus Centrioles Middle piece Golgi complex Tail Axial filament Fig 02 HUMAN EMBRYOLOGY -03-
    • The fluid medium, which transports spermatozoa, consists of secretions from the seminal vesicles, prostate and bulbourethral glands and is termed semen. Spermatozoa from both testes pass through a duct system consisting of seminiferous tubules, rete testis, efferent ducts, epididymis, vas deferens, ejaculatory duct and urethra. The urethra provides a common passage for urine and semen. About 200-300 million spermatozoa are deposited in the female genital tract during coitus. Although 300-500 spermatozoa reach the site of fertilization only one sperm fertilizes one ovum and the rest die off. Very rarely two ova may be produced and fertilized by two spermatozoa. ■ Each ejaculation contains 3 - 4 ml of seminal fluid. ■ Sperm count - 100 million sperms per ml ■ 200 - 300 million sperms are deposited in the female genital tract. ■ 300 - 500 sperms reach the site of fertilization. ■ Only one sperm is needed for fertilization. Structure of the sperm Fig 03 Acrosomal Head cap HUMAN EMBRYOLOGY Neck Middle piece Principal piece Tail/end piece •04
    • The com ponent parts of a mature sperm are shown diagram atically in figure 3. A mature sperm consists of a head, neck and a tail bounded by a cell membrane. The head is oval in shape and flattened anteriorly and the nuclear material (chrom osom es) is condensed within it. Covering two thirds of the anterior part of the head is the acrosomal cap. The acrosom e contains enzyme acrosin and trypsin like substance needed to penetrate the ovum. The tail consists of a middle piece, principal piece and an end piece. In the middle piece mitochondria are arranged as a helical sheath, the principal piece has a structure of a flagellum and the end piece consists only of an axoneme. FE A M LE R P O U TIV S S M E R D C E Y TE The functions of the female reproductive system include the following. ■ Production of ova (oogenesis) ■ Reception of spermatozoa ■ Fertilization ■ Secretion of hormones ■ Development of the fertilized ovum ■ Implantation ■ Nutrition of the developing foetus ■ Expulsion of the foetus to the environment ■ Nutrition of the new born HUMAN EMBRYOLOGY -05-
    • The component parts of the female reproductive system include the ovaries, the genital tracts and the breast. The ovaries and the genital tracts are diagrammatically represented in figure 4a and 4b. Fallopian tube Bladder Vagina Ovary Uterus Rectum Vulva Fallopian tube Ovary Uterus Vagina Vulva The ovaries are oval shaped structures about 3 to 5 centimeters in length and are situated close to the lateral wall of the pelvic cavity. The ovary has a stroma consisting of spindle shaped fibroblast like cells, collagen fibers, smooth muscle and ground substance. The surface epithelial lining of the ovary is a continuation of the peritoneum. During development, the germ cells from the wall of the yolk sac migrate to the developing ovarian cortex at the 6thweek intra uterine life. These cells become oogonia around 4 to 5 months intra uterine life. Some of these cells enlarge and become primary oocytes. HUMAN EMBRYOLOGY -06-
    • At 7,hmonth intra uterine life these primary oocytes are covered by a single layer of flattened cells called follicular cells and are referred to as the primordial follicles. During foetal life 700,000 to 2 million primordial follicles nre present. Several of these degenerate during foetal life and at birth 400,000 primordial germ cells are present. The primary oocytes do not complete their first meiotic division until puberty. From birth to puberty •everal of these undergo further degeneration. After puberty the primary oocyte gives rise to a secondary oocyte and a polar body. The ova are produced in the ovaries by the process of oogenesis. The changes that take place in the primordial germ cells in the ovary after puberty are shown in figure 5. The maturation of the primordial germ cells to the graafian follicle is influenced by the follicle stimulating hormone (FSH) and luteinising hormone (LH) secreted by the anterior pituitary. At the time of ovulation the graafian follicle is about 15 millimeters in diameter Imd the primary oocyte has completed its first meiotic division. Ovulation is the process whereby the mature ovum is shed into the peritoneal cavity. The remaining part of the graafian follicle forms the corpus luteum. The maturation of a primordial germ cell into a graafian follicle, ovulation and the formation of the corpus luteum is collectively known as the ovarian cycle. During each ovarian cycle about 5 to 15 primordial germ cells starts lo mature but only one primordial germ cell undergoes full maturity and the others become atretic. The ovaries shed the mature ovum with its r.urrounding cells (cumulus oophorus) into the peritoneal cavity. Zona pellucida Oocyte Primordial germ cell Oocyte Zona pellucida Follicular cells Theca externa and interna Follicular antrum Zona pellucida Oocyte Mature graaffian follicle HUMAN EMBRYOLOGY -07-
    • The genital tract consists of a pair of fallopian tubes (uterine tubes or oviducts), uterus, cervix, vagina and vulva. The fallopian tube has finger like processes at its proximal end called fimbriae. The broadest area of the tube is the ampulla, which is the normal site for fertilization. The fallopian tubes open into the uterus, a muscular organ that consists of three layers namely, endometrium- the innermost mucosal lining ■ myometrium - the middle smooth muscle layer ■ perimetrium - outermost connective tissue peritoneal covering Uterus is the site for implantation and development of the placenta and the foetus. The uterus consists of the fundus, body and cervix. The cervix opens into the vagina through the internal and external os. At the external opening of the vagina is the vulva which consists of the labia majora, labia minora and the clitoris. C yclic changes in the endom etriumafter puberty At puberty the first cyclical change of the endometrium occur with a bleeding of 3 to 4 days and is called menarche. These cyclical changes take place under the influence of hormones produced by the ovary (oestrogen and progesterone). From there onwards the endometrium undergoes cyclical changes every 28 days until menopause. This is known as the menstrual cycle. The menstrual cycle is defined as the period from one bleeding to another. The endometrium consists of a basal cell layer with a rich blood supply. The blood vessels present in this layer are branches of the ovarian and uterine arteries and are morphologically straight arteries. During the cyclical changes the endometrium pass through three stages namely menstrual phase, proliferative or follicular phase and secretory or progestational phase. HUMAN EMBRYOLOGY -08-
    • The menstrual phase lasts 3 to 4 days with bleeding and breakdown of the cell layers of the endometrium leaving only the basal cell layer. The menstrual phase is followed by the proliferative or follicular phase where the thickness of the endometrium increase by the proliferation and increase of epithelial cells and blood vessels, influenced by the oetrogens secreted by the theca interna of the developing follicle. The layer thus formed is called the spongy layer. During the secretory phase further thickening of the endometrium by proliferation of the epithelium forming simple tubular glands and increase in the number and size of straight and spiral arteries takes place. This happens in response to progesterone secreted by the corpus luteum. The compact layer of the endometrium is lormed during the secretory phase. At this stage the endometrium consists of all three layers namely the basal cell layer, spongy layer and compact layer. The endometrium is now ready to receive the fertilized ovum (implantation). The changes that take place in the endometrium are shown In figure 6. HUMAN EMBRYOLOGY -09-
    • S tru c tu re o f th e o vu m Zona pellucida Corona radiata The component parts of the maWre ovum are shown diagrammatically in figure 7. The ovum at the time of shedding into the peritoneal cavity is at the 2n meiotic division. The second meiotic division of the oocyte is not d completed until after penetration by the spermatozoa. The oocyte at this stage is called the secondary oo^Yte- The oocyte and the polar body are covered by the zona pellucida a^d corona radiata (protective cell layer) forming the cumulus oophorus. T E M M A YG N H A M R LA D The mammary gland or breast is a component part of the female reproductive system. It is a modif'ed apocrine sweat gland. It consists of several compound tubular acinar glands embedded in adipose and collaginous tissue, separated intc? lobes by thick collaginous septa. About 15 to 20 lactiferous ducts draining these secretory units separately open to the surface of the nipple. Before entering to the surface the ducts form dilatations called lactiferous sinusesThe breast develops from the ectoderm at any point along the mammary line (milk line) extending from the ax> a to the groin. In the human only one H breast develops on one side in the thoracic wall in the mammary line. An ectodermal thickening grows ar,d proliferates into the underlying mesoderm and forms a compound tubular acinar gland. The acinar cells and lining of ducts are ectoderrY>al in origin. The connective tissue, the smooth muscle in the areolar and adipose tissue are mesodermal in origin. HUMAN EMBRYOLOGY
    • I I UTILIZATION I firtilization takes place in the ampullary region of the fallopian tube. At ovulation the mature oocyte (secondary oocyte) is shed into the peritoneal ( -ivity with cumulus oophorus. The fimbriated end of the uterine tube (Itillopian tube) lie very close to the ovary and the ovum is swept into the ulorine tube. The ovum is carried along the tube by ciliary action and pciristaltic waves to the ampullary region of the fallopian tube where Inrtilization takes place. Al fertilization the male and female gametes (sperm and ovum) fuse and form a fertilized ovum containing male and female pronuclei. I he sperms that enter the female genital tract undergo capacitation and wcrosome reaction before fertilization. During capacitation the glycoprotein and several plasma proteins covering the head of the sperm Is removed. This provides the sperm to penetrate corona radiata and undergo acrosome reaction, which occur after binding with zona pellucida. During acrosome reaction acrosin and trypsin like substance produced by the sperm head helps to penetrate the zona pellucida. The sperm head fuse with the cell membrane of the oocyte and the nuclear material of the Hperm is injected into the cytoplasm of the ovum, completing fertilization. The fertilized ovum contains the male and female pronuclei and become diploid. Results of fertilization are as follows ■ Restoration of diploid number of chromosomes ■ Determine the chromosomal sex of an individual ■ Initiation of cleavage (cell division) If fertilization takes place the corpus luteum continue to secrete progesterone and become the corpus luteum of pregnancy upto the 4th month and afterwards become atretic. By this time the developing placenta starts to secrete progesterone and maintain the pregnancy. HUMAN EMBRYOLOGY -1 1
    • If fertilization fails to occur, the corpus luteum cease to secrete progesterone and after about 9 days it becomes atretic and form a scar tissue called corpus albicans. Due to the low levels of progesterone, menstrual bleeding takes place and a new menstrual cycle is initiated. FU TH R D V LO M N O TH FE TILIZE O U R E E E P E T F E R D V M Fimbriae follicle Corpus luteum The fertilized ovum undergoes cell division while travelling along the fallopian tube towards the uterine cavity. This is facilitated by the ciliary action and peristalsis of the fallopian tube. The path taken by the fertililized ovum to the site of implantation is shown in figure 8. The following changes take place during its passage in the fallopian tube. ■ Zygote- the male and female pronuclei mix and divide giving rise to a two-cell stage called the zygote. Each of these cells consists both maternal and paternal chromosomal material. ■ M orula- the two cells of the zygote undergo several mitotic divisions giving rise to a cell ball with 12 to 16 cells called the morula. The covering cells of the morula are called the outer cell mass and the cells within are called the inner cell mass. HUMAN EMBRYOLOGY -12-
    • • Blastocystfluid filled cavities appear within the inner cell mass which later fuse and give rise to a cavity called the blastocoele. The cells covering the blastocyst remain as the outer cell mass or trophoblast and the cells within it remain as the inner cell mass. The inner cell mass give rise to the embryo proper and the outer cell mass or trophoblast give rise to the foetal membranes (chorion and amnion) and placenta. At this stage the blastocyst is within the uterine cavity and is ready for implantation. The structure of the zygote, morula and blastocyst is shown in figure 9. Zygote (2 cells) Morula (12-16 celis) Blastocyst Im plantation Implantation takes place in the posterior wall of the upper uterine segment around the 8thday of development. Implantation can take place in several abnormal sites. These sites are given below. HUMAN EMBRYOLOGY
    • ■ Intra abdominal commonest site is the recto uterine region (pouch of Douglas) ■ Ampullary region ■ Tubal within the fallopian tube. ■ Interstitial in the narrowest part of the fallopian tube. ■ Internal os ■ Ovarian At this time the trophoblast (outer cell mass of the blastocyst) is differentiated in to two distinct cell layers namely ■ Cytotrophoblast- inner cell layer consisting of mononucleated cells showing mitotic figures. ■ Syncytiotrophoblast- outer cell layer consisting of multinucleated cells without mitotic figures. A cavity appears within the inner cell mass and is known as the amniotic cavity. At the floor of this cavity two cell layers are seen. ■ Hypoblast layer-cuboidal cells adjacent to the blastocyst cavity. ■ Epiblast layer- high columnar cells adjacent to the amniotic cavity. This stage is referred to as the bilaminar germ disc. The lining cells of the rest of the amniotic cavity are called amnioblasts. The structure of the bilaminar disc is shown in figure 10a and 10b. HUMAN EMBRYOLOGY -14-
    • G astrulation Gastrulation occur at the 3rdweek intra uterine life and is characterized by the establishment of the three germ layers namely ectoderm, endoderm and mesoderm. In the bilaminar embryonic disc the primitive streak, a thickening in the midline extending from the caudal end towards the cephalic end, forms on the surface of the epiblast. At its cephalic end an elevated area called the prmitive node appears with a small pit known as the primitive pit. Cells from the epiblast invaginate through the primitive pit between the epiblast and the hypoblast and gives rise to the intra embryonic mesoderm. In two areas at the cephalic and caudal ends these cells do not pass through the epiblast and the hypoblast. These two areas where the two layers are fused are known as the prechordal plate and cloacal membrane. Some of these invaginating epiblast cells displace the hypoblast layer and gives rise to the endoderm. Once the mesoderm is formed the remaining cells of the epiblast is known as the ectoderm. The direction of the invagination of the epiblast cells is shown in figure 11. HUMAN EMBRYOLOGY 15.
    • Prehordal plate No co rd Prnitives node Prnitive streak Clacal membrane The invaginating cellss that traverse forwards in th m idline in a cephalic direction up to the p orechordal plate gives rise j th e notochord. The prechordal plate (fu uture buccopharyngeal meibra ne) and cloacal membrane are devoid Id of mesoderm. A small diverfculum from the caudal end of the yolk sac a p p e a rs and extends into the conecting stalk, which is known as the allantoioc diverticulum. Now the emtyonic disc consists of three germ layers ancrd is called the trilaminar disc! fig. 12a and 12b). At this stage three dist'nmct areas are present at th cephalic end of the embryo namely the septum transversum, prco cardiac area and prechordal plate. Thiss is shown in a longitudinabection of the embryo given in figure 12c. Notochord Ectoderm Mesoderm Fig 12a Trilaminar disc (coronal section) Endoderm Body stalk Ectoderm Endoderm Primitive knot HUMAN EMBRYOLOGSY -16-
    • Notochord Prechordal plate YS Fig 12c Trilaminar disc (longitudinal section) Cloacal membrane Within the intra embryonic mesoderm a cavity appears and is called the (inolomic cavity. I loxion of the em bryo At the beginning of the 4th week intra uterine life flexion of the embryo (folding) takes place. This includes the cranial fold (head fold), caudal fold (tail fold) and two lateral folds. Due to the flexion of the embryo the ©ndoderm lined gut is formed. The gut is now connected to the yolk sac by n small tube called the vitello-intestinal duct. The septum transversum, protocardiac area and prechordal plate acquires its final position due to the flexion of the embryo. The different stages of the flexion of the cranial part of the embryo (head fold) and the final position of the septum transversum, prechordal plate and the protocardiac area are shown in ______________ figures 13a to 13c. T Neural tube Protocardiac area Precordal plate HUMAN EMBRYOLOGY Septum transversum
    • Precordal plate Septum transversum Protocardiac area Fig 13b Septum transversum HUMAN EMBRYOLOGY
    • Mu' prechordal plate is now called the buccopharyngeal membrane, 'V h begins to rupture around 41 weeks intra uterine life. At this time the iii« /2 '!"V<>|opment of the heart is seen in the protocardiac area. The septum iMiisversum later gives rise to the central tendon of the diaphragm, • i innective tissue and kupffer cells of the liver, lesser omentum and falciform ligament. A coronal section of the embryo showing the arrangement of the i>i toderm, endoderm and different types of mesoderm at the 4thweek intra uterine life afterflexion is shown in figure 14. Neural tube Ectoderm Notochord Paraxial mesoderm Coelomic cavity Intermediate mesoderm Gut endoderm Visceral -i Lateral . r plate Somatic mesoderm HUMAN EMBRYOLOGY -19-
    • The distribution of the embryonic mesoderm and derivatives are given below ■P araxial m esoderm mesoderm surrounding the developing neural tube. The paraxial mesoderm shows segmentations, which are called somites. In each segment a pair of somites are seen. Each of these somites further divide into three parts namely the sclerotome, dermatome and myotome. The sclerotome gives rise to the vertebrae, annulus fibrosus of intervertebral disc and ribs of each segment. The dermatome gives rise to the dermis of the same segment. The myotome gives rise to the muscles supplied by the segmental nerve. ■ Lateral plate m esoderm - Somatic mesoderm (somatopleuric intraembryonic) lies beneath the ectoderm giving rise to the body wall muscles. - Splanchnic mesoderm (splanchnopleuric intraembryonic) lies beneath the endoderm giving rise to the muscles of the gastro intestinal tract and bladder. ■ Interm ediate m esoderm between the lateral plate mesoderm and the paraxial mesoderm giving rise to parts of the genitourinary system. D evelopm ent of the neural tube The nervous system starts to develop from the ectoderm as early as the 3rd week intrauterine life immediately after the three germ layers are formed. In the midline an ectodermal thickening forms due to the inductive effect of the notochord and extends in the cranial and caudal directions. This thickening is called the neural plate and these cells constitute the neuroectoderm. Further growth of this neural plate forms a depression and is called the neural groove. The lateral edges of the neural groove are called neural folds (neural crests), which later fuse together and give rise to the neural tube. Some of the cells of the neural folds separate and form neural crest cells. Figure 15 gives a diagrammatic representation of the HUMAN EMBRYOLOGY
    • Neural groove Notochord Endoderm D erivatives of the three germ layers E ctoderm ■ Central nervous system including neural crest cells ■ Peripheral nervous system ■ Sensory epithelium of ear, eye and nose ■ Skin (epidermis) and its appendages (hair, nails, sebaceous glands and sweat glands) ■ Mammary gland (secretory component and ducts) ■ Pituitary gland (Rathke's pouch and neuroectoderm) ■ Secretory acini and ducts of parotid salivary gland ■ Enamel ofteeth ■ Lens of the eye HUMAN EMBRYOLOGY -21-
    • Endoderm ■ Lining epithelium of the gastro-intestinal tract ■ Secretory acini and ducts of submandibular and sublingual salivary glands ■ Alveolar paranchyma and lining epithelium of the respiratory tract ■ Liver paranchyma and epithelium of the gall bladder ■ Exocrine pancreas ■ Epithelium of the urinary bladder and urethra ■ Paranchyma of the thyroid and parathyroid glands ■ Epithelial lining of the tympanic cavity and auditory tube ■ Epithelium of the palatine tonsil ■ Hassel's corpuscles of the thymus MesodermP araxial m esoderm - forms somites segmentally. Each somite consist of a sclerotome, myotome and a dermatome. ■ Sclerotome - vertebral bodies, ribs, annulus fibrosus of intervertebral discs ■ Myotome - muscles supplied by the segmental nerves ■ Dermatome - skin supplied by the segmental nerves HUMAN EMBRYOLOGY
    • (himm ediate m esodem - Parts of the genito urinary system l <iloi.il plate m esoderm • Somatic mesoderm- body wall muscles, parietal pericardium and parietal pleura • Splanchnic mesoderm- muscles and connective tissue of the gastro­ intestinal tract, visceral pericardium and visceral pleura Iti m ichial m esodermgives rise to the mesodermal structures in the head and neck region, supplied by the cranial nerves. HUMAN EMBRYOLOGY
    • D erivatives of neural crest cells ■ Sensory and autonomic ganglia (sympathetic and parasympathetic) ■ Ganglia of cranial nerves (V, VII, IX, and X) ■ Melanocytes ■ Schwann cells and adrenal medulla ■ Pia mater and arachnoid mater ■ Islets of Langerhans ■ Endocrine secreting cells of the gut ■ Dentine, pulp, cementum and periodontal membrane of the teeth T IN IN W N G Development of two or more individuals at the same time inside a uterine cavity is known as twinning. There are two types of twins namely. ■ Dizygotic (fraternal)-70% ■ Monozygotic (Identical)-30% Dizygotic twins are produced by the fertilization of two ova by two separate sperms giving rise to two separate zygotes. The dizygotic twins, ■ do not resemble each other ■ can have the same or opposite sex ■ have a different genetic constitution ■ always have two placentae, two amniotic cavities and two chorionic cavities. HUMAN EMBRYOLOGY -24-
    • Monozygotic twins are produced by the fertilization of one ovum by a «l mrrn giving rise to a zygote. The monozygotic twins, • resemble each other ■ always have the same sex ■ have a similar genetic constitution I ho zygote acts in the following ways in producing monozygotic twins. ■ The two cells of the zygote can separate and give rise to two individuals. These twins will have two placentae, two amniotic cavities and two chorionic cavities. ■ The zygote undergoes cell division giving rise to morula and blastocyst, with two separate inner cell masses. These twins will have one placenta, two amniotic cavities and one chorionic cavity. ■ The zygote undergoes cell division giving rise to morula and blastocyst, with two fused inner cell masses. These twins will have a common placenta, amniotic cavity and chorionic cavity. C N E IT L A N R A O G N A B O M LITIE S Congenital abnormalities include gross structural defects to anomalies at cellular or molecular level. 'S °/o to 3% of all live births have congenital abnormalities. Of these 10% are genetic, 10% are environmental and the rest include both. HUMAN EMBRYOLOGY
    • Factors that causes congenital abnorm alities ■ Infections eg: rubella, syphillis, viruses, toxoplasmosis etc... ■ Physical eg: radiation ■ Chemical eg: thalidomides, anti-convulsant drugs etc.. ■ Nutritional eg: vitamin deficiencies, iodine deficiency, cretinism ■ Hormonal eg: maternal diabetes, sex hormones, cortisones etc.. ■ Genetic eg: Duchenne muscular dystrophy, hemophilias, neurofibromatosis, Down syndrome (mongols) and Turners syndrome HUMAN EMBRYOLOGY -26-
    • PLACENTA During early embryonic life the developing embryo gets its nutrition through diffusion. After the three germ layers are formed the placenta develops, supplying nutrition to the embryo. In the human, implantation is Interstitial. After implantation the uterine wall is called the decidua. The ilnddua consists of three parts namely the deicidua basalis, decidua I mrietalis and decidua capsularis. This is shown in figure 16. Decidua basalis (m - Decidua capsularis Decidua parietalis ' Fig 16 I he functional elements of the placenta are called villi. Villi form all over the trophoblast at first, but later the villi in the decidua basalis develop and form h disc shaped mass called the placenta. Syncytiotrophoblast Cytotrophoblast Trabeculae Cytotrophoblast HUMAN EMBRYOLOGY Decidua Extra embryonic mesoderm Lacunar spaces (maternal) Extra embryonic mesoderm
    • Syncytiotrophoblast Cytotrophoblast Extra embryonic mesoderm Trabeculae Decidua Cytotrophoblast Extra embryonic mesoderm Syncytiotrophoblast Blood vessels Decidua Cytotrophoblastic shell Blood vessels Anchoring villus Fig 18d Extra embryonic mesoderm After implantation the trophoblast consists of syncytiotrophoblast (outer layers) and cytotrophoblast (inner layers) and extra embryonic mesoderm (Figure 17a). Rapid multiplication of cells in the syncytiotrophoblast takes place and cavities (lacunae) appear within it (figure 17b). Due to the invasion of the syncytiotrophoblast cells into the decidua, uterine blood enter the lacunae spaces. HUMAN EMBRYOLOGY -28
    • I Proliferation of the cytotrophoblast cells into the syncytiotrophoblast forms Imboculae between lacunar spaces. This is called a primary villus. A primary villus consists of a central core of cytotrophoblast cells surrounded I •y •.yncytiotrophoblast cells (Figure 18a). A fiocondary villus is formed due to the proliferation of extraembryonic inrmoderm and growing into the cytotrophoblast in the primary villus. A 'tncondary villus consists of a central core of extraembryonic mesoderm surrounded by cytotrophoblast cells and syncytiotrophoblast cells (Figure 1Bb). Wood vessels developing in the extraembryonic mesoderm enter the uocondary villus thus giving rise to a tertiary villus. A tertiary villus consists of a central core of extraembryonic mesoderm with blood vessels Hurrounded by cytotrophoblast cells and syncytiotrophoblast cells (Figure 18c). The cytotrophoblast cells of the tertiary villus penetrate the syncytiotrophoblast cells and spread along the decidua forming a cytotrophoblastic shell (Figure 18d). This leads to the formation of nnchoring villi. Branchings of the anchoring villi form and increase the surface area of the placenta for exchange of substances. Figure 19 shows the placenta with anchoring villi and branches. Branchings of anchoring villi Umbilical cord On the maternal side connective tissue septa grow into the lacunar spaces thereby separating the maternal side of the placenta into several elevations known as cotyledons (Figure. 20a). HUMAN EMBRYOLOGY
    • The full term placenta has, ■ 15 to 20 cotyledons ■ adiameterof6to8inches ■ a weight of 500 to 600 grams ■ a total exchange area of 14 square meters Cortyledon Maternal surface Septa Ifl Further development of placenta (septa formation from maternal tissue) Fig 20a Figure 20b shows the structure of a fully formed placenta. At the 4thmonth intra uterine life the placental barrier is formed due to the thinning of the trophoblast cells separating the maternal and foetal blood. The placental barrier consists of a syncytiotrophoblast cell layer, basement membrane and an endothelial cell layer. The placenta performs hormonal, respiratory, nutritive and immunological functions. HUMAN EMBRYOLOGY -30-
    • Cortyledon Maternal surface Septa Intervillus space Umbilical cord Placenta at term Fi9 2°b Abnorm alities of the placenta • Placenta praevia- formation of palcenta in the lower uterine segment. There are different degrees of placenta praevia. The most serious condition is when the palcenta forms on the internal os obstructing it. • Placenta acreta- formation of palcenta as separate parts (present in pieces). • Velamentous insertionfoetus. HUMAN EMBRYOLOGY blood vessel branching before entry into -31-
    • 3 C1 RCU LATORY SYSTEM DEVELOPMENT OR THE HEART I Me b lo o d v e s s e ls s t a r t to d e v e lo p a t t h e t h ir d w e e k : in t r a u t e r in e life in t h e r i lo s o d e r m b e f o r e t h e f le x io n o f t h e e m b r y o . * C o lle c tio n s o f a n g io g e n ic c e lls d e v e lo p in th e s p la n c h n ic m e s o d e r m in t h e p r o t o c a r d ia c a r e a ( c a r d io g e n ic a r e a ) . T h e s e c e lls fo rm a n g io b la s t c e ll ■ lu s t e r s w h ic h la t e r g iv e r is e to tw o e n d o c a r d ia l h e a r t tu b e s ( p r im it iv e h e a r t tu b e s ) . D u e to f le x io n o f t h e e m b r y o t h e tw o e n d o c a r d ia l h e a r t t u b e s ip p r o a c h e a c h o t h e r a n d f u s e to f o r m t h e p r im a r y h e a r t t u b e . T h e p r im a r y I to a r t t u b e h a s a d o r s a l m e s o c a r d t u m ( d o r s a l m e s e n t e r y ) b u t d o e s n o t h a v e a v e n t r a l m e s o c a r d iu m . L a t e r a c a v it y a p p e a r s w it h in t h e d o r s a l I I v e s o c a r d iljm a n d f o r m s t h e t r a n s v e r s e s in u s ( T S ) . T h e f u s e d p r im a r y I lo a r t t u b e is s u s p e n d e d in th e p e r ic a r d ia l c a v it y b y th e c r a n ia l a n d c a u d a l h lo o d v e s s e ls . T h e t u b e f u r t h e r b e n d s a n d f o r m s a c a r d ia c lo o p a t th e 4 th w e e k in tr a u t e r in e life d u e to t h e f le x io n o f t h e e m b r y o . E x p a n s io n s a p p e a r In th e c a r d ia c lo o p t h r o u g h o u t its le n g t h . ( >ne e n d o f t h e b e n t t u b e b e c o m e s t h e a r t e r ia l e n d a n d t h e o t h e r t h e v e n o u s e n d . R ro m t h e a r t e r ia l e n d th e e x p a n d e d p a r t s o f t h e t u b e a r e t h e I r u n c u s A r t e r io s u s ( T A ) , O o n u s O o r d is ( C C ) B u lb o u s C o r d is ( B O ) P r im it iv e V e n t r ic le (F */) P r im it iv e A t r iu m ( FV) a n d S in u s / e n o s u s (S /). 1 h e s in u s v e n o s u s c o n s is t s o f tw o h o r n s ( le f t a n d r ig h t ) in to w h ic h d r a in t h e lo ft a n d r ig h t u m b ilic a l v e in s , v it e llin e v e in s a n d c o m m o n c a r d in. a I v e in s . I ig u r e 2 2 a to d s h o w 's t h e f o r m a t io n o f t h e c a r d ia c lo o p . I lUIVlAfsl EMBRYOLOGY -3 3
    • TA CC BC PV PA SV Fig 22b TS OS Parietal Pericardium { Visceral Fig 22d S epta form ation in the heart PT BC O Spiral septum - - PV PA ( RV Inter ventricular septum RA p r_ | | ~ r Inter atrial — ~' septum Fig 23a Fig 23b Figures 23a and 23b give a diagrammatic representation of the compartments of the primitive heart tube and the separation into different chambers. HUMAN EMBRYOLOGY -34-
    • IA lmncus arteriosus V-sinus venosus I < i:onus cordis : AA-ascending aorta HC bulbous cordis PT-pulmonary trunk !'/ primitive ventricle RV-right ventricle I'A primitive atrium LV- left ventricle I ‘ .»transverse sinus RA- right atrium • )S- oblique sinus LA- left atrium I hivelopm ent of the atria an the interatrial septum d I he primitive atrium and the primitive ventricle are separated by the formation of myocardial cushions that form the atrioventricular septum, these myocardial cushions later give rise to the mitral and tricuspid vnlves. The primitive atrium is divided into left and right atria by the formation of the interatrial septum. The sinus venosus opens into the right wide of the primitive atrium. Two valves namely the left and right valves of Hie sinus venosus are present at its opening. These two valves are fused together and are attached to the roof of the primitive atrium by a thick band called the septum spurium (Figure 24). from the roof of the primitive atrium a thick muscular septum namely the primum grows down separating the primitive atrium partly into left and right sides. The blood flows from the right side of the primitive atrium to the left side through a space between the septum primum and the atrioventricular septum (Figure 24). This space is known as the foramen primum (primary foramen/osteum primum). As soon as the septum primum grows further down and approaches the atrioventricular septum it closes the ostium primum detaching from its attachment to the roof of the atrium. Now the blood flows from the right atrium to the left atrium through the new opening, which is called the foramen secondum (secondary foramen/osteum secondum). nupturn HUMAN EMBRYOLOGY -35-
    • Another septum known as the septum secondum grows from the roof of the primitive atrium on the right side of the septum primum and overlaps the septum primum thereby closing the ostium secondum and creating an oblique pathway for blood to flow from the right to the left side. This oblique pathway persists up to birth and is called the foramen ovale (Figure 25). Septum primum Septum spurium Sinus J venosus r Rt valve Ostium primum Lt valve Myocardial cushions Absorbed septum spurium Crista terminalis Foramen ovale Valve of IVC Septum secondum Valve of coronary sinus Septum primum Fig 25 Due to the shunting of blood from left to right side of the sinus venosus during the 4thweek intra uterine life, the right sinus venosus and the veins enlarge and directly connect with the right side of the primitive atrium. Later the right horn of the sinus venosus gets absorbed into the right atrium and gives rise to the smooth part of the right atrium. The remaining part of the right side of the primitive atrium forms the rough part of the right atrium. Left horn of the sinus venosus remains rudimentary and gives rise to the coronary sinus and the oblique vein of the left atrium. The rough part of the left atrium is formed from the left side of the remaining primitive atrium and the smooth part is formed from the absorbed pulmonary veins. HUMAN EMBRYOLOGY -36-
    • ......... . the absorption of the sinus v^nosus to the ri9ht side of ,he Primitive itrlum, the left valve of the sinus v ^nosus and the sePtum sPurium fuse with the septum secondum. The upPer Part of the ri9ht valve of the sinus vnnosus gives rise to the crista termina'is. which is seen as the sulcus .....nlnalis on the outer surface o f the ri9ht atrium - This separates the month and the rough parts of the r®ht atrium - The lower P ^ o f the right ilvo of the sinus venosus gives ris ^ t0 the valve of the ,nferior vena cava nid the coronary sinus. M birth with the initiation of the p u l'"™ 81? circulation, blood pressure in .... left atrium increases compare^ t0 the ri9ht atrium ' Due to this the •upturn primum and septum secondum < with the fused left valve of the .Inus venosus and the septum *P urium > approach each other and lunctionally close the foramen o v ^|e- Structural closure of the foramen . iv.ile takes place within the first ye*>r o f life- The closed foramen ovale is presented as the fossa ovalis in t*!e interventricular septum in the fully iloveloped heart. I ho embryological structures that cjive rise to the interventricular septum include, ■ Septum primum ■ Septum secondum ■ Septum spurium ■ Left valve of sinus venosus Developm ent of the ventricles an< ^*he int® rven,r'cularseP ,um I he primitive ventricle, bulbous ^ordis and conus cordis forms the common ventricle which is s e p a ra t^ int° 'eft and right ventricles by the formation of the interventricular sePtum- The interventricular septum is i loveloped from four embryonic s tru ^ ures namely, ■ Muscular septum- this forms P e main bulk of the interventricular septum and is formed from the flo^r of the Primitive ventricle HUMAN EMBRYOLOGY
    • DEVELOPMENT OF ARTERIAL SYSTEM Aortic arches fThe aortic arches appear during the fourth and fifth weeks intrauterine life when the pharyngeal arches develop at the head end of the embryo. Initially six pairs of aortic arches form in the developing neck region. These aortic arches arise from the left and right ventral aortae (developed from the truncus arteriosus) and end in the left and right dorsal aortae. The two dorsal aortae fuse to form a common dorsal aorta caudal to the pharyngeal arches(Figure 27a). Several changes in the aortic arches occur during development to achieve the final arterial system. Of the six pairs of aortic arches, the fifth pair persists only for a short period and regress. The first and the second pairs of aortic arches disappear leaving their remnants as the maxillary and stapedial arteries respectively. The third aortic arch on both sides gives rise to the common carotid artery and the proximal part of the internal carotid artery. The fourth aortic arch on the left side enlarges and becomes the arch of aorta and that on the right forms the proximal part of the right subclavian artery. The proximal part of the sixth aotic arch on both sides becomes the left and right pulmonary arteries and the distal part of the sixth aortic arch on the left side becomes the ductus arteriosus and that on the right side disappears. The ductus arteriosus persists up to birth and after birth becomes the ligamentum arteriosum (Figure 27b). The recurrent laryngeal nerve, a branch of the vagus nerve is given off in the developing neck region and descends down on either side of the aortic arches. The left and right nerves hook around the left and right sixth aortic arches and ascend up to supply the developing larynx. During further development due to the changes that take place in the fifth and sixth aortic arches, the final course of the left and right recurrent laryngeal nerves differ on the two sides. HUMAN EMBRYOLOGY -40-
    • The changes that take place on the right side include ■ Disappearance of the fifth aortic arch Diappearance of the distal part of the sixth aortic arch leaving only the proximal part intact giving rise to the right pulmonary artery, Due to the above changes the right recurrent laryngeal nerve gets pulled up and finally hooks around the fourth aortic ar£h which later becomes the right subclavian artery. Therefore the right recurrent laryngeal nerve is seen in the neck region. The changes that take place on the left side include Disappearance of the fifth aortic arch ■ Persistence of the sixth aortic arch (proximal part giving rise to the left pulmonary artery and distal part to the ductus arteriosus finally becoming the ligamentum arteriosum after birth) Due to the above changes the left recurrent larYn 9eal nerve remains in its original position related to the sixth aortic aroh, which becomes the left pulmonary artery and ligamentum arteriosum (figure 27b). I igure 27c shows a diagrammatic representation of the arteries derived from the aortic arches, at birth. External carotid Internal carotid Arch of aorta Ductus arteriosus Descending aorta Developing lung HUMAN EMBRYOLOGY -41-
    • Lt subclavian External carotid Arch of aorta Rt. subclavian Ductus arteriosus Rt. pulmonary Pulmonary trunk Descending aorta Lt pulmonary Fig 27c Summary o f the fate of A ortic Arches ■ First- left and right maxillary artery ■ Second- left and right stapedial artery ■ Third- left and right Internal carotid artery (proximal) and Common carotid artery ■ Fourth- left- arch of aorta, right- right subclavian ■ Fifth- disappear on both sides ■ Sixth- left- pulmonary artery and ductus arteriosus right- pulmonary artery ■ Dorsai aortaleft and right internal carotid artery (distal) descending aorta right subclavian artery (distal) HUMAN EMBRYOLOGY -42-
    • ■ Ventral aorta- left and right external carotid artery left and right common carotid artery Left and right pulmonary artery D V LO M N O T E V N U S S E E E P E T F H E O S Y T M Sinus venosus Liver Duodenum Sinus venosus Lt. Umbilical vein The sinus venosus receives three pairs of veins at the fifth week intrauterine life. These are the vitelline veins draining the yolk sac, umbilical veins draining the chorionic villi (placenta) and the cardinal veins draining the rest of the body (Figure 29a). HUMAN EMBRYOLOGY -43-
    • Rt. hepato cardiac channel (posthepatic IVC) Ductus venosus Portal vein Rt. Vitelline v. (sup. mesenteric v.) Lt. Umbilical vein Fig 30 The left and right vitelline veins lie close to the developing liver and form sinusoidal spaces within it. In the region of the gut they anastomose with each other. The proximal part of the left vitelline vein disappears while that on the right enlarges and becomes the main source of venous drainage to the heart from the gut and liver (Figure 29b). This part of the right vitelline vein is called the right hepatocardiac channel, which later forms the post hepatic part of the inferior vena cava (Figure 30). The vitelline anastomosis forms the portal vein and the distal part of the right vitelline vein forms the superior mesenteric vein. The right umbilical vein and the proximal part of the left umbilical vein disappears while the distal part of the left umbilical vein obtains connections with the hepatic sinusoids (Figure 30). The venous blood from the left umbilical vein is therefore shunted across the liver via the ductus venosus to the right hepatocardiac channel. At birth when the placental circulation ceases the left umbilical vein is obliterated and later becomes the ligamentum teres. The ductus venosus becomes the ligamentum venosum. At the fourth week intrauterine life two pairs of cardinal veins are seen. These are the anterior cardinal veins draining the cranial part and the posterior cardinal veins draining the caudal part of the developing embryo. These two pairs drain into the common cardinal veins which in turn drain into the left and right horns of the sinus venosus. At the fourth week intrauterine life three other pairs of cardinal veins appear. The subcardinal veins draining the kidneys, the sacrocardinal veins draining the pelvic area and the lower extrimities and the supra cardinal veins draining the body wall (Figure 3 1 ) . _____________________________ HUMAN EMBRYOLOGY -44-
    • Ilm posterior cardinal vein disappears leaving few remnants when the |M|tmc:ardinal veins appear which take over the venous drainage of the wall including the intercostal areas. Anastomoses appear between lit. i lull and right cardinal veins and gradually a left to right shunt of venous ....... I takes place. These anastomoses are the anterior cardinal .in.i-.tomosis, supracardinal anastomosis, subcardinal anastomosis and |H< locardinal anastomosis (Figure 31). Ant. Cardinal anastomosis Ant. Cardinal vein Common Cardinal vein Supracardinal vein Post. Cardinal vein Subcardinal vein Bmxacardinal anastomosis Sacrocardinal anastomosis Uibcardinal anastomosis Sup. Vena cava Azygos vein Inf. Vena cava Inf. Vena cava Distal part Common iliac vein HUMAN EMBRYOLOGY Sacrocardinal vein
    • The left anterior cardinal vein and anterior cardinal anastomosis gives rise to the left brachiocephalic vein. Superior vena cava is derived from the right anterior cardinal vein and the right common cardinal vein. Lower part of the left supra cardinal vein and the supracardinal anstomosis gives rise to the hemi azygos vein while the right supracardinal vein and the proximal part of the right posterior cardinal vein give rise to the azygos vein. The left renal vein is derived from the subcardinal anastomosis. The left subcardinal vein gives rise to the left gonadal vein. Hepatic and renal parts of the inferior vena cava are derived from the right subcardinal vein. The left sacrocardinal vein and sacrocardinal anastomosis gives rise to the common iliac vein. The distal most part of the inferior vena cava is derived from the right sacrocardinal vein (Figure 32). S m um ary of the developm ent of the venous system Embryonic structure Adult structure Right hepato cardiac channel- Post hepatic inferior vena cava Ductus venosus- Ligamentum venosum Right vitelline vein- Portal vein; Superior mesenteric vein Left umbilical vein- Ligamentum teres Anterior cardinal anastomosis- Left brachiocephalic vein Left posterior cardinal vein- Left superior intercostal vein Right common cardinal veinSuperior vena cava Right anterior cardinal vein- HUMAN EMBRYOLOGY -46-
    • Supracardinal anastomosisLeft supracardinal vein- Hemiazygos vein Right supracardinal veinAzygos vein Right posterior cardinal veinSubcardinal anastomosis- Left renal vein Left subcardinal vein(distal part)-Left gonadal vein Right subcardinai vein- Renal segment of the inferior vena cava Sacrocardinal anastomosis- Left common iliac vein Right sacrocardinal vein- Sacrocardinal segment of the inferior vena cava F E A C C LA IO O T L IR U T N Ductus arteriosus Pul. artery Descending aorta Ductus venosus Portal vein h u m a n e m b r y o lo g y -47-
    • The main source of exchange of gases and nutrition during foetal life is the placenta. A diagrammatic representation of the foetal circulation is shown in figure 33. The deoxygenated blood from the foetus is carried along the umbilical arteries to the placenta for oxygenation. After oxygenation the oxygenated blood, 80% saturated with oxygen returns to the foetus through the left umbilical vein, which enters the liver and is shunted across the ductus venosus to the post hepatic part of the inferior vena cava. The portal vein also enters the liver carrying deoxygenated blood from the gut (from superior and inferior mesenteric veins). Mixing of oxygenated and deoxygenated blood takes place in the liver. The blood that enters the right atrium from the inferior vena cava is shunted across the foramen ovale to the left atrium. The superior vena cava, which carries deoxygenated blood from the head and neck region, also enters the right atrium and the majority of this blood is directed through to the right ventricle and then to the pulmonary trunk. Mixing of oxygenated and deoxygenated blood also occurs in the right atrium. The oxygenated blood that enters the left atrium passes to the left ventricle and then to the aorta. The pulmonary veins drain deoxygenated blood from the lungs to the left atrium which mixes with the oxygenated blood. Most of the blood that enters the pulmonary trunk is shunted to the arch of the aorta through the ductus arteriosus and mixes with the oxygenated blood in the aorta. As the lungs are not performing the function of oxygenation, very little blood from the pulmonary trunk enters the left and right pulmonary arteries. The head and neck region receives oxygenated blood before it mixes with the blood from the ductus arteriosus. The descending aorta containing the mixed blood, supplies the thoracic and abdominal organs. The umbilical arteries finally carry blood with about 58% oxygen saturation to the placenta for oxygenation. HUMAN EMBRYOLOGY
    • lii the foetal circulation mixing of deoxygenated and oxygenated blood Irikos place in the following areas. ■ Liver- oxygenated blood from the left umbilical vein and deoxygenated blood from the portal vein ■ Right atrium- oxygenated blood from the inferior vena cava and deoxygenated blood from the superior vena cava ■ Left atrium- oxygenated blood from the right atrium and deoxygenated blood from the lungs (left and right pulmonary veins) ■ Arch of aorta- oxygenated blood from the left ventricle and deoxygenated blood from the pulmonary trunk (via ductus arteriosus) A U C C LA D LT IR U TIO N Ligamentum arteriosum / Sup. vena cava Inter atrial septum Pulmonary vein i Inferiror vena cava Pulmonary trunk L Ligamentum teres Medial umbilical ligament Descending aorta Portal vein >/ / Fig 34 Sup. Vesical artery Diagrammatic representation of the adult circulation is shown in figure 34. At birth when the umbilical cord is cut the placental blood flow ceases. With the first breath of the infant the lungs get inflated and the release of bradykinin, mediate the contraction of the ductus arteriosus and close It functionally. Structural closure of the ductus arteriosus takes 1 to < months. HUMAN EMBRYOLOGY
    • The obliterated ductus arteriosus is represented in the adult as the ligamentum arteriosum. The closure of the ductus arteriosus leads to an increase in the blood flow to the pulmonary arteries. The pulmonary veins carry more blood to the left atrium leading to an increase in the pressure within the left atrium compared to that of the right atrium. The difference in the pressure in the two atria causes apposition of the septum primum and septum secondum and temporarily closes the foramen ovale. Structural closure of the foramen ovale takes about a year. The closed foramen ovale is represented by the fossa ovalis in the adult. Contraction of the umbilical arteries occurs due to the cessation of blood flow and thermal and mechanical stimulation. The distal parts of the umbilical arteries become the medial umbilical ligaments and the proximal parts persist as the superior vescical arteries. The obliterated left umbilical vein and the ductus venosus form the ligamentum teres and the ligamentum venosum respectively. Foetal vessels that become ligaments at birth include ■ Left umbilical vein- ligamentum teres ■ Ductus arteriosus- ligamentum arteriosum ■ Ductus venosus- ligamentum venosum ■ Left and right umbilical arteries (distal part)- left and right medial umbilical ligaments HUMAN EMBRYOLOGY 50-
    • 4 RESPIRATORY SYSTEM I he respiratory system starts to develop at the fourth week intrauterine life it!, an endodermal diverticulum from the ventral aspect of the primitive foregut. This is called the tracheobronchial diverticulum or the lung bud which grows along with the splanchnic mesoderm to the coelomic cavity. I his part of the coelomic cavity is specifically known as the (Miricardioperitoneal cavity which lies on either side of the developing gut. I he tracheobronchial diverticulum and its relationship to the foregut is r.hown in figure 35. The pericardial and peritoneal cavities are separated from the pericardioperitoneal cavity by the formation of the pleuropericardial and pleuroperitoneal membranes respectively. The mmaining part of the pericardioperitoneal cavity forms the left and right pleural cavities. diverticulum The tracheobronchial diverticulum loses its connection with the foregut leaving its proximal attachment intact. The proximal part of the bud gives rise to the larynx and the trachea. As it grows it divides into two parts, which later give rise to the left and right principal bronchi (main bronchi). The left bud divides into two parts and the right bud into three parts giving rise to the secondary bronchi (lobar bronchi). The two divisions on the left and the three divisions on the right demarcate the two lobes of the left lung and the three lobes of the right lung respectively. Each of these divisions are covered by the splanchnic mesoderm (Figure 36a and 36b). HUMAN EMBRYOLOGY
    • Tracheo bronchial . diverticulum Somatic mesoderm (parietal pleura) Intra embryonic Splanchnic mesoderm (visceral pleura) c o e lom (pleural cavity) Fig 36 a Lobar bronchi Main bronchus Lung alveoli Somatic mesoderm (parietal pleura) Splanchnic mesoderm (visceral pleura) Intra embryonic coelom (pleural cavity) Further development of these bronchi occur in a dichotomous (always dividing into two) manner giving rise to 10 tertiary bronchi in the right and 8 in the left lung. The terminal parts of these divisions give rise to the lung alveoli (alveolar sacs). At sixth month intrauterine life about 17 subdivisions are formed and another 6 subdivisions are formed postnatally. The alveolar sacs increase in size and number during the last 2 months of prenatal life and few years after birth (up to 10 years). The alveolar sacs are lined by squamous cells called Type I alveolar epithelial cells and these with the capillary endothelial cells form the blood air barrier (respiratory diffusion barrier). At the end of the sixth month prenatal life the Type II cells or surfactant cells develop which produce a surfactant that helps to lower the surface tension at the air-blood interface. The lining epithelium of the respiratory tract and the Type I (pneumocytes) and Type II (surfactant) cells are derived from the endoderm. The mesodermal components namely the cartilages, smooth muscle, connective tissue and alveolar macrophages are derived from the splanchnic mesoderm. HUMAN EMBRYOLOGY -52-
    • ! he larynx is developed from the proximal part of the tracheobronchial dlvorticulum. The epithelium of the larynx is derived from the endoderm. I he mesodermal structures of the larynx namely the thyroid, cricoid, mytenoids, cuneiform and corniculate cartilages are derived from the fusion of the fourth and sixth branchial arch cartilages. The membranes niid ligaments of the larynx, connective tissue components, intrinsic muscles (thyroarytenoid, posterior and lateral crycoarytenoid, Inlerarytenoid and vocalis) and extrinsic muscle (cricothyroid) are derived (torn the sixth branchial arch mesoderm. Hence the intrinsic muscles are nupplied by the recurrent laryngeal nerve which is the sixth branchial arch norve. The extrinsic muscle is supplied by the external laryngeal branch of the superior laryngeal nerve which is the fourth branchial arch nerve. The visceral pleura is derived from the splanchnic mesoderm and the parietal pleura is derived from the somatic mesoderm. The pleural cavity is derived from the coelomic cavity. Summary of the development of the respiratory system Derivatives of the endoderm■ Epithelial lining of the larynx, trachea, bronchi, bronchioles and alveoli ■ Type I (pneumocytes) and type II (surfactant) cells Derivatives of the splanchnic mesoderm■ Cartilages of trachea, bronchi and bronchioles ■ Smooth muscle of bronchi and bronchioles ■ Connective tissue of the bronchial tree ■ Alveolar macrophages ■ Visceral pleura HUMAN EMBRYOLOGY
    • Derivatives of the branchial arch cartilages (fourth and sixth arches)Thyroid Cricoid Arytenoid > Cartilages of the larynx Cuneiform Corniculate ' Derivative of the somatic mesoderm■ Parietal pleura Derivative of the coelomic cavity (pleuroperitoneal cavity)■ Pleural cavity Developmental defects of the respiratory system ■ Agenesis of the lung- non development of the tracheobronchial diverticulum ■ Tracheo-oesophageal fistulaimproper separation of the tracheobronchial diverticulum from the foregut ■ Respiratory distress syndrome (hyaline membrane disease)occurs in premature infants when insufficient surfactant is present. Due to this air-blood surface tension increases resulting in collapse of the lung during expiration. This is a common cause of death in premature infants (20% of infant deaths). This is also called hyaline membrane disease due to accumulation of fluid consisting of a high level of proteins and hyaline membrane bodies. Artificial sufactant treatment is possible in reducing mortality. ■ Ectopic lung lobes- due to formation of additional divisions of the lung bud (tracheobronchial diverticulum). HUMAN EMBRYOLOGY -54-
    • i > VELOPMENT OF THE DIAPHRAGM i I he diaphragm is a musculotendinous structure separating the thorax from the abdomen. It consists of muscles originating from the lumbar vortebrae (crura), sternum and lateral body wall. These muscles converge towards the central tendon. A coronal section of the embryo showing the development of the diaphragm is shown in figure 37. The diaphragm is developed from four embryonic components. ■ Septum transversum- gives rise to the central tendon and is supplied by the phrenic nerve (anterior primary rami of 3rd 4thand 5th , cervical spinal nerves) ■ Left and right pleuroperitoneal membranes ■ Left and right lateral body wall muscles- originating from the lower six intercostal muscles. The left and right pleuroperitoneal membranes and the lateral body wall muscles give rise to the left and right domes of the diaphragm and is supplied by the lower six intercostal nerves. HUMAN EMBRYOLOGY
    • ■ Dorsal mesentery of t h e oesophagus- gives rise to the left and right crura. The diaphragm develops around the existing inferior vena cava, oesophagus and the descending aorta. The inferior vena cava, oesophagus and the descend ing aorta pass through the diaphragm at the 8th 10,hand 12ththoracic vertebral levels respectively. , HUMAN EMBRYOLOGY -56-
    • GASTRO INTESTINAL TRACT AND RELATED ORGANS PRIMITIVE GUT FORMATION AND BLOOD SUPPLY Ai ihe 4lhweek intrauterine life due to the flexion of the embryo the primitive gut is formed by the incorporation of a part of the endoderm lined yolksac. Ihe alantoic diverticulum and the remaining part of the yolk sac remain outside the gut. They are also lined by the endoderm. The primitive gut is divided into the fore gut, mid gut and hind gut and at this stage three niteries arising from the descending abdominal aorta supply the gut. I hese arteries are the coeliac trunk supplying the fore gut, the superior mesenteric artery supplying the mid gut and the inferior mesenteric artery ■upplying the hid gut (Figure 38). Foregut Coeliac artery Midgut Sup. Mesenteric artery Hindgut Inf. Mesenteric artery The foregut is the part extending form the rupturing bucco-pharyngeal membrane to a point just distal to the hepatic diverticulum. The foregut is further divided into the proximal part (pharyngeal gut) and distal part. The proximal part extend from the bucco-pharyngeal membrane to the origin of the tracheo-bronchial diverticulum. The distal part extend from the tracheo-bronchial diverticulum to the hepatic diverticulum. The mid gut extends from the origin of the hepatic diverticulum to the junction of the proximal 2/3 and the distal 1/3 of the transverse colon as seen in the adult. HUMAN EMBRYOLOGY
    • The mid gut loop is connected to the umbilical cord by the vitello intestinal duct which is connected to the yolk sac. The hind gut extends from the future junction of the proximal 2/3 and the distal 1/3 of the transverse colon to the cloacal membrane. Development of the stomach and duodenum The derivatives of the distal part of the foregut include the oesophagus, stomach and part of the duodenum up to the duodenal papilla. The glands that develop from the foregut are the liver, gall bladder and the pancreas. After separation of the tracheo-bronchial diverticulum from the ventral aspect of the foregut the dorsal part lengthens and become the oesophagus. Dorsal mesentery Gut tube Lt. and Rt. vagi Ventral mesentery Lesser curvature Greater curvature HUMAN EMBRYOLOGY -58-
    • Al the 4th week intrauterine life the part of the foregut representing the Itomach appear dilated and is attached to the dorsal and ventral body walls by the dorsal and ventral mesogastrium. The left and right vagi lie on loft and right sides of the foregut. The relationship of the ventral and dorsal mosogastrium to the vagi is shown in figure 38. In this part of the gut the dorsal wall (posterior surface) enlarges rapidly than the ventral wall (anterior surface) thereby giving rise to the greater curvature and the losser curvature of the stomach respectively ( figure 39). Another change that occur is the 90° rotation of the stomach in a clockwise direction. Due to this the left vagus nerve moves to an anterior position and innervates the (interior wall (left wall before rotation) and the right vagus nerve moves to a posterior position and innervates the posterior wall (right wall before rotation). At the same time the dorsal mesogastrium lengthens rapidly and the two leaves fuse together giving rise to the greater omentum. The ventral mesogastrium forms the lesser omentum. The space behind the ■tomach becomes the omental bursa. As a result of the rotation of the gut and due to the different rate of growth of the wall, the stomach obtains its final position. The pyloric part lies on the right side of the mid line in an upward position while the cardiac part lies on Ihe left side of the mid line in a downward direction. The duodenum is developed from the distal part of the foregut and the proximal part of the midgut. The adult “C” shape and position of the duodenum is due to the rotation of the stomach and the growth of the pancreatic buds. The dorsal mesentery of the duodenum fuses with the posterior abdominal wall along with the head of the pancreas and becomes retroperitoneal (secondarily retroperitoneal). The stomach after rotation with the location of the left and right vagi is shown in figure 40. Rt. vagus Mesogastrium HUMAN EMBRYOLOGY
    • • Epithelial lining of the gut including the secretory components of glands are derived from the endoderm. • The musculature of the gut wall (inner circular and outer longitudinal muscles), lamina propria, muscularis mucosa, submucosa and adventitia are derived from the splanchnic mesoderm. DEVELOPMENT OF THE LIVER, GALL BLADDER AND SPLEEN The liver develops around the 3rdto 4lhweek intra uterine life from the distal part of the foregut as an endodermal outgrowth called the hepatic diverticulum or the liver bud (Figure 41a). The liver bud rapidly grows into the septum transversum, a mesodermal mass in the ventral mesogastrium. The proximal part of this bud gives rise to the bile duct and the distal part further proliferates and grows into the septum transversum giving rise to the liver parenchyma (hepatocytes). Another outgrowth, the cystic diverticulum grows ventrally from the proximal part of the developing bile duct and gives rise to the gall bladder (Figure 41a). As the liver bud grows into the septum transversum, the mesoderm of the septum transversum between the developing stomach and liver and that between the liver and the anterior abdominal wall become membranous giving rise to the lesser omentum and falciform ligament respectively (Figure 41 b and 42). Liver bud Cystic diverticulum Ventral pancreatic bud Dorsal pancreatic bud HUMAN EMBRYOLOGY -60-
    • Falciform liga Ment Ligament urn teres Midgut loop Diaphragm Stomach Lesser omentum Dorsal mesentry Allantois Cloaca Duodenum Hindgut Kidney Lieno renal ligament Lessr omentum Liver Falciform ligament Spleen Greater omentum and gastrosplenic ligament Stomach The remaining part of the septum transversum gives rise to the connective tissue, kupffer cells and the haemopoietic tissue of the liver. Within the dorsal mesentery due to the condensation of its mesoderm the spleen develops and the part of the dorsal mesentery between the stomach and spleen lengthens and becomes the greater omentum and the gastrosplenic ligament (Figure 42). The kidney develops in the posterior abdominal wall (in the intermediate mesoderm) in relation to the dorsal mesentery and becomes retroperitoneal. The part of the dorsal mesentery between the spleen and the kidney becomes the lienorenal ligament. During foetal life the liver performs a haemopoietic function and due to this it weighs 10% of the total body weight at the 10thweek intra uterine life. At birth due to the cessation of the haemopoietic function it weighs 5% of the total body weight. A transverse section and a longitudinal section of the developing embryo showing the position of the kidney, spleen, stomach, liver and derivatives of the dorsal and ventral mesenteries are shown in figures 41 b and 42. HUMAN EMBRYOLOGY 81
    • Derivatives of the septum transversum (ventral mesentery or ventral mesogastrium) ■ Connective tissue and haemopoietic tissue of the liver ■ Kupffer cells of the liver (liver macropharges) ■ Connective tissue of the wall of the gall bladder ■ Falciform ligament ■ Lesseromentum ■ Coronary ligaments of the liver ■ Central tendon of the diaphragm Derivatives of the dorsal mesentery or dorsal mesogastrium ■ Spleen ■ Greater omentum ■ Gastro splenic ligament ■ Lieno renal ligament DEVELOPMENT OF THE PANCREAS The pancreas consists of a head,uncinate process,neck, body and tail. The head is situated in relation to the duodenum and the tail extend to the hilum of the spleen.The pancreas consists of an exocrine (serous) and an endocrine (islets of Langerhans) component. The embryological structures that contribute to the development of the pancreas are the dorsal and ventral pancreatic buds. The dorsal bud develops as an outgrowth from the endoderm of the dorsal aspect of the foregut and grows into the dorsal mesentery. The ventral bud develops from the endoderm near the junction between the hepatic diverticulum and the foregut (Figure 43). -62-
    • Liver bud Pancreatic Buds Dorsal Ventral Developing Gall bladder Ventral pancreatic bud Dorsal pancreatic bud Due to the rotation of the gut (stomach and midgut loop) the ventral bud rotates and fuses with the ventral aspect of the dorsal bud (Figure 44). Due to this the ducts of the two buds also fuse. The uncinate process and the lower part of the head of the pancreas develop from the ventral bud and the rest of the head, body and tail of the pancreas develops from the dorsal bud. The main duct is developed from the distal part of the dorsal duct and the whole of the ventral duct and opens at the major papilla in the second part of the duodenum along with the bile duct. The proximal part of the dorsal duct sometimes persists as the accessory duct (Santorini) of the pancreas (Figure 45). Stomach Gall bladder Accessory duct (santorini) Dorsal bud Combined duct Ventral bud The exocrine pancreas develops from the dorsal and ventral pancreatic buds and the islets of Langerhans develop from the migrating neural crest cells.The bile duct, cystic duct and the pancreatic duct open via one opening onto the major papilla of the second part of the duodenum. HUMAN EMBRYOLOGY
    • Developmental defects of the fore gut ■ Pyloric stenosis- narrowing of the pyloric region of the stomach due to improper recanalisation of the gut. ■ Atresia of the bile duct- failure to recanalise the duct which is solid initially. ■ Duplication of the gall bladder- due to the formation of two cystic diverticula. ■ Annular pancreas- due to mal fusion of the dorsal and ventral pancreatic buds the fused tissue surrounds the duodenum. This could lead to stenosis of the duodenum. DEVELOPMENT OF THE MIDGUT The derivatives of the midgut loop includes the distal part of the duodenum, jejunum, ileum, caecum, appendix, ascending colon and the transverse colon up to the junction between the proximal 2/3 and distal 1/3. The midgut loop receives its arterial supply from the superior mesenteric artery. At the 5thweek intrauterine life the midgut loop is suspended by the dorsal mesentery to the posterior abdominal wall. Ventrally the midgut loop is connected to the yolk sac by the vitello-intestinal duct (Figure 46a).The midgut loop consist of a cephalic and a caudal limb separated by the vitello intestinal duct. Changes that occur in the midgut loop include lengthening and rotation. The cephalic limb of the midgut loop lengthens rapidly compared to the lengthening of the caudal limb. The caecal bud appears at the 6thweek intra uterine life as a small dilated area. HUMAN EMBRYOLOGY -64-
    • Also an anticlockwise 270° rotation occur around the axis formed by the superior mesenteric artery. Figure 46b shows the direction of rotation of the midgut loop and its relation to the other abdominal organs. Due to the lengthening of the gut and also due to the small size of the abdominal cavity the midgut loop herniate to the extraembryonic coelom at the ()"'week intrauterine life. This is called the physiological umbilical herniation. During its herniation it rotates 90° anticlockwise and the remaining 180° rotation takes place at the time of return of the midgut to the abdominal cavity at the 10th week intra uterine life. The return of the Intestinal loops is due to the increase in the size of the abdominal cavity and completion of the development of the liver and kidney. Pancreas Jejuno ileal loops Hindgut Superior mesenteric artery The jejunum is the first part to return to the abdominal cavity and lie on the left side. Intestinal loops returning later gradually occupy a position to the right. The last to return is the caecum which lies just below the right lobe of the liver (right upper quadrant).Later the caecum descends to the right iliac fossa and due to this the ascending colon and the hepatic flexture finally come to lie on the right side of the abdomen. During its descent at the 10th week intra uterine life, the appendix develops as an outgrowth from the caecal bud. After the return of the intestinal loops the mesenteries of the ascending and descending colon fuse with the posterior abdominal wall and become retroperitoneal. Mesenteries of the appendix, caecum and sigmoid colon retain as it is and the mesentery of the transverse colon fuses with the greater omentum but retain its mobility. HUMAN EMBRYOLOGY
    • D e v e lo p m e n ta l d e fe c ts o f th e m id g u t ■ Vitelline fistula-persistence of the vitello intestinal duct (remain ' patent). Faecal discharge could be seen from the umbilicus at birth. ■ Meckels diverticulum- persistence of a small part of the vitello intestinal duct connected to the gut. ■ Vitelline cyst and volvulus- the vitello intestinal duct becomes ligamentous on either side with a central cyst (vitelline cyst) I thereby causing strangulation of the intestinal loops. ■ Abnormal rotation of the gut- could lead to changes in the normal I position of the parts of the gut. ■ Stenosis of intestine- narrowing of the gut could occur at any place I leading to several complications. ■ Omphalocoele- this is also called persistent physiological umbilical hernia. If the herniated intestinal loops (6th week intra I uterine life) do not return to the abdominal cavity at the 10th week I intra uterine life it will persist in the umbilical region covered by the amnion. DEVELOPMENT OF THE HIND GUT HUMAN EMBRYOLOGY -6 6
    • The distal end of the hind gut opens into the cloaca and end at the cloacal membrane which is endodermal (inner) and ectodermal (outer) in origin. The anterior part of the cloaca is also connected to the allantois which is also lined by the endoderm. The urorectal septum, a mesodermal thickening forms in the hindgut region and separate the cloaca into an anorectal part and a urogenital part (Figure 47). This seotum meets the cloacal membrane and forms the perineal body separating the cloacal membrane into a urogenital membrane and an anal membrane. The anal membrane ruptures around the 7lhweek intrauterine life. Mesoderm around the distal part of the hind gut region condenses and form mesodermal elevations. The ectodermal covering of these invaginate into the developing anal canal and give rise to the ectodermal lining of the lower part of the anal canal. Thus the derivatives of the hind gut (ano rectal part) lined by the endoderm include the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and the upper part of the anal canal. The lower part of the anal canal is derived from the ectoderm. Therefore the epithelial lining of the anal canal is derived from both the endoderm (upper part) and ectoderm (lower part). Developmental defects of the hindgut ■ Imperforate anus- non rupture of the anal membrane ■ Urorectal fistula- due to the defective formation of the urorectal septum an epithelial lined connection persist between the urinary tract and rectum HUMAN EMBRYOLOGY
    • GENITO-URINARY SYSTEM D E V E L O P M E N T O F T H E U R IN A R Y S Y S T E M I lie u r i n a r y s y s t e m s t a r ts to d e v e l o p a t th e 4 ' w e e k in t r a u t e r in e life. T h r e e I iciney s y s t e m s a p p e a r in th e h u m a n a n d e x t e n d s f r o m th e c e r v ic a l to th e II ii n b a r re g io n . T h e s e a r e th e p r o n e p h r o s , m e s o n e p h r o s a n d m e t a n e p h r o s ( F ig u r e 4 8 ) . Pronephros mesonephros Mesonephric Urogenital sinus Cloaca duct Metanephros F ig . 4 8 or permenent kidney Metanephros Ureteric bud (Intermediate mesoderm) Urogenital sinus (urinary bladder) Rectum F ig . 4-9 T h e p r o n e p h r o s d e v e l o p s a t t h e b e g in n in g o f t h e 4 th w e e k in t r a u t e r in e life a t t h e c e r v ic a l r e g io n in a s e g m e n t a l m a n n e r . P r o n e p h r o s is n o n f u n c t io n a i a n d la s ts o n ly a w e e k a n d d e g e n e r a t e s . T h e m e s o n e p h r o s w h ic h a p p e a r s e g m e n t a l l y in t h e t h o r a c i c a n d l u m b a r r e g io n s c o n s is t o f e x c r e t o r y u n its h a v in g a b o w m a n s c a p s u le a t its m e d ia l e n d a n d th e d is ta l e n d c o n n e c t e d to a c o m m o n m e s o n e p h r i c d u c t. T h e le ft a n d r ig h t m e s o n e p h r i c d u c t s o p e n in to t h e d is t a l p a r t o f t h e u r o g e n it a l s in u s (F ig u re 4 8 ). HUMAN EMBRYOLOGY < .■ t
    • At the 5thweek intrauterine life the third urinary sys tern, the metanephros on the permanent kidney develops. The ureteric bud, an outgrowth from th* lower part of the left and right mesonephric ducts- appears and grows Into the intermediate mesoderm in the genital ridge o n the posterior abdominal wall (Figure 49). The ureteric bud divides upto twelve generations and tha intermediate mesoderm is induced to form th e metanephric blastemi around this. The metanephric tissue form metanephic caps which are condensations around these divisions of the ureteric buds and latei develop into tubules. These tubules, lengthen rapi dly and the distal ends ol these form the bowmans capsules while the proximal ends connect with the collecting components (Figure 50). ppA n - Fig 50a Ureteric bud Metanephric tissue component Collecting Excretory system system component The ureteric bud give rise to the collecting component of the kidney and consists of the ureter, pelvis of the kidney, major and minor calyces and 1 to 3 million collecting ducts. HUMAN EMBRYOLOGY -70-
    • I ho metanephric blastema give rise to the excretory component of the kidney (nephron) which consists of the bowman capsule, proximal and dlHlal convoluted tubules and loop of Henle (Figure 50b). Both excretory Mnd collecting components of the kidney are intermediate mesoderm in origin. The connection between the excretory and collecting components mptures and as early as the 10thweek intrauterine life the kidney starts to function. During infancy there is further development of the nephrons but Iho number does not increase. I he kidney develops from the intermediate mesoderm in the pelvic region. After its development the kidney moves to its final position in the posterior nbdominal wall and becomes retroperitoneal. This is called the 'ascent of the kidney'. The urinary bladder develops from the upper part of the urogenital sinus which is connected to the allantois. The allantois is later called the urachus which becomes fibrous and form the median umbilical ligament. The middle part of the urogenital sinus called the pelvic part becomes the membranous and prostatic urethra in the male and the urethra in the female. The lower part of the mesonephric ducts gets absorbed into the posterior wall of the bladder there by forming separate openings of the ureters and mesonephric ducts. The absorbed parts of the mesonephric duct form the trigone of the bladder which is intermediate mesoderm in origin. Later the endoderm of the bladder grows over the trigone and in the adult the epithelium of the whole bladder isendodermal in origin. Developmental abnormalities of the urinary system ■ Renal agenesis- due to the non development of ureteric bud the metanephric tissue do not form leading to renal agenesis which could be unilateral or bilateral. ■ Double ureter- this may be due to the formation of two ureteric buds from one mesonephric duct or early division of a single ureteric bud. HUMAN EMBRYOLOGY
    • ■ Horse shoe kidney- during the ascent of the kidney the two inferior poles may fuse together and form a horse shoe shaped kidney which is seen trapped between the root of the inferior mesenteric artery and aorta. ■ Pelvic kidney- persistence of the kidney in the pelvic region without ascent to its final position. ■ Congenital polycystic kidney- multiple cysts could form within the kidney which will leads to renal insufficiency. This may be inherited (autosomal recessive or autosomal dominant) or abnormal formation of the tubular system in the kidney. Sometimes cysts may form if the excretory and collecting tubular parts do not fuse together. ■ Aberrant renal arteries / Accessory renal arteries- during development the kidney gets its arterial supply from the pelvic region of the aorta. Later due to its ascent it obtains its arterial supply from the abdominal aorta. If the embryonic arteries persist it is called aberrant / accessory renal arteries. DEVELOPMENT OF THE GENITAL SYSTEM The genital system includes the gonads, genital ducts and external genitalia. Although the chromosomal sex of an individual is determined at the time of fertilization, the morphological features appear late in the development (7thweek intrauterine life). At first both in the male and in the female an indifferent stage of gonads, ducts and external genitalia appear. Indifferent gonads Aorta Mesonephric duct (wolffian duct) Migrating germ cells Sex cords HUMAN EMBRYOLOGY _______ Fig. 51 Paramesonephric duct (mullerian duct) -72-
    • I ho gonads develop in the genital ridges in the posterior abdominal wall ih Tlved from the intermediate mesoderm. The coelomic epithelium in this flfllo n proliferates into the mesoderm and form primitive sex cords. The (jorm cells migrate from the endoderm of the wall of the yolk sac to the i fir|ion of primitive sex cords at the 6thweek intra uterine life (Figure 51). Indifferent genital ducts Degenerating mesonephros Developing gonads Mesonephric duct Paramesonephric duct Two pairs of ducts develop longitudinally in the intermediate mesoderm. They are the mesonphric and paramesonephric ducts. The mesorephric ducts (wolffian ducts) which are parts of the mesonephric urinary system open into the posterior aspect of the urogenital sinus. The paramesonephric ducts (Mullerian ducts) cranially lie parallel and lateral to the mesorephric ducts and cross horizontally anterior to the mesonephric ducts lying parallel and close to each other opening into the urogenital sinus. A thickening, the Mullerian tubercle forms on the wall of the urogenital sinus where the paramesonephric ducts open (Figure 52). Indifferent external genitalia Genital tubercle Genital tubercle Cloacal fold Genital Swelling Urethral folds Cloacal membrane Anal membrane Anal folds Genital swelling Fig. 53a HUMAN EMBRYOLOGY
    • Migration of the primitive streak mesoderm around the cloacal membrane forms two swellings on either side and is called the cloacal folds. The two cloacal folds unite dorsally and form the genital tubercle. This is later called the phallic tubercle. Lateral to the cloacal folds another pair of swellings, the genital swellings appears (Figure 53a). The formation of the urorectal septum separates the cloacal membrane into an anterior urogenital membrane and a posterior anal membrane. Due to this the cloacal folds gets separated into urethral folds (around urogenital membrane) and anal folds (around anal membrane) as shown in figure 53b. Differentiation of indifferent gonads into Testis and ovary Male (XY) Female (XX) Indifferent Gonad r Y influence (Testis) ■ Medullary cords appear ■ No cortical cords ■ Tunica albugenea form 1 r Absence of Y (Ovary) ■ Cortical cords appear - Medullary cords disappear . No tunica albugenia Development of the Testis The primitive sex cords in the gonads continue to proliferate into the medulla under the influence of the SRY gene (sex determining region) on the Y chromosome of a genetically determined male. These cords are called medullary cords which later become testis cords (Figure 54a). At the hilum of the developing testis they form a plexus called the rete testis. A thick fibrous tissue capsule, the tunica albugenea forms between the surface epithelium and testis cords. HUMAN EMBRYOLOGY -74-
    • The cortical cords degenerate. The testis cords later become the seminiferous tubules. The sertoli cells are derived from the surface epithelium and the interstitial cells of Leydig are derived from the mesoderm. Testosterone secretion starts at the 8th week intrauterine life and this influence the differentiation of the genital ducts and the external genitalia In the male. At puberty the seminiferous tubules are formed by canalisation of the testis cords and these open into the efferent ducts which are remnants of the excretory mesonephric system. The efferent ducts open into the mesonephric duct (Wolffian duct) which becomes the vas deferens (Figure 54b). Mesonephric duct Medullary cords Tunica albugenea (dense fibers) Paramesonephric duct V Efferent ductules 1 Tunica albugenea Testis cords Mesonephric duct . Paramesonephri duct Y f 1 I 'Fig. 54b Rete testis Development of the ovary The primitive sex cords in the gonads breakdown and form cell clusters with germ cells in the medulla in a genetically determined female. These later degenerate and another set of cortical cords develop at the T week intrauterine life (Figure 55a). These cortical cords form cell clusters and surround the germ cells which lie close to the surface. The cells of the cords develop into follicular cells and the germ cells form the oogonia (Figure 55b). HUMAN EMBRYOLOGY 7fi
    • Degenerating medullary cords H Cortical cords Fig. 55a Surface epithelium Primary oocytes Degenerating medullary cords Mesonephric duct Paramesonephric duct Surface epithelium U " Fig. 55b Development of the genital ducts in the female The genital ducts in the female are derived from the pararmesonephric ducts. The paramesonephric ducts lie lateral to the mesonephric ducts at the cranial end and crosses the mesonephric ducts horizontally and lie vertically close to each other medial to mesonephric ducts opening into the urogenital sinus. This caudal vertical part of the paramesonephric ducts fuse together and form a common canal which later enlarge and become the uterine canal (Figure 56). The wall of the urogenital sinus shows a thickening at the point where the two paramesonephric ducts open and is called the Mullerian tubercle (sino vaginal bulb). The sinovaginal bulb is ectodermal in origin. It further grows by lengthening and forms a thick plate called the vaginal plate within which a cavity appears and gives rise to the lower 2/3 of the vagina (Figure 57a). The upper 1/3 of the vagina and the fornices are derived from the lower part of the uterine canal. Hymen is represented as a thin endodermal membrane atthe lower end of the vaginal plates (Figure 57b). The proximal vertical part and the middle horizontal parts of the paramesonephric ducts give rise to the fallopian tube. The fused distal parts of the paramesonephric ducts (uterine canal) give rise to the fundus, body, fornices and cervix of the uterus. HUMAN EMBRYOLOGY -76-
    • Fused paramesonephric (mullerian) ducts Sinovaginal bulb Fig. 56 Uterus Cervix Vaginal plate Fornix Vagina Hymen Development of genital ducts in the ma,e The SRY gene on the Y chromos^me induce the production of Mullerian inhibiting substance produced py the Sertoli cells and Testosterone produced by the Leydig cells of t h e seminiferous tubules. This leads to the suppression of paramesorephric duct development and stimulation of the development of mesonephric ducts- The mesonephric ducts thus develop into epididymis and vas deferens. Development of the external ger1^3*'3 in the male The genital tubercle undergoes raP'd elongation under the influence of the dihydrotestosterone formed by t h e conversion of testosterone in the testis. The elongated genital tubercle Known as the phallus pulls the urethral folds and forms the urethral groov^ The urethral groove is lined by urethral plate which is of endodermal origin- The urethral grooves fuse together and form the penile urethra. T h e ectoderm at the tip of the developing phallus proliferates and invaginates into it as an epithelial plug. This later canalises and form part of the penile urethra in the glans penis. Thus the lining of the external urethral m e ^ us a* the tip of the glans is developed from the ectoderm. The genital sWellings grow further caudally and fuse to form the scrotum into which th^ testis descends from an abdominal position. _____ _______ HUMAN EMBRYOLOGY H
    • Develonment of the external genitalia in the female In the female a slight elongation occur in the genital tubercle and give rise to the clitoris. The urethral folds form the labia minora and the genital swellings enlarge and form labia majora. The vestibule is derived from the urogenital groove. Male and female derivatives from the embryonic structures Embryonic Structure Homologous structures Male Indifferent gonads Female testis ovary Genital tubercle (phallus) penis clitoris Urethral fold penile urethra labia minora Genital swellings scrotum labia majora Mesonephric ducts epididymis epoophoron seminal vesicle paroophoron ductus deferens Paramesonephric ducts appendix testis upper1/3 vagina, utriculus prostaticus vaginal fornices, uterus, fallopian tubes Ul IMAM EMDPVm n f iV _7ft_
    • I Congenital abnormalities associated with the development of the genital system ■ Uterus didelphys- non fusion of the paramesonephric ducts giving rise to two uteri. ■ Uterus bicornis- two horns of the uterus opening into a common vagina. ■ Uterus bicornis unicollis- uterus having two horns one of which is rudimentary and opening into a common vagina. ■ Hypospadias- non fusion of urethral folds in the male where urethral meatus extends along scrotal raphe. ■ Epispadias- development of genital tubercle at the anal end in which urethral meatus is found on the dorsum of the penis.
    • HEAD AND NECK PHARYNGEALARCHES I le x io n o f t h e e m b r y o t a k e s p la c e at th e b e g in n in g o f t h e 4 th w e e k in tra u te r in e life. I m m e d i a t e l y a f t e r fle x io n th e h e a d e n d o f th e e m b r y o s h o w s ■•overal e l e v a t i o n s and d e p re s s io n s . T h e s e in c lu d e th e fro n ta l p r o m in e n c e , p e r ic a r d ia l s w e llin g , s ix p h a r y n g e a l a r c h e s a n d p h a r y n g e a l < le fts . F ig u r e 5 3 a s h o w s t h e e x t e r n a l a p p e a r a n c e o f th e h e a d e n d o f th e u m b r y o . F ig u r e 5 8 b s h o w s th e in te rn a l a p p e a r a n c e (g u t) w ith h u c c o p h a r y n g e a l m e m b r a n e a n d e n d o d e r m a l lin e d p h a r y n g e a l p o u c h e s . Bucco pharyngeal membrane Frontal prominence Pharyngeal pouches Pharyngeal Arches and clefts Pericardial swelling C artilage Nerve Artery II III IV Closing m e m b ra n e V I Cleft Pouch I IUMAN EMBRYOLOGY C oronal se ction o f p h aryn g e al g u t Fig 59
    • Although six pharyngeal (branchial) arches are seen in the developing neck region of the embryo at the 4th week intrauterine life, only somo arches contribute to the-development of structures in the head and neck. These include 1s 2n 3rd 4th and 6th pharyngeal arches. The 5th arch t d disappears early in life. Each pharyngeal arch consists of an ectodermal covering (surface) and an endodermal lining (gut wall) within which lie the branchial mesoderm with an arch cartilage, an artery and a nerve (Figure 59). Each arch has its own nerve (post trematic nerve) and a branch of the nerve from the adjoining arch (pre trematic nerve). The pharyngeal arches are connected to each other by the closing membrane. On either side of the closing membrane lie depressions, an ectodermal cleft (surface) and an endodermal pouch (gut wall). Closing membrane is a thin membrane consisting of ectoderm and endoderm with a little mesoderm through which the pretrematic nerve pass to the next arch. During development the second pharyngeal arch grows over the other pharyngeal arches and closes the second; third, fourth and sixth pharyngeal clefts, leaving only the first pharyngeal cleft intact. Due to this all closing membranes disappear except the first. Nerves of the pharyngeal arches Each pharyngeal arch is supplied by a nerve and therefore the derivatives of each arch obtain the nerve supply from the arch nerve or its branches. ■1s arch- Mandibular division of Trigeminal nerve (fifth cranial nerve) t ■2n arch- Facial nerve (seventh cranial nerve) d ■3rdarch- Glossopharyngeal nerve (ninth cranial nerve) ■4tharch- Superior laryngeal branch of Vagus nerve (tenth cranial nerve) ■6tharch- Recurrent laryngeal branch of Vagus nerve (tenth cranial nerve) HUMAN EMBRYOLOGY -82-
    • DEV'ELOPMENT OF THE FACE Development of the face starts at the 4thweek intrauterine life. The fontal view' of the head end of the embryo shows a central depression called the t.lornodeum bounded by several prominences. These include the frontal prominence above, two maxillary processes superolaterally and two mandibular processes inferolaterally(Figure 60a). Inferiorly the cardiac nwelNing is seen. The maxillary and mandibular processes are derivatives of th e first pharyngeal arch. On either side of the frontal prominence two ectodermal thickenings, the nas^l placodes develop. These placodes further develop into nasal pits. On either side of each nasal pit two swellings appear (Figure 60b). The medial swelling is called the medial nasal fold (process) and the lateral swelling is called the lateral nasal fold (process). At this stage the development of the eye is seen on the lateral aspect of the frontal prominence. The two medial nasal processes grow rapidly downwards and merge towards each other forming the intermaxillary segment. At the 7"’ week intra uterine life the two maxillary and mandibular processes grow towards the midline. The left and right mandibular processes grow towards each other meeting at the midline(Figure 60c).As the intermaxillary segment grows downwards it gets trapped between the two medially growing maxillary processes. Nasal pit | ] Nasal pit Maxillary swelling 1 ■ Lateral nasal fold Stomodeum j Medial nasal fold Mandibular swelling Cardiac buldge 5wks IUL Fig 60a HUMa n EMBRYOLOGY 6wks IUL Fig 60b -P
    • Medial nasal fold Lateral nasal fold Maxillary swelling Mandibular swelling Nasolacrimal groove Philtrum During the 9thand 10thweek intra uterine life fusion between the following processes takes place. ■ Left and right mandibular processes (median fusion)- these give rise to the lower lip and lower cheek. ■ Left and right maxillary processes with the intermaxillary segment- the left and right maxillary processes give rise to the upper lip and cheek excluding the philtrum. ■ Left and right lateral nasal processes with the left arid right maxillary processes (oblique fusion)- the left and right lateral nasal processes give rise to the lateral wall of the nose. ■ Left and right maxillary processes with the left and right mandibular processes During the oblique fusion the epithelium of the two processes fuse and form an epithelial cord which extends from the developing eye to the nasal cavity. Later this epithelial cord canalise and form the nasolacrimal duct which extends from the developing medial canthus of the eye to the lateral wall of the nose (Figure 60d). Its proximal part enlarges and form the lacrimal sac which lies in the lacrimal fossa on the medial aspect of the orbit. The migration of the neural crest cells contributes to the development and fusion processes of the craniofacial structures including the face. A summary of the embryological structures contributing to the development of the face and their derivatives are given below. HUMAN EMBRYOLOGY
    • Embryological structure ■ Medial nasal processes- Derivative crest and tjp of n0S6) phiitmm of the (intermaxillary segment) ■ Lateral nasal processes- alae and lateral wall of nose - Fronto nasal process- nasal bridge and forehead - Maxillary processes - upper lip and cheek excluding philtrum ■ Mandibular processes- lower lip and cheek Congenital abnormalities associated with the development of tho face / f -------FTg. 61 e V'-FJg! 61 d HUMAN EMBRYOLOGY
    • ■ Median cleft lip- non fusion of the two mandibular processes or non fusion of the two maxillary processes in the midline (Figure 61a and b). ■ Unilateral cleft lip- non fusion of the intermaxiliary segment with the left or right m axillary processes (Figure 61c) ■ Bilateral cleft lip- non fusion of the intermaxilary segment with the left and right maxillary processes (Figure 61 d) ■ Oblique facial cleft- non fusion of the lateral nasal process with the maxillary processes (Figure 61 e) ■ Macrostomia- non fusion of the maxillary and mandibular processes ■ Microstomia- excessive fusion of the maxillary and mandibular processes ■ Mandibulofacial dysostosis or first arch syndrome- under development of one or both sides of the first pharyngeal arch affecting the maxilla, mandible, external ear and lower eyelids. DEVELOPMENT OF THE TONGUE The development of the tongue starts at the 4th week intrauterine life from the floor of the primitive oral cavity. Initially the first, second, third and fourth pharyngeal arches contribute to the development of the tongue. From the first pharyngeal arch two lateral lingual swellings and a median swelling, tuberculum impar forms. The endodermal lining of the two lateral lingual swellings and tuberculum impar fuse togther and give rise to the epithelium of the anterior 2/3 of the tongue. Lateral lingual swellings of the second, third and fourth arches appear from the respective pharyngeal arches. HUMAN EMBRYOLOGY -86-
    • A medial swelling in relation to the third and fourth pharyngeal arches appear and is called the hypobranchial eminence or copula (Figure 62a). Although initially lateral lingual swellings of the second branchial arch is seen, this does not contribute to the development of the tongue. The lingual swellings of the third pharyngeal arch grows over the lingual swellings of the second pharyngeal arch and approach the developing anterior 2/3 of the tongue. The endoderm of the lateral lingual swellings and the median hypobranchial eminence of the third and fourth branchial arches contributes to the formation of the epithelium of the posterior 1/3 of the tongue.The epiglottal swelling, a median swelling from the 4th arch gives rise to the epiglottts. The endoderm of the anterior 2/3 and posterior 1/3 fuse in a V' shaped groove called sulcus term inalis (Figure62b).The apex of the V shape groove is the foramen caecum from which the thyroid gland develops. The intrinsic and extrinsic muscles of the tongue except the palatoglossus are derived from the migrating occipital myotomes. Hence the nerve supply by the hypoglossal nerve. After the development of the tongue, it separates from the floor of the mouth by epithelial degeneration. T b rcu m im a u e lu p r H o ra ch l yp b n ia e in n m e ce E ig tta s e p lo l w llin g HUMAN EMBRYOLOGY -87-
    • Congenital abnormalities associated with the development of the tongue ■ Bifid tongue- non fusion of the left and right lingual swellings of the first arch. ■ Tongue tie / Ankyloglossia- fusion of the tongue to the floor of the mouth. Embryological derivatives of the tongue Mucous membrane ■ Anterior 2/3- endoderm of first branchial arch (lateral lingual swellings and tuberculum impar) ■ Posterior 1/3- endoderm of third and fourth branchial arches (lateral lingual swellings and hypobranchial eminence) Musculature ■ All extrinsic and intrinsic muscles except palatoglossus muscle are derived from the occipital myotomes HUMAN EMBRYOLOGY -88- j
    • Nerve supply Anterior 2/3General sensation from lingual nerve branch of the mandibular division of trigeminal nerve (first arch nerve). Taste (special) sensation from chorda tympani nerve (pretrematic nerve of the first arch). ■ Posterior 2/3general and taste (special) sensations from glossopharyngeal nerve (third arch nerve) and superior laryngeal nerve of vagus nerve (fourth arch nerve) ■ Muscles-AII extrinsic and intrinsic muscles supplied by hypoglossal nerve except palatoglossus which is supplied by the pharyngeal plexus. DEVELOPMENT OF THE PALATE Intermaxillary segment Palatine process Maxillary process Nasal septum Pre maxilla Palatine process Nasal septum Tongue Palatine shelves (vertical) Palatine Shelves (Horizontal) HUMAN EMBRYOLOGY 89
    • The development of the palate starts at the 6th week intrauterine life. The embryological structures that contribute to the formation of the palate are the pre maxillary process of the intermaxillary segment and the left and right palatine processes of the maxillary processes(Figure 63a). The primary palate is derived from the pre maxillary process and the secondary palate from the two palatine processes(Figure 63b). The two palatine processes initially grow obliquely downwards (vertical) on either side of the developing tongue at the 6thweek intrauterine life(Figure 63c), but elevates and become horizontal at the 7th week intrauterine life due to the increase in the size of the oral cavity and the depression of the developing tongue(Figure 63d). The premaxillary process from the intermaxillary segment develops horizontally backwards towards the palatine processes and all three structures fuse together in a T shaped manner starting from before backwards. Later ossific centres appear in the future hard palate area and mesoderm in the posterior part form the muscles of the soft palate. The fusion of the palate occur at the 9-10thweek intrauterine life. Migrating neural crest cells contributes to the fusion of the palate. Congenital abnormalities associated with the development of the palate ■ Bifid uvula- non fusion of the posterior most part of the palatine processes ■ Median cleft palate- non fusion of the left and right palatine processes ■ Unilateral cleft palate- non fusion of the pre-maxillary process with either left or right palatine processes ■ Bilateral cleft palate- non fusion of the pre-maxillary process with both left and right palatine processes HUMAN EMBRYOLOGY -90-
    • DEVELOPMENT OF THE THYROID GLAND The thyroid gland is developed from the endoderm of the floor of the primitive oral cavity in the region of the future foramen caecum. Endoderm proliforates and grows as an epithelial cord through the tongue and is called the thyroglossal duct. This further grows in front of the body of the hyoid, between the body of the hyoid and thyrohyoid membrane, passing in front of the thyrohyoid membrane to the front of the thyroid cartilage and divides into two parts. These give rise to the left and right lobes of the thyroid gland (Figure 64a and b).The distal end of the thyroglossal duct form the pyramidal lobe. The derivatives of the thyroglossal duct include only the follicular cells. The parafollicular cells are derived from the endoderm of the ventral wing of the fused 4thand 6thpharyngeal pouches (ultimobranchial body). After the formation of the thyroid gland, the thyroglossal duct degenerates and looses its connection with the oral cavity. F re u o g t F ra e c e u o m n a c m T y g s a d c h ro lo s l u t T y g s a d c h ro lo s l u t (e d d rm n o e ) Ho b n y id o e T y h o m m ra e h ro y id e b n T y id c rtila e h ro a g C o c rtila e ric id a g HUMAN EMBRYOLOGY -91-
    • Congenital abnormalities associated with the development of the thyroid gland ■ Ectopic thyroid tissue- persistance of the remnants of the thyroglossal duct in any place along its course can give rise to ectopic thyroid tissue. This could be within the tongue, behind the body of the hyoid bone, in the neck or in the superior mediastinum. ■ Lingual thyroid- persistance of thyroid tissue in the foramen caecum area of the dorsum of the tongue. ■ Retrosternal goitre- migration of the thyroglossal duct to the superior mediastinum (behind sternum) giving rise to thyroid tissue. ■ Thyroglossal cysts or fistulae- proliferation of thyroglossal remnants giving rise to epithelial lined cavities or tubes. DEVELOPMENT OF THE PITUITARY GLAND The pituitary gland develops from two embryonic sources. These are the Rathkes' pouch an outgrowth from the ectoderm of the roof of the primitive oral cavity and a down growth from the neuroectoderm of the ventral aspect of the fore brain. These two embryonic structures grow towards each other and fuse(Figure 65a and b). Later the connection with the oral ectoderm is lost and the pituitary gland is seen connected to the inferior aspect of the forebrain. The anterior pituitary is developed from the Rathkes' pouch (ectoderm) and the posterior pituitary from the neuroectoderm. HUMAN EMBRYOLOGY -92-
    • Fore brain Down growth from neuro ectoderm Rathke's pouch Oral ectoderm Fig 65a Fig 65b DEVELOPMENT OF SALIVARY GLANDS The parotid gland develops from the oral ectoderm and the submandibular gland and sublingual glands develop from the oral endoderm. These salivary glands develop as an epithelial bud which branches, canalize and differentiate and grows into the mesenchyme beneath them. The acinar cells and the lining of the ducts are derived from the ectoderm (parotid) or endoderm (submandibular and sublingual) and the connective tissue components from the mesenchyme. FATE OF PHARYNGEALARCHES Each pharyngeal arch consists of a mesoderm (branchial mesoderm), cartilages, pharyngeal clefts (ectodermal), pharyngeal pouches (endodermal) and closing membranes (ectoderm and endoderm). All pharyngeal pouches, first pharyngeal cleft and the first closing membrane persist and give rise to adult structures (Figure 66a, b and c). Derivatives of each of these are given below. HUMAN EMBRYOLOGY -9 3
    • Derivatives of the branchial mesoderm 1starch- Muscles of mastication (medial pterygoid, lateral pterygoid, masseter and temporalis), anterior belly of digastric, myolohyoid, tensor tympani and tensor palati muscles. Mandible and maxilla, zygomatic bone and zygomatic process and squamous part of temporal bone. 2n archd Muscles of facial expression (orbicularis oris, orbicularis occuli, zygomaticus major and minor, occipito frontalis etc.), Buccinator, platysma, auricular muscles, stapedius, stylohyoid and posterior belly of digastric muscles. 3rdarch- Stylopharyngeus muscle 4tharch - Cricothyroid, pharyngeal constrictors and levator palati muscle 6tharch- All intrinsic muscles of the larynx Derivatives of the branchial cartilages (Fig 66d) 1s archt Cartilage is the Meckel's cartilage. Incus and malleus (ossicles of the middle ear) Spheno-mandibular ligament and sphenomalleolar ligament 2n archd Cartilage is the Reichert's (hyoid) cartilage. Stapes (ossicle of the middle ear), styloid process, stylohyoid ligament, lesser horn and upper part of the body of hyoid bone 3rdarch - Greater horn and lower part of the body of hyoid bone 4th and 6th arch- Thyroid, cricoid, arytenoid, corniculate and cuneiform cartilages HUMAN EMBRYOLOGY -94-
    • Derivatives of the pharyngeal clefts 1s cleft- epithelial lining of the external auditory meatus and the outer l lining of the tympanic membrane Second, third, fourth and sixth pharyngeal clefts disappear Derivatives of the pharyngeal pouches 1s pouch t Inner lining of the tympanic membrane, epithelial lining of the middle ear cavity and the lining of the eustachian tube 2n pouch d Epithelium of the palatine tonsil- stratified squamous non keratinizing epithelium 3rdpouch - This divides into dorsal and ventral wings.The epithelial lining of the ventral wing gives rise to the Hassel's corpuscles of the thymus. The epithelium of the dorsal wing gives rise to the parathyroid tissue which will form the inferior parathyroid gland 4thand 6thpouches - Both these pouches fuse together and divide into dorsal and ventral wings. The epithelium of the ventral wing is called the ultimobranchial body and gives rise to the parafollicular cells of the thyroid gland and that of the dorsal wing gives se to the parathyroid tissue of the superior parathyroid gland Derivatives of the closing membranes First closing membrane develops into the tympanic membrane. Second, third, fourth and sixth closing membranes disappear. HUMAN EMBRYOLOGY
    • Eustachian tube External ear Tympanic Middle ear m m ra e e b n Palatine tonsil Ectoderm Endoderm r Inferior parathyroid Thymus Fig 66a Superior parathyroid Ultimo branchial body Palatine tonsil Pharynx Superior parathyroid Inferior parathyroid Thyroid gland Thymus Fig 66c 1 In u c s 6 1 23456789- Fig 66 d HUMAN EMBRYOLOGY Malleus Stapes Styloid process Stylohyoid lig. Upper 12 of the body & lesser horn of hyoid / Lower Vz of the body & greater horn of hyoid Thyroid cartilage Cricoid, arytenoid, cuneiform & corniculate cartilages -96-
    • Development of the Nervous System The development of the neural tube is described in chapter 1. The fusion of the neural tube begins in the future neck region and extends cephalocaudally. The cranial and caudal openings of the tube are called cranial and caudal neuropores. The cranial neuropore fuse before the caudal neuropore. The cephalic end of the neural tube further enlarge and gives rise to the brain consisting of the procencephalon (forebrain), mesencephalon (midbrain) and (rhombencephalon) hind brain. The remaining part of the neural tube gives rise to the spinal cord. The procencephalon gets divided into the telencephalonA (cerebrum and basal ganglia), the mesencephalon remain as it is (mid brain) and rhombencephalon gets divided into metencephalon(pons and cerebellum) and myelencephalon(medulla oblongata). The final position of the different parts of the brain is due to several flexures. These are the cervical flexure (between rhombencephalon and spinal cord),cephalic flexure (in midbrain),pontine flexure (in mid rhombencephalon) and telencephalic flexure (between telencephalon and diencephalons). Due to these flexures and further development of the brain ,the central canal changes its shape in different areas giving rise to the following; ■ Lateral ventricles in the cerebral hemispheres (telencephalon) ■ Third ventricle in relation to thalamus (diencephalon) ■ Aqueduct in mid brain (mesencephalon) ■ Fourth ventricle in relation to pons, medulla oblongata and cerebellum (rhombencephalon) ■ Central canal in the spinal cord HUMAN EMBRYOLOGY -97-
    • The neural tube consists of three distinct layers. ■ Ependymal layer form the lining of central canal and ventricles, germinal cells, some nerve cells and glial cells ■ Mantle layer form nerve cells and glial cells (grey matter) ■ Marginal layer contain cell processes of glial cells and connective tissue (white matter) Roof plate jvj v_ w_ / Alar plate (Sensory) Basal plate < (Motor) /* % /'. p Sulcus limitans fS[f Fig 67 Across section of the neural tube is shown in figure 67. It has a dorsal part called the alar plate and a ventral part called the basal plate separated by the sulcus limitans. Alar plate gives rise to the sensory component and the basal plate gives rise to the motor component of the nervous system. DEVELOPMENT OF THE EYE The eye develops from three embryonic sources. These include the surface ectoderm, neuroectoderm and the mesoderm. At first the neuroectoderm on either side of the forebrain form an optic groove which later bulges out as an optic vesicle. This vesicle grow towards the surface ectoderm and Induce it to form a lens placode and give rise to the lens vesicle (Figure 68 a and b). The lens vesicle separates from the surface ectoderm at the 5thweek intrauterine life and form the lens of the eye. HUMAN EMBRYOLOGY -98-
    • Further growth of the optic vesicle leads to an invagination and the formation of a double walled optic cup. The proximal end of the optic vesicle becomes the optic stalk and the invagination extends on the inferior aspect of the stalk and is called the choroid fissure. The hyaloid artery passes through the choroid fissure and supply the developing eye. The choroid fissure fuse at 7thweek intrauterine life (Figure 68 c and d). F re b in o ra L n p co e e s la d ..^.L n v s le e s e ic O tic g o e p ro v ..-^..O tic v s le * O tic c p p e ic — p u L n v s le e s e ic O tic c p p u O tic c p p u H a id a y lo rte ry O tic s lk p ta O te la e u r y r In e la e n r y r The optic cup consists of an inner and outer layer and the derivatives are given below. ■ Outer layer of optic cup forms the pigment layer of retina ■ Inner layer - posterior 4/5 (pars optica retinae) give rise to the rods and cones, ganglion layer, plexiform layers and nuclear layers ■ Inner layer - anterior 1/5 (pars ceca retinae) give rise to the inner layer of the iris and the ciliary body. HUMAN EMBRYOLOGY -99-
    • O tic c p p u L n e s E to e c d rm R tin e a L n e s C rn a o e Irrid p p o a illa ry m m ra e e b n P m n la e ig e t y r A t. c a b r n h m e E to e c d rm Fig 69b The opening at the anterior end of the optic cup forms the pupil. Vacuolation within the mesoderm in front of the pupil forms the anterior chamber of the eye. The mesoderm posterior to the anterior chamber forms the irridopupillary membrane which later ruptures. The mesoderm anterior to the anterior chamber with the surface ectoderm forms the cornea. The mesoderm surrounding the optic cup develops into the ciliary muscle, suspensory ligament, choroid (pigmented layer) and the sclera. The optic stalk becomes the optic nerve and the hyaloid artery becomes the central artery of the retina. Congenital abnormalities associated with the development of the eye ■Congenital cataract - lens becomes opaque during prenatal life ■Anophthalmia - absence of eyeball ■Cyclopia - development of a single eye ■Coloboma - non fusion of choroid fissure ■Persistence of irridopupillary membrane HUMAN EMBRYOLOGY -100-
    • DEVELOPMENT OF THE EAR The external ear develops from the ectoderm of the first pharyngeal cleft, the middle ear and the eustachian tube develops from the endoderm of the first pharyngeal pouch the tympanic membrane develops from the first closing membrane (ectoderm and endoderm), incus and malleus from the first arch cartilage (Meckel's cartilage) and stapes from the second arch cartilage. The internal ear develops from the Surface ectoderm in relation to the rhombencephalon area. Ectodermal thickenings, the otic placodes develop and later separate from the Surface ectoderm and form the otic vesicles (Figure 70a to c). The otic vesicle divides into two parts giving rise to the membranous labyrinth. This consists of a ventral part, the saccule and cochlear duct and a dorsal part, the utricle, semicircular canals and endolymphatic duct (Figure 70d). The auricle is developed by the formation of six mesodermal thickenings from the first and second arches surrounding the first pharyngeal cleft. These fuse together and give rise to th§ definitive auricle. -101-
    • Otic pit Fig T O b 1st pharyngeal cleft (external ear) "1 st pharyngeal pouch (middle ear) HUMAN EMBRYOLOGY
    • Rh om boncephalon Otic placode P h a ry n x Otic vesicle R ig 7 0 c ndoiymphatic ducts Utricle ITI I 1c [ tC ~ > Inner ear Saccule C C Q 3C Q - F ig 7 O d -1 0 2 -