Embryology studies the changes occurring from fertilization to adulthood, focusing on prenatal development. It encompasses two main aspects: ontogeny (individual development) and phylogeny (evolutionary history), and it introduces essential concepts like zygote, morula, implantation, and embryo. Understanding embryology helps clarify human developmental processes and their implications for various medical fields, including obstetrics and pathology.

1. Embryology

2. INTRODUCTION
 Embryology: In its widest sense is applied to the various changes which take place during
the growth of an animal from the egg to the adult condition. It is, however, usually restricted
to the phenomena, which occur before birth.
 Embryology may be studied from two aspects (1) that of ontogeny which deals with
the development of the individual; and (2) that of phylogeny, which concerns itself with the
evolutionary history of the animal kingdom.
 Developmental Periods:
 Human development is divided into two:
1. Prenatal (before birth): the main developmental changes occur during prenatal phase.
2. Postnatal: (after birth).
 Embryological Terminology:
 The following terms are commonly used in discussions of developing humans:
• Oocyte (ovum, egg): the female germ or sex cell produced in the ovaries. When mature,
the oocyte is called a secondary oocyte or mature oocyte.
• Sperm (spermatozoon): Refers to the male germ cell produced in the testes (testicle).
• Zygote: This cell results from the union of an oocyte and a sperm during fertilization. A
zygote is the beginning of a new human being.
• Cleavage: This is the series of mitotic cell divisions of the zygote that result in the formation
of early embryonic cells – blastomeres. The size of the cleaving zygote remains unchanged
because at each succeeding cleavage division the blastomeres becomes smaller.

3. • Morula: This solid mass of 12 to about 32 blastomeres is formed by cleavage of a zygote.
• Blastocyst: After the morula enters the uterus from the uterine tube (fallopian tube), a fluid
filled cavity– develops inside it. This change converts the morula into a blastocyst.
• Implantation: The process during which the blastocyst attached to the endometrium and
subsequently embeds in it.
• Gastrula: During gastrulation (transformation of a blastocyst into a gastrula), a three layered
embryonic disc forms (third week).
 The three germ layers of the gastrula ectoderm, mesoderm and endoderm, subsequently
differentiate into the tissues and organs of the embryo.
• Neurula: The early embryo during the third and fourth weeks, when the neural tube is
developing from the neural plate.
• Embryo: The developing human during its early stages of development. The end of the eighth
week ( 56 days.
• Conceptus: The embryo and associated membranes.
• Primordium: The beginning or first discernible indication of an organ or structure.
• Fetus: After the embryonic period (eight weeks), the developing human is called a Fetus:
During the fetal period (ninth week to birth), differentiation and growth of the tissues and organs
formed during the embryonic period occur.
• Abortion (miscarry): A premature stoppage of development and expulsion of a Conceptus
from the uterus.
• Trimesto: A period of three calendar months during a pregnancy. The most critical stages of
development occur during the first trimester (13 weeks).
• Congenital Anomalies or Birth Defects: Abnormalities of development that are present at birth
(born with).

4. • Postnatal period: The changes occurring after birth – the development of teeth and breasts, for
example, are more or less familiar to most people.
• Infancy: Refers to the earliest period of extra uterine life; roughly the first year after birth – An infant
aged 1 month or less is called a new born or neonate.
• Childhood: Is the period from about 13 months until puberty.
• Puberty: Is the period, usually between the age 12 and 15 years in girls and 13 and 16 years in boys.
• Adolescence: Is the period from 11 to 19 years of age, which is characterised by rapid physical and
sexual maturation.
• Adulthood: Attainment of full growth and maturity – is generally reached between the ages of 18 and
21 years. Ossification and growth are virtually completed during early adulthood.
States Of Development
About 280 days after fertilization a new individual is born. These 10 lunar months are divided into 3 periods
(or stages).
1. Zygote (fertilized ovum) stage: The first two weeks.
2. Embryonic stage: From the beginning of the third week to the end of the second month (eighth
week) 3rd week – birth
What is the value of studying Embryology?
1. Bridges the gap between prenatal development and obstetrics, prenatal medicine, paediatrics, and
clinical anatomy.
2. Develops knowledge concerning the beginnings of human life and the changes occurring during prenatal
development.
3. Is of practical value in helping to understand the causes of variations in human structure.

5. 4. Illuminates gross anatomy and explains how normal and abnormal relations develop.
5. It is important to pathology, because abnormalities which may occur during development result in
malformations which appear in the adult.
Branches of Embryology:
• Developmental anatomy: Is the field of embryology concerned with the changes that cells, tissues,
organs and the body as a whole undergo from a germ cell of each parent to the resulting adult.
• Teratology: Is the division of embryology and pathology that deals with abnormal development (birth
defects): This branch of embryology is concerned with various genetic and/or environmental factors that
disturb normal development and produce birth defects.
Pregnancy Symptoms: What to expect during the first trimester.
• The first few months of pregnancy are marked by an invisible – yet amazing transformation. Knowing
what pregnancy symptoms to expect can help you face the months ahead with confidence.
Your Body:
Within two weeks of conception, hormones triggers your body to begin nourishing the baby – even
before tests and a physical examination can confirm the pregnancy. Here are some common physical
changes you may notice early on:
• Tender breast: Increased hormone production may make your breasts unusually sensitive. Your
breasts will probably feel fuller and heavier wearing a more supportive bra or a sport bra may help.
• Bouts of nausea: Many women have queasiness, nausea or vomiting in early pregnancy – probably
due to normal hormonal changes. Nausea tends to be worse in the morning, but it can last all day.
• Unusual fatigue: You may feel tired as your body prepares to support the pregnancy your heart will
pump faster and harder. Make sure you are getting enough iron and protein. Include phsical activity in
your daily routine.

6. THE THREE STAGES OF DEVELOPMENT
 the first two weeks are known as the ZYGOTE STAGE. At this stage
all cells look like each other
 From the beginning of the third week to the end of the second month(
eighth weeks) 3rd week- 2nd month is called EMBRYONIC STAGE,
at this stage the tissues, organs and systems are formed
 The period from the beginning of the third month till the time of birth
is called
THE FETAL STAGE, at this stage the body grows in length and in
weight

8. • Increased urination: You may need to urinate more often as your enlarging uterus presses on your
bladder during the first few months. The same pressure may cause you to leak urine when sneezing,
coughing or laughing.
• Dizziness: Normal circulatory changes in early pregnancy may leave you feeling a little dizzy, stress,
fatigue and hunger also may play a role.
Your Emotions:
Pregnancy may leave you feeling delighted, anxious, exhilarated and exhausted – sometimes all at
once.
It’s natural to .worry about your baby’s health, your adjustment to motherhood and and the financial
demands of raising a child. You may wonder how the baby will affect your relationship with your partner
or what type of parent you will be.
Your relationship with your partner:
Becoming a mother takes time away from other roles and relationships. You may lose some of your
psychological identity as a partner and lover but good communication can help you keep intimacy a live.
 Be honest.
 Be patient.
 Be supportive.

9. THE BIGINNING OF HUMAN DEVELOPMENT
First Week
 Human development begins at fertilization when a male gamete (or sperm) unites with a female gamete
(or ovum) to form a single cell – a zygote.
Sperm + Ovum zygote
fertilization
 The zygote marked the beginning of each of us a unique individual it contains chromosomes and genes
that are derived from the mother and father.
 The unicellular zygote divides many times and becomes progressively transformed into a multi cellular
human being through cell division, migration, growth and differentiation.
 Although development begins at fertilization, the stages and duration of pregnancy described in clinical
medicine are calculated from the commencement of the mothers last normal menstrual period (LNMP),
which is about 14 days before conception occurs.
Gametogenesis:
Gametogenesis (gamete formation): Is the process of formation and development of specialized
generative cells (gametes). This process involving the chromosomes and cytoplasm of the gametes,
prepares these sex cells for fertilization.
During gametogenesis, the chromosome number is reduced by half and the shape of the cells are
altered.
Gametogenesis (gamete formation)
Spermatogenesis (sperm formation Oogenesis (egg formation)

10.  What is the purpose of spermatogenesis? Two things:
1. To reduce the number of chromosomes from 46 to 23.
2. To change the shape of the male sex cell in order to become ready for fertilization.
Site: Spermatogenesis takes place in the somniferous tubules of the uterus.
occurrence: Spermatogenesis occurs continuously from puberty to old age.
Stages: Spermatogenesis is divided into two stages:
A. Spermato - Cyto - genesis
B. Spermio - geneis
A. Spermato-cyto-genesis = Spermatogonium
Primary spermatocyte
Secondary spermatocyte
Spermatid
B. Spermio – genesis = Spermatid Sperm
Spermatogonium Primary Spermatocyle Seconday spermatocyle
Spermotids
Sperm

11.  Spermatogonia, which have been dormant in the seminiferous tubules of the testes since the fetal period
begin to increase in number at puberty. After several mitotic divisions, the spermatogonia grow and
undergo changes that transform them into primary spermatocytes, the largest germ cells in the
seminiferous tubules.
 Each primary spermatocyte subsequently undergoes a reduction division – the first meiotic division to
form two haploid secondary spermatocytes, which are about half the size of primary spermatocytes.
 Subsequently the secondary spermatocytes undergo a second meiotic division to form four haploid
spermatids, which are about half the size of secondary spermatocytes.
 The spermatids are gradually transformed into four mature sperms by a process known as
spermiogenesis.
N.B:
 The entire process of spermatogenesis takes about two months.
 When spermiogenesis is completed the sperms enter the lumina of the seminiferous tubules.
 Sertoli cells lining the seminiferous tubules support and nurture the germ cells and may be involved in
the repulation of spermatogenesis.

13. .
Oogenesis
Oogenesis (ovogenesis): Is the sequence of events by which oogenia are transformed into mature
oocytes.
 This maturation process begins before birth and is completed after puberty (12-15 years) and continues
to menopause.
Prenatal Maturation of Oocytes:
 During early fetal life, oogonia proliferate by mitotic division.
 Oogonia enlarge to form primary oocytes before birth.
 As a primary oocyte forms, connective tissue cells surround it and form a simple layer of flattened
follicular epithelium cells.
 The primary oocyte enclosed by this layer of cells constitute a primordial follicle.
 As the primary oocyte enlarges during puberty, the follicular epithelium cells become cubical in shape
and columnar, forming primary follicle.
 The primary oocyte soon becomes surrounded by a covering of amorphous a cellular glyco-protein
material – the zona pellucida.
 When the primary follicle has more than one layer of follicular cells, it is called a secondary follicle.
 Primary oocytes begin the first meiotic division before birth but completion of prophase does not occur
until adolescence (11-19 years).
 The follicular cells surrounding the primary oocyte are believed to secrete a substance, oocyte
maturation inhibitor (OMI), which keeps the meiotic process of the oocyte arrested.

14. Postnatal Maturation of Oocytes
 Beginning during puberty usually one follicle matures each month and ovulation occurs, except when
oral contraceptives are used.
 As a follicle matures, the primary oocyte increases in size and, shortly before ovulation, completes the
first meiotic division, however, the division of cytoplasm is unequal – the secondary oocyte receives
almost all the cytoplasm and the first polar body receives very little.
 The polar body is a small, non functional cell that soon degenerates.
 At ovulation, the nucleus of the secondary oocyte begins the second meiotic division, but progresses
only to metaphase, when division is arrested.
 If a sperm penetrates the secondary oocyte, the second meiotic division is completed and most
cytoplasm is again retained by one cell, the fertilized oocyte.
 The other cell, the second polar body also a small non-functional cell, soon degenerates.
 There are about two million primary oocytes in the ovaries of a new born female, but many regress
during childhood so that by adolescence nor more than 40 thousand remain of these only about 400
become secondary oocytes and are explelled at ovulation during the reproductive period

16. The Female Genital Organs
Uterus:
The uterus (womb) is a thick-walled, pear-shaped muscular organ that varies considerably in size.
 The uterus averages 7 to 8 cm in length, 5 to 7 cm in width at its superior part, and 2’ to 3cm in
thickness.
 The uterus consists of two major parts:
1. Body: The expanded superior two-thirds.
2. Cervix: The cylindrical inferior third.
 The body of the uterus narrows from the fundus (the rounded, superior part of the body) to the isthmus
(the 1cm long constricted region between the body and cervix (neck).
 The cervix of the uterus is the tapered vaginal end that is nearly cylindrical in shape.
 The lumen of the cervix, the cervical canal has a constricted opening at each end.
 The internal os communicates with the cavity of the uterine body and the external os communicates with
the vagina.
 The walls of the body of the uterus consist of three layers:
1. Perimetrium: The thin external layer.
2. Myometrium: The thick smooth muscle layer.
3. Endometrium: The thin internal layer.

17. Uterine (Fallopian) Tubes:
 The uterine tubes, 10 to 12 cm long and 1 cm in diameter, extend laterally from the horns of the uterus.
 The tubes carry oocytes from the ovaries and sperms entering from the uterus to reach the fertilization
site in the ampulla of the uterine tube.
 The uterine tube also conveys the cleaving zygote to the uterine cavity.
 Each tube opens at its proximal end into the horn of the uterus and into the peritoneal cavity at its distal
end.
 For descriptive purposes, the uterine tube is divided into four parts:
1. infundibulum:-is the part which opens in to the peritoneal cavity
2. Ampulla: is the wide and tortuous part which follows the infundibulum
3. isthmus: is a narrow and straight part and has a thick wall
4. Interstitial part: is the part which pierces the wall of uterus
Ovaries:
 The ovaries are almond – shaped reproductive glands located close to the lateral pelvic walls on each
side of the uterus.
 The ovaries produce estrogens and progesterone, the hormones responsible for the development of
secondary sex characteristics and regulation of pregnancy.
 The ovaries are also responsible for producing and maintaining oocytes.

19. Uterine (Fallopian) Tubes:
 The uterine tubes, 10 to 12 cm long and 1 cm in diameter, extend laterally from the horns of the uterus.
 The tubes carry oocytes from the ovaries and sperms entering from the uterus to reach the fertilization
site in the ampulla of the uterine tube.
 The uterine tube also conveys the cleaving zygote to the uterine cavity.
 Each tube opens at its proximal end into the horn of the uterus and into the peritoneal cavity at its distal
end.
 For descriptive purposes, the uterine tube is divided into four parts:
1. Incunabulum.
2. Ampulla.
3. Uterine part (interstitial part)
Ovaries:
 The ovaries are almond – shaped reproductive glands located close to the lateral pelvic walls on each
side of the uterus.
 The ovaries produce estrogens and progesterone, the hormones responsible for the development of
secondary sex characteristics and regulation of pregnancy.
 The ovaries are also responsible for producing and maintaining oocytes.

20. Female Reproductive Cycle:
 Commencing at puberty and normally continuing throughout the reproductive years. Females undergo
monthly reproductive cycles (sexual cycles), involving activities of the hypothalamus of the brain,
pituitary gland, ovaries, uterus, uterine tubes, vagina and mammary glands.
 These monthly cycle prepare the reproductive system for pregnancy.
 A gonadotropin – releasing hormone (Gn RH) is synthesized by neurosecretory cells in the hypothalamus
and is carried by the hyperphysical portal system to the anterior tube of the pituitary gland.
 Gn RH stimulates the release of two hormones produced by this gland that act on the ovaries.
o Follicle: Stimulating hormone (F SH) stimulates the development of ovarian follicles and the
production of estrogen by its follicular cells.
o Luteinizing hormone (LH) serves as the trigger for ovulation (release of secondary oocyte) and
stimulates the follicular cells and corpus luteum to produce progesterone.
These hormones also induce growth of the endometrium.
Ovarian Cycle:
FSH and LH produce cyclic changes in the ovaries
1. Development of follicles.
2. Ovulation.
3. Corpus luteum formation

22. Follicular Development
 Development of an ovarian follicle is characterized by:
- Growth and differentiation of primary oocyte.
- Proliferation of follicular cells.
- Development of theca folliculi.
 As the primary follicle increases in size, the adjacent connective tissue organizes into a capsule, the
theca folliculi.
 The theca soon differentiate into two layers, an internal vascular and glandular layer – the theca interna
and a capsule like layer, the theca externa.
 Thecal cells are thought to produce an angiogenesis factor that promotes growth of blood vessels in the
theca interna which provide nutritive support for follicular development.
 The follicular cells divide actively producing a stratified layer around the oocyte.
 The ovarian follicle soon becomes oval and the oocyte eccentric in position.
 Subsequently fluid – filled spaces appear around the follicular cells, which coalesce to form a single
large cavity, the autrum, which contains follicular fluid.
 After the autrum forms the ovarian follicle is called a vesicular or secondary follicle.
 The primary oocyte is pushed to one side of the follicle, where it is surrounded by a mound of follicular
cells, the cumulus oophorus, that projects into the autrum.
 The follicle continues to enlarge until it reaches maturity and produces a swelling on the surface of the
ovary.
 The early development of ovarian follicles is introduced by FSH, but final stages of maturation require
LH as well.
 Growing follicles produce estrogen.

23. Ovulation
 Around mid cycle (about 14 days in a 28 day menstrual cycle), the ovarian follicle under the influence of
FSH and LH undergoes a sudden growth spurt, producing a cystic swelling or bulge on the surface of
the ovary.
 A small a vascular spot, the stigma, soon appears on this swelling.
 Prior to ovulation, the secondary oocyte and some cells of the cumulus oophorus detach from the
interior of the distended follicle.
 Ovulation is triggered by a surge of LH production.
 Ovulation usually follows the LH peak by 12 to 24 hours.
 The LH surge, elicited by the high estrogen level in the blood, appears to cause the stigma to balloon
out, forming a vesicle.
 The stigma soon ruptures, expelling the secondary oocyte, with the follicular fluid.
 The expelled secondary oocyte is surrounded by the zona pellucid a and one or more layers of follicular
cells, which are radially arranged as the corona radiata, forming the oocyte cumulus complex.
Corpus Luteum
Shortly after ovulation the walls of the ovarian follicle and theca folliculi collapse and thrown into
folds:
 Under LH influence they develop into a glandular structure – the corpus luteum – which secretes
progesterone, and some estrogen.
 These hormones particularly progesterone cause the endometrial glands to secrete and prepare the
endometrial for implantation of the blastocyst.

24.  If the oocyte is fertilized, the corpus luteum enlarges to form a corpus luteum of pregnancy and
increases its hormone production.
 When pregnancy occurs, degeneration of the corpus luteum is prevented by human chorionic-
gonadotropin (HCG),a hormone secreted by the syncytio-trophoblast of the blasto cyst, which is r.ch.n
LH.
 The corpus luteum of pregnancy remains functionally active throughout the first 20 weeks of pregnancy.
 By this time, the placenta has assumed the production of the estrogen and progesterone that is
necessary for the maintenance of pregnancy.
Menstrual Cycle:
 The menstrual cycle is the period during which the oocyte matures, is ovulated and enters the uterine
tube.
 The hormones produced by the ovarian follicles and corpus luteum (estrogen and progesterone)
produce cyclic changes in the endomatrium.
 These monthly changes in the internal layer of the uterus constitute the endometrial cycle, commonly
referred to as the menstrual cycle.
Phases of the Menstrual Cycle:
1. Menstrual phase.
2. Proliferative phase.
3. Luteal phase

25.  Menstrual phase: The first day of menstruation is the beginning of the menstrual cycle’
- The functional layer of the uterine wall is sloughed off and discharged with the menstrual flow –
menses (monthly bleeding) – which usually lasts 4 to 5 days.
 Proliferative phase: Follicular (estrogenic) phase lasting about 9 days, coincides growth of ovarian
follicles and is controlled by estrogen secreted by these follicles.
- There is a two – to three – fold increase in the thickness of the endometrium and in its water content
during this phase of repair and proliferation.
- During this phase the surface palladium reforms and covers the endometrium.
- The glands increase in number and length and the spiral arteries elongate.
 Luteal phase: Secretory (Progesterone) phase, lasting about 13 days, coincides with the formation,
functioning and growth of the corpus luteum.
- The progesterone produced by the corpus luteum stimulates the glandular epillalium to secrete a
glycogen-rich material.
- The glands become wide, tortuous and secular.
- Endometrium increases in thickness.
- Venous spaces develops.
- Direct arterio-venous anastomose are prominent features of this stage.

26. Fertilization:
Fertilization is a complex sequence of coordinated molecular events that begins with contact between a
sperm and oocyte and ends with the intermingling of maternal and paternal chromosomes at metaphor of
the first mitotic division of the zygote.
Phases of Fertilization:
1. Passage of sperm through corona radiate .
2. Penetration of zona pellucid a
3. Fusion of plasma membranes of the oocyte and sperm.
4. Completion of second meiotic division of oocyte and formation of female pronucleus.
5. Formation of male pronucleus.
6. As the pronucli fuse in a single diploid aggregation of chromosomes, the Ootid becomes a zygote.
7. The chromosomes in the zygote become arrayed on a cleavage spindle, in preparation for cleavage of the
zygote.
N.B: An early pregnancy factor (EPF), an immuno suppressant protein, is secreted by the trophoblastic cells
and appears in the maternal serum within 24 to 48 hours after fertilization.
 EPF forms the basis of a pregnancy test during the first 10 days of development.
Importance of Fertilization:
 Stimulates the penetrated oocyte to complete the second meiotic division.
 Restores the normal diploid number at chromosomes (46) in the zygote.
 Results in variation of the human species through mingling of maternal and paternal chromosomes.
 Determines chromosomal sex of the embryo.
 Causes metabolic activation of the ootid and initiates cleavage of the zygote.

27. Cleavage of Zygote:
 Cleavage consists of repeated mitotic divisions of the zygote, resulting in a rapid increase in the number
of cells.
 These embryonic cells – blastameres – become smaller with each cleavage division.
 First the zygote divides into two blastomeres, which then divide into four blastomeres, eight blastomeres
and so on.
 Cleavage normally occurs as the zygote passes along the uterine tube toward the uterus.
 During cleavage, the zygote is within the rather thick zone pellucida that is translucent under the light
microscope.
 Division of the zygote into blastomeres begins about 30 hours after fertilization.
 After the nine-cell stage, the blastomeres change their shape and tightly align themselves against each
other to form a compact ball of cells. This phenomenon – Compaction – probably mediated by cell
surface adhesion glycoprotein.
 When there are 12 to 32 blastomeres, the developing human is called a morula
 Internal cells of the morula are surrounded by a layer of cells that form the outer cell layer.
 The spherical morula forms about 3 days after fertilization and enters the uterus.

29. Formation of Blastocyst:
 Shortly after he morula enters the uterus (about 4 days after fertilizaion).
 A fluid-filled space called the blasto cystic cavity appears inside the morula.
 The fluid passes from the uterine cavity through the zona pellucida to form this space.
 As fluid increases in he blasto cystic cavity it separates the blastomeres into two parts:
1. A thin, outer cell layer – the trophoblast – which gives rise to the embryonic part of the placenta.
2. A group of centrally located blastomeres – the inner cell mass – which gives rise to the embryo:
Because it is the primordium of the embryo, the inner cell mess is called the embryoblast.
 About 6 days ater fertilization, the blasto cyst attaches to the endometrial epithelium, usually adjacent to
the embryonic pole.
 As soon as it attaches to the endometrial epillalium, the trophoblast starts to proliferate rapidly and
gradually differentiates into two layers:
1. An inner layer of cyto trophoblast.
2. An outer mass of syncytiotrophoblast.

30. Second Week
Formation of Bilaminar Embryonic Disc:
 Completion of implantation and continuation of embryonic development.
 Implantation of the blastocyst which commenced at the end of the first week, is completed by the
end of the second week.
 The erosive syncytiotrophoblast involves the endometrial connective tissue which supports the
endometrial capillaries and glands.
 As this occurs, the blastocyst slowly embeds itself in the endometrium.
 The endometrial cells undergo apoptosis (programmed cell death), which facilitates the invasion of
the maternal endometrium during implantation.
 Proteolytic enzymes produced by the syncytiotrophoblast, as well as COX -2 derived prostacyclin
and fas ligual present at the implantation site, are involved in this process.
 The connective tissue cells around the implantation site accumulate glycogen and lipids and
assume a polyhedral appearance, some of cells – decidual cells – degenerate adjacent to the
penetrating syncytiotrophoblast.
 The syncytiotrophoblast engulfs these degenerating cells, providing a rich source of embryonic
nutrition.

32. Formation of Amniotic Cavity Embryonic Disc and Yolk Sac:
 As implantation of the blasto cyst progresses, a small space appears in the embryo blast which is the
primordium of the amniotic cavity.
 Rapid morphological changes occur in the embryo blast that result in the formation of a flat almost
circular bilaminar plate of cells, the embryonic disc consisting of two layers:
a) Epiblast: ( ectoderm)The thicker layer consisting of high columnar cells related to the amniotic
cavity.
b) Hypoblast( endoderm) Consisting of small cuboidal cells adjacent to the exocoelomic cavity.
 The epiblast forms the floor of the amniotic cavity and is continuous peripherally with the amnion.
 The hypoblast forms the roof of the exocoelomic cavity and is continuous with the thin exocoelomic
membrane.
 Exocoelomic membrane plus hypoblast forms the primary yolk sac.
 The embryonic disc now lies between the amniotic cavity and the primary yolk sac.
 Cells from the yolk sac endoderm form a layer of connective tissue, the extra embryonic mesoderm,
which surrounds the amnion and yolk sac.
 As the amnion, embryonic disc and primary yolk sac form isolated cavities – lacunae appear in the
syncytiotrophobllast.
 The 10-day human conceptus embryo and extra embryonic membrane is completely embedded in the
endometrium.
 As the conceptus implants, the endometrial connective tissue cells undergo a transformation – the
decidual reaction.
 After the cells swell because of the accumulation of glycogens and lipid in the cytoplasm, they are
known as decidual cells.
 In a 12-day embryo, adjacent syncytiotrophoblastic lacunae have fused to form lacunar networks.

33. Development of Chorionic Sac:
 The end of the second week is characterized by the appearance of primary chorionic vile. Proliferation
of cytotrophoblastic cells produces cellular extensions that grow into the syncytiotrophoblast.
 The growth of these extensions is though to be induced by the underlying extra embryonic somatic
mesoderm. The cellular projections form primary chorionic vile, the first stage in the development of the
chorionic vile of the placenta.
 The extra embryonic coelom splits the extra embryonic mesoderm into two layers:
1. Extra embryonic somatic mesoderm: Lining the trophoblast and covering the amnion.
2. Extra embryonic splanchnic mesoderm: Surrounding the yolk sac.
 The extra embryonic somatic mesoderm and the two layers of trophoblast form the chorionic, the
chorion forms the wall of the chorionic sac, within which the embryo and its amniotic and yolk sacs are
suspended by the connecting stalk.

35. Formation of Germ Layers and Early Tissue and Organ Differentiation: Third Week:
 Gastrulating: Formation of germ layers.
 Gastrulating is the formation process by which the three germ layers and axial orientation are
established in embryos.
 During gestrulation, the bilaminar embryonic disc is converted into a trilaminar embryonic disc.
 Gastrulation is the beginning of morphogenesis (development of body form).
 Each of the three germ layers (ectoderm, mesoderm and endoderm) gives rise to specific tissues and
organs.
1. Embryonic ectoderm: Gives rise to the epidermis, central and peripheral nervous systems,
retina of the eye
2. Embryonic endoderm:. Is the source of the epithelial linings of the reparatory passages and
gastro intestinal tract.
3. Embryonic mesoderm: Gives rise to smooth muscular coats, connective tissues and vessels
associated with the tissues and organs, cardiovascular system the skeletal and striated muscles,
reproductive and excretory organs.
Primitive Streak:
 The fist signs of gastrulations is the appearance of the primitive streak.
 The primitive streak result from the proliferation and migration of cells of the epiblast to the median plane
of the embryonic disc.
 As the streak elongates by addition of cells to its caudal end, its cranial end proliferates to form a
primitive node.
 Concurrently a narrow groove – primitive groove – develops in the primitive streak that is continuous
with a small depression in the primitive node – the primitive pit.

36.  As soon as primitive streak appears, it is possible to identify the embryos craniocandal axis, its cranial
and candal ends, its dorsal and ventral surfaces and its right and left sides.
 Shortly after the primitive streak appears cells leave its deep surface and form mesenchyone, a tissue
consisting of loosely arranged cells suspended in gelatinous matrix. `
 Mesenchymel cells are ameboidal and actively phagocytic, they form the supportive tissues of the
embryo, such as most of the connective tissues of the body and the connective tissue frame work of
glands.
 Some mesenchyone forms mesoblast (un differentiated mesoderm), which forms the intra embryonic
or embryonic mesoderm.
 Cells from the epiblast displace the hypoblast, forming the intra embryonic or embryonic endoderm
in the root of the yolk sac.
 The cells remaining in the epiblast form the intra embryonic or embryonic ectoderm.
Fate of Primitive Streak
 The primitive streak actively forms mesoderm until the early part of the fourth week, thereafter,
production of mesoderm slows down.
 The primitive streak diminishes in relative size and becomes an insignificant structure in the sacro
ccygeal region of the embryo.
 Normally the primitive streak undergoes degeneration changes and disappears by the end of the fourth
week.

38. Notochordal Process and Notochord:
 Some mesenchymal cells migrate cranially from the primitive node and pit forming a median cellular
cord, the notochordal process.
 The process soon acquires a lumen, the notochondel canel.
 The notochordel process grows cranially between the ectoderm and endoderm until it reaches the
prechondral plate, a small circular area of columnar endodermal cells where the ectoderm and
endoderm are in contact.
 The prechordal plate is the primordium of the oropharyngeal membrane.
 Some mesenchyonal cells from the primitive streak migrate cranially on each side of the notochordal
process and around the prechordal plate. Here they meet cranially to form cardiogenic mesoderm in the
cardiogenic area, where the heart primordium begins to develop at the end of the third week.
 caudal to the primitive streak there is a circular area, the cloacal membrane which indicates the future
site of the anus.
 The notochord is a cellular rod that develops by transformation of the notochordal process.

39. THE NOTOCHORDAL DEVELOPMENT
 the notochordal process elongates by invagination
of cells from the primitive pit
 The primitive pit extends in to the notochordal
process, forming the notochordal canal
 The notochordal process is now a cellular tube that
extends from the primitive node to the prechordal
plate
 The floor of the notochordal process fuses with the
underlying embryonic endoderm
 The fused layers gradually undergo degeneration,
resulting in the formation, which brings the
notochordal canal in to communication with the york
sac
 The opening rapidly become confluent and the floor
of the notochordal canal disappears

40.  The remains of the notochordal process form a flattened,
grooved notochordal plate
 Beginning at the cranial end of the embryo , the notochordal
cells proliferate and the notochordal plate in folds to form the
notochord
 The proximal part of the notochordal canal persists
temporarily as the NEURENTERIC CANAL, which forms the
communication between the amniotic and york sac cavities
 The notochord becomes detached from the endoderm of the
york sac
N.B . The notochord is an intricate structure around which the
verterbral column forms. It extends from the oropharyngeal
membrane to the primitive node
 The notochord functions as the primary inductor in the early
embryo

42. ALLANTOIS
 The allantois appears on about day 16 as a small, sausage-shaped
diverticulum( out pouching) from the caudal wall of the yolk sac that
extends in to the connective stalk
 In human embryo the allantois remains very small , because the
placenta and amniotic sac take over its functions
 The allantois is involved with the early blood formation in the human
embryo and is associated with development of the urinary bladder
 As the bladder enlarges , the allantois becomes the URACHUS, which is
represented in adults by the MEDIAN UMBILICAL LIGAMENT
 The blood vessels of the allantois become the umbilical arteries and
veins
 ALLANTOIC CYSTS: Is the remnants of the extraembryonic portion of
the allantois , are usually found between the fetal umbilical vessels and
can be detected by ultrasonography. They are most detected in the
proximal part of the umbilical cord, near its attachment to the anterior
abdominal wall

43. ALLANTOIC CYST

44. NEURULATION
 The processes involved in the formation of the formation of the neural
plate and neural folds and the closure of the folds to form the neural tube
constitute neurulation
 These processes are completed by the end of the fourth week
 During the neurulation , the embryo may be referred to as a neurula
 As the notochord develops , the embryonic ectoderm over it thickens to
form an elongated , slipperlike plate of thickened epithelial cells, THE
NEURAL PLATE.
 The ectoderm of neural plate gives rise to the brain and spinal cord
(CNS)
 At first the elongated neural plate corresponds in lengths to the
underlying notochord.
 It appears cranial to the primitive node and dorsal to the notochord and
the mesoderm adjacent to it
 As the notochord elongates, the neural plate broadens and eventually
extends cranially as far as the oropharyngeal membrane

45.  On the 18th day, the neural plate invaginates along its central axis to
form a longitudinal median NEURAL GROOVE, which has neural
folds on each side
 The neural folds becomes prominent at the cranial end of the embryo
and are the first signs of brain development
 By the end of the third week, the neural folds fuse , converting the
neural plate in to a NEURAL TUBE, the primordium of the CNS

46. NEURAL CREST FORMATION
 As the neural folds fuse to form the neural tube , some
neuroectodermal cells lying along the crest of each neural fold lose
their epithelial affinities and attachments to neighboring cells
 As the neural tube separates from the surface ectoderm, NEURAL
CREST CELLS, migrate dorsolaterally on each side of the neural
tube
 They soon form a flattened irregular mass, the neural crest
 The neural crest soon separates in to right and left parts that migrate
to the dorsolateral aspects of the neural tube; here they give rise to
the sensory ganglia of the spinal and cranial nerves
 The neural crest cells also contribute the formation of pigment cells,
the suprarenal medulla and several skeletal and muscular
components of the head

47. NEURAL TUBE DEFECTS
 Neural tube defects ( NTDs) are among the most common congenital
anomalies
 MEROANENCEPHALY: partial absence of the brain

49. The derived MESODERM will be splitted in to three parts
 Paraxial Mesoderm: which gives rise to SOMITES, somites will be
divided in to SCLEROTOME and DERMO-MYOTOME
 Intermediate Mesoderm: which gives rise to the kidneys, testes,
ovaries and cortex of the suprarenal glands
 Lateral Plate Mesoderm: it gives rise Intra-embryonic Coelomic
Cavity

50. DEVELOPMENT OF SOMITES
 About the 17th day of development the paraxial mesoderm
starts to be formed on either side of the notochord
 About the 20th day the paraxial mesoderm starts to be
divided in to little blocks( paired cuboidal bodies) called
somites
 The division of paraxial mesoderm in to somites begins at
the 20th and continues up to the 35th or even 40th day
 Most somites are clearly seen by the end of the first months
 42 to 44 somites are formed , However, the only the first 30
somites can be easily seen
 Because the somites are so prominent during the fourth and
fifth weeks , they are used as one of several criteria for
determining an embryo's age

52. FORMATION OF INTRAEMBRYONIC COELOM
 The coelom( cavity) with in the embryo arises as isolated spaces in
the lateral mesoderm and cardiogenic mesoderm
 The coelomic vesicles subsequently coalesce to form a single,
horseshoe- shaped cavity that eventually gives rise to the body
cavity, the peritoneal cavity for example

54. FORMATION OF BLOOD VESSELS AND BLOOD
 Blood vessels first appear in the wall of the yolk sac, allantois and
chorion
 They develop within the embryo shortly thereafter
 Spaces appear within aggregations of mesenchyme, known as
BLOOD ISLANDS.
 The spaces soon become lined with endothelium derived from the
mesenchymal cells
 These primordial tubules sprout and unite with other vessels to form
a primordial cardiovascular system
 Toward the end of the third week, the heart is represented by paired
endocardial heart tubes that are joined to blood vessels in the
embryo and in the extraembryonic membranes ( yolk sac, umbilical
cord and chor-
ionic sac)
 By the end of the third week , the heart tubes have fused to form a
tubular heart that is joined to vessels in the embryo

55.  The primordial blood cells – HEMANGIOBLASTS, are derived
mainly from the endothelial cells of blood vessels in the walls of the
yolk sac and allantois
 Fetal and adult erythrocytes probably develop from different
hematopoietic precursors

56. COMPLETION OF CHORIONIC VILLI FORMATION
 Primary chorionic villi become secondary chorionic villi as they
acquire mesenchymal cores
 Before the end of the third week , capillaries develop in the
secondary chorionic villi, transforming them in to tertiary chorionic
villi
 Cytotrophoblastic extensions from these stem villi join to form a
CYTOTROPHOBLASTIC SHELL, that anchors the chorionic sac to
the endometrium
 The rapid development of chorionic villi during the third week greatly
increases the surface area of the chorion for the exchange of oxygen
and nutrients and other substances between the maternal and
embryonic circulations.

57. DERIVATIVES OF THE MESODERMAL GERM LAYER
1. Connective tissue
2. Cartilage
3. Bone
4. Joints
5. Striated muscles
6. Smooth muscles
7. The heart
8. Blood and lymph vessels
9. Red and white blood corpuscles
10. Spleen
11. Serous membrane
12. Cortex of suprarenal glands
13. Kidneys
14. Tests and ovaries

58. DERIVATIVES OF THE ENDODERMAL GERM LAYER
1. All epithelium lining of the
 Digestive tube
 Respiratory system
 Middle ear and pharyngotympanic tube
 Urinary bladder and urethra
2. The parenchyma of the
 Tonsil thyroids, parathyroid and thymus
 Liver
 pancreas

59. DERIVATIVES OF THE ECTODERMAL GERM LAYER
1. The nervous system
2. The sensory epithelium of the sense organs
3. The pituitary gland
4. The epithelium of the skin including the hairs, nails and skin
glands
5. Mammary glands
6. Subcutaneous glands
7. Enamel of teeth

60. ORGANOGENETIC PERIOD: FOURTH TO EIGHTH WEEKS
 The fourth to eight weeks of development constitute most of the
embryonic period
 By the end of the Organogenetic period , the main organ systems
have begun to develop; however, the function of most of them is
minimal, except for the cardiovascular system
 As the tissues and organs form, the shape of the embryo changes
and by the eighth week it has a distinctly human appearance.

61. PHASES OF EMBRYONIC DEVELOPMENT
1. The first phase is GROWTH, which involves cell division and
elaboration of cell products
2. The second phase is MORPHOGENESIS; ( development of
shape, size or other features of a particular organ or part or the
whole of the body
3. The third phase is DIFFERENTIATION; ( maturation of
physiological processes, which results in the formation of tissues
and organs that are capable of performing specialized functions

62. FOLDING OF THE EMBRYO
 A significant event in the establishment of body form is folding of the
flat trilaminar embryonic disc in to a somewhat cylindrical embryo
 Folding occur in both median and horizontal planes and results from
rapid growth of the embryo
 At the time when the somites are developing, the embryonic disc
begins to bulge in to the amniotic cavity and starts to be folded in an
antero-posterior(cephalo-caudal) direction
 When the embryo reaches the 7 somites stage a head fold as well
as tail fold are formed
 The head fold arises as result of growth of the embryo in a cranial
direction while the tail fold arises as a result of growth of the embryo
in a caudal direction
 While the embryo is undergoing folding in an antero-posterior
direction, the embryonic disc becomes gradually lifted from the yolk
sac and in this way two lateral folds are formed

63. EMBRYONIC FOLDS

64. FOURTH WEEK
 Major changes in body form occur during the fourth week
 At the beginning , the embryo is almost straight and has 4 to 12
somites that produce conspicuous surface elevations
 The neural tube is formed opposite the somites, but is widely open at
the rostral( anterior and posterior NEUROPORES ,
 By 24 day the pharyngeal arches are visible
 The first ( mandibular arch) and the second ( hyoid arch) are distinct
 Three pairs of pharyngeal arches are visible by 26 days, and the
rostral neuropores are closed
 The forebrain produces a prominent elevation of the head, and the
folding of the embryo has given the embryo a C-shaped curvature
 Along , curved caudal eminence( tail like structure ) is present
 The upper limb buds; become recognizable by day 26 or 27 as
small swellings on the ventrolateral body walls
 The lower limbs ; are visible by the end of the fourth week

65. FOURTH WEEK EMBRYO

67. FIFTH WEEK
 Changes in body form are minor during the fifth week compared with
those that occurred during the fourth week
 But growth of the head exceeds that of other regions
 Enlargement of the head is caused mainly by the rapid development
of the brain and facial prominence
 The rapid growing second pharyngeal arch overgrows the third and
fourth pharyngeal arches, forming a lateral ectodermal depression on
each side the CERVICAL SINUS

68. FIFTH WEEK EMBRYO

69. SIXTH WEEK
 The upper limbs begin to show regional differentiation as the elbows and
large hand plates develops
 The primordia of the digits called DIGITAL RAYS, begin to develop in the
hand plates, which indicate the formation of digits
 The embryo at the six week show spontaneous movements, such as twitching
of the trunk and limbs
 Development of the lower limbs occurs some what later than that of the upper
limbs
 Several small swellings- AURICULAR HILLOCKS, develop around the
pharyngeal groove or cleft between the first two pharyngeal arches
 This groove becomes the EXTERNAL ACOUSTIC MEATUS, ( external
auditory canal) and the auricular hillocks around it fuse to form the auricle, the
shell shaped part of the external ear
 The eyes are obvious
 The head is much larger than the trunk and is bend over the HEART
PROMINENCE
 The embryo at sixth week show reflex responses to touch.

70. SIXTH WEEK EMBRYO

71. SEVENTH WEEK
 The limbs undergo considerable change during the seventh week
 Notches appear between the digital rays in the hand plates, clearly
indicating the future digits
 The communication between the primordial gut and york sac is now
reduced to a relatively slender duct, the YOLK STALK.
 The intestines enter the extraembryonic coelom in the proximal part
of the umbilical cord
 This UMBILICAL HERNIATION is a normal event in the embryo
 The herniation occurs because the abdominal cavity is too small at
this age to accommodate the rapidly growing intestine
 By the end of the seventh week, ossification of the bones of the
upper limbs has begun

72. SEVENTH WEEK EMBRYO

73. UMBILICAL HERNIATION

74. EIGHTH WEEK
 At the beginning of this final week of the embryonic period, the digits
of the hand are separated but noticeable webbed
 Notches are now clearly visible between the digital rays of the fan-
shaped feet
 The tail like caudal eminence is still present but short
 The SCALP VASCULAR PLEUS has appeared and forms a
characteristic band around the head
 By the end of the eighth week, all regions of the limbs are apparent
 Ossification begins in the lower limbs during the eighth week and is
first recognizable in the femur
 The hands and feet approach each other
 At the end of the eighth week, the embryo has distinct human
characteristics

75. EIGHTH WEEK EMBRYO

76. ESTIMATION OF EMBRYONIC AGE
 During the somite period ( i.e. between 20 and 30 days) the age of
the embryo can be roughly estimated by counting the number of
somites.
Number of somites:- 1 4 7 10 13 16 19 22 25 28 31
Age in days: 20 21 22 23 24 25 26 27 28 29 30
 Because embryos of the third and early fourth weeks are straight,
measurement of them indicate the GREATEST LENGTH ( GL)
 The setting height or CROWN-RUMP length ( CRL) is most
frequently used for older embryo
 The standing height , or CROWN-HEEL LENGTH ( CHL), is some
times measured for 8 week embryo

77. CROWN-RUMP LENGTH

78. ULTRASOUND EXAMINATION OF THE EMBRYO
Most women seeking obstetrical care have at least one ultrasound
examination during their pregnancy for one or more of the following
reasons:
 Estimation of gestational age for confirmation clinical dating
 Evaluation of embryonic growth
 Examination of a clinically detected pelvic mass
 Suspected ectopic pregnancy
 Possible uterine abnormality
 Detection of congenital abnormalities

79. THE FETAL PERIOD( NINTH WEEK TO BIRTH)
NINE TO TWELVE WEEKS
 At the beginning of the ninth week, the head constitutes half the crown-heal
length of the fetus
 Subsequently, growth in body length accelerates rapidly and by the end of the
12 weeks the CRL has more than double
 At the 9 week the face is broad , the eyes are widely separated, the ears are
low-set and the eyelids are fused
 By the end of 12 weeks, primary ossification centers appear in the skeleton
,especially in the cranium and long bones
 Early in the 9 week, the legs are short and the thighs are relatively small
 By the end of the 12 weeks, the upper limbs have almost reached their final
relative lengths, but the lower limbs are still not so well developed and are
slightly shorter than their final relative length
 The external genitalia of males and females appear similar until the end of
the ninth week
 Their mature fetal form is not established until the twelfth week
 Intestinal coils are clearly visible in the proximal end of the umbilical cord until
the middle of the 10th week
 By the end of the 11th week the intestines have returned to the abdomen

80. NINE TO TWELVE WEEKS CONTINUE
 At the 9th week the liver is the major site of erythropoiesis (formation
of red blood cells)
 By the end of the 12th, this activity has decreased in the liver and has
begin in the spleen
 Urinary formation begins between the 9th and 12th weeks, and urine
is discharged in to the amniotic fluid
 The fetus reabsorbs some amniotic fluid after swallowing it
 Fetal waste products are transferred to the maternal circulation by
passing across the placental membrane

81. THIRTEEN TO SIXTEEN WEEKS
 Growth is rapid during this period
 By 16 weeks the head is relatively small compared with that of the 12
week fetus and the lower limbs have lengthened
 Limb movements, which first occur at the end of the embryonic
period , become coordinated by the 14th week but are too slight to be
felt by the mother
 Ossification of the fetal skeleton is active during this period and the
bones are clearly visible in ultrasound images by the beginning of the
16th week
 Slow eye movement occur at 14th week
 Scalp hair patterning is also determined during this period
 By 16th weeks the ovaries are differentiated and contain primordial
ovarian follicles that contain oogonia
 The sex of the external genitalia can be recognized by 12 to 14
weeks in most cases
 By the 16 weeks the eyes face anteriorly rather than anterolaterally

82. SEVENTEEN TO TWENTY WEEKS
 Growth slows down during this period but the fetus still increases its
CRL by about 50 mm
 The limbs reach their final relative proportions and fetal movements-
QUICKENING- are commonly felt by the mother
 The mean time that intervenes between a mother's first detection of
fetal movements and delivery is 147 days, with standard deviation of
+5 or -5 days
 The skin is now covered with a greasy, cheese like material-VERNIX-
CASEOSA, it consists of a mixture of a fatty secretion from the fetal
sebaceous glands and dead epidermal cells
 The vernix caseosa protects the delicate fetal skin from abrasions,
chapping and hardening that could result from exposure to the
amniotic fluid
 Eyebrows and head hairs are also visible at 20 weeks
 The bodies of 20-week fetuses are usually covered with fine downy
hair-LANUGO- which helps to hold the vernix caseosa on the skin
 Brown fat forms during this period and is the site of head production

83. CONTINUE
 By the 18 weeks the uterus is formed and canalization of the vagina
has begin
 By the 20 weeks the testes have begun to descend, but they are still
located on the posterior abdominal wall, as are the ovaries in female
fetuses

85. TWENTY-ONE TO TWENTY-FIVE WEEKS
 There is substantial weight gain during this period
 The skin is usually smooth and more translucent, particularly during
the early part of this period
 At 21 weeks rapid eye movements begin and blink-startle responses
have been reported
 By 24 weeks the secretory epithelial cells in the interalveolar walls of
the lungs have begin to secrete surfactant( a surface active lipid that
maintains the patency of the developing alveoli of the lungs)
 Fingernails are present by 24 weeks
 Although a 22 to 25 weeks fetus born prematurely may survive if
given intensive care, it may die during early infancy because its
respiratory system is still immature.

87. TWENTY SIX TO TWENTY-NINE WEEKS
 At this stage the fetus survives if born prematurely and given
intensive care
 The lungs and pulmonary vasculature have developed
 The central nervous system has matured to the stage where it can
direct rhythmic breathing movements and control body temperature
 The eye lids are open at 26 weeks and head hairs are well
developed
 Toe nails become visible and considerable subcutaneous fat is now
present under the skin
 The fetal spleen is now an important site of hematopoiesis
 Erythropoiesis in the spleen ends by 28 weeks, by which time bone
marrow has become the major site of this process

88. THIRTY TO THIRTY –FOUR WEEKS
 The pupillary light reflex of the eyes can be obtained by 30 weeks
 By the end of this period the skin is pink and smooth
 The upper and lower limbs have a overweight appearance
 At this stage the quantity of white fat is 8% of the body weight
 Fetuses 32 weeks and older usually survive if born prematurely
 If a normal –weigh fetus is born during this period , it is premature by date
but not premature by weight

89. THIRTY-FIVE TO THIRTY-EIGHT WEEKS
 The central nervous system is sufficiently mature to carry out some
integrative functions
 At this stage the circumferences of the head and abdomen are approximately
equal
 There is a slowing of growth as the time of birth approaches
 By full term, most fetuses usually reach a CRL of 360 mm and weight about
3400 gm
 The amount of white fat is about 16% of body weight
 A fetus adds about 14 gm of fat a day during these last weeks of gestation
 In general , male fetuses are longer and weight more at birth than females
 The chest is prominent and breasts often protrude slightly in both sexes
 The testes are usually in the scrotum in full-term male infants
 Premature male infants commonly have undescended tests
 The head is smaller than the rest parts of the body but it is still one of the
largest regions of the fetus, this is an important consideration related to its
passage through the birth canal

92. EXPECTED DAY OF DELIVERY
 The expected day of delivery ( EDD) of a fetus is 266 days or 38
weeks after fertilization. i.e. 280 days or 40 weeks after LNMP
 About 12% of babies , however, are born 1 to 2 weeks after the
expected time of birth

94. THE PLACENTA
 The placenta is the primary site of nutrient and gas exchange between the
mother and fetus
 The placenta is fetomaternal organ that has two components
 A fetal part that develops from the chorionic sac
 A maternal part that is derived from the endometrium
 The placenta and umbilical cord form a transport system for substances
passing between the mother and fetus
 Nutrients and oxygen pass from the maternal blood through the placenta
to the fetal blood, and waste materials from the fetus to the maternal
blood
 The placenta and fetal membranes perform the following functions
1. Protection
2. Nutrition
3. Respiration
4. Excretion
5. Hormone production

95. DECIDUA
 The decidua is name given to the endometrium after the blastocyst is
completely embedded in it
 In other words, the decidua is the endometrium of pregnancy
 The decidua is divided in the following three parts
 Decidua basalis
 Decidua capsularis
 Decidua parietalis
 Decidua basalis: is the most important part of the decidua. It is the
part which lie between the blastocyst and the wall of the uterus
 Decidua capsularis: this is thin layer of endometrium (decidua) which
covers the blastocyst and forms a thin capsule for it.
 Decidua parietalis: this is the decidua which lines the remaining part of
the uterine cavity
 As the embryo grows only decidua basalis develops and forms the
maternal part of the placenta, while the other parts of the decidua
degenerate

96. DEVELOPMENT OF PLACENTA
 Placenta is formed as result of proliferation of the trophoblast and the
development of the chorionic sac and chorionic villi
 By the end of the third week, the anatomical arrangements
necessary for physiological changes between the mother and her
embryo are established
 A complex vascular network is established in the placenta by the end
of fourth week, which facilitates maternal-embryonic exchanges of
gases, nutrients and metabolic waste products
 Chorionic villi cover the entire chorionic sac until the beginning of the
8th week
 As this sac grows the villi associated with the decidua capsularis are
compressed, reducing the blood supply to them, these villi soon
degenerate , producing a relatively avascular bare area , the
SMOOTH CHORION
 As the villi disappear, those associated with the decidua basalis
rapidly increase in number

97. PLACENTA CONTINUES
 The fully developed placenta covers 15 to 30% of the decidua and
weights about one-sixth that of the fetus
 The placenta has two parts
 The fetal part of the placenta: is formed by the villous chorion, the
chorionic villi that arise from it project in to the intervillous space
containing the maternal blood.
 The maternal part of the placenta:- is formed by the decidua
basalis
 By the end of the 4th month , the decidua basalis is completely
replaced by the fetal part of the placenta
 Fetomaternal Junction:- the fetal part of the placenta ( villous
chorion) is attached to the maternal part of the placenta( decidua
basalis) by the CYTOTROPLASTIC SHELL- the external layer of the
trophoblast

98. INTERVILLOUS SPACE
 The intervillous space containing maternal blood is derived from the
lacunae that developed in the syncytiotrophoblast during the second
week of development
 This large blood filled space results from the coalescence and
enlargement of the lacunar networks
 The intervillous space of the placenta is divided in to compartments
by the PLACENTAL SEPTA; however, there is free communication
between the compartments, because the septa do not reach the
chorionic plate
 The maternal blood enters the intervillous space from the spiral
endometrial arteries in the decidua basalis

99. PLACENTAL CIRCULATION
 The branch chorionic villi of the placenta provide a large surface area
where materials may be exchanged across the very thin
PLACENTAL MEMEBRANE ( barrier), inserted between the fetal
and maternal circulations
 It is through the numerous branch villi, which arise from the stem villi.
That the main exchange of material between the mother and fetus
takes place
 The circulation between the mother and the fetus is separated by the
placental barrier( placental membrane)

100. FETAL PLACENTAL CIRCULATION
 Poorly oxygenated blood leaves the fetus and passes through the
UMBILICAL ARTERIES to the placenta
 At the site of attachment of the cord to the placenta, these arteries
divide in to several radially disposed CHORIONIC ARTERIES that
branch freely in the chorionic plate before entering the chorionic villi
 The blood vessels form an extensive ARTERIO- CAPILLARY-
VENOUS SYSTEM within the chorionic villi, which brings the fetal
blood extremely close to the maternal blood
 Large vessel called UMBILICAL VEIN carries oxygen rich blood to
the fetus

101. MATERNAL PLACENTAL CIRCULATION
 The maternal blood in the intervillous space is temporarily outside
the maternal circulatory system
 It enters the intervillous space through 80 to 100 spiral endometrial
arteries in the decidua basalis
 These vessels discharge in to the intervillous space through the
gaps in the Cytotrophoblastic shell
 The blood flow from the spiral arteries is pulsatile and is propelled I
jet like sprays by the maternal blood pressure

104. PLACENTAL MEMBRANE
 Placental membrane ( placental barrier): is a complex structure that
consists of the extrafetal tissues separating the maternal and fetal
blood
 Until about 20 weeks, the placental membrane consists of four layers
 Syncytiotrophoblast
 Cytotrophoplast
 Connective tissue of villus
 Endothelium of fetal capillaries

106. FUNCTIONS OF THE PLACENTA
 Metabolism( e.g. synthesis of glycogen)
 Transport of gases and nutrients
 Endocrine secretion( e.g. human chorionic gonadotropin)

107. ABNORMALITIES OF THE PLACENTA
 Bilobed or Trilobed placenta: formed of two or three lobes but
having one umbilical cord only. This is not an important abnormality
 Succenturiate Placenta: this is small accessory placenta
 Twin Placenta: a placenta with two separate umbilical cords, this is
due to twins or multiple pregnancy
 Placenta previa: it is a placenta which develops in the lower part
of the uterus NEAR THE INTERNAL OS: normally implantation
takes place on the posterior wall of the uterus and the normal
placenta develops in the upper part of the uterus, in this case the
implantation occurs at the lower part of the uterus, the placenta will
then develop lower down near the internal os

108. PLACENTA PREVIA

109. BILOBED PLACENTA

110. UTERINE GROWTH DURING PREGNANCY
 The uterus of nonpregnant women lies in the pelvis minor( lesser
pelvis)
 To accommodate the growing conceptus, the uterus increases in size
and weight
 During the first trimester, the uterus moves out of the pelvis
 By 20 weeks it reaches the level of umbilicus
 By 28 to 30 weeks, the uterus reaches the epigastric region– the
area between the xiphoid process of the sternum and the umbilicus
 The increase in size of the uterus largely results from hypertrophy of
preexisting smooth muscular fibers, and partly from the development
of new fibers

111. PARTURITION ( CHILDBIRTH)
 Parturition: is the process during which the fetus, placenta and
fetal membranes are expelled from the mother's reproductive tract
 Labor: is the sequence of involuntary uterine contractions that
result in dilation of the uterine cervix and the expulsion of the fetus
and placenta from the uterus. There are four stages of labor
 Dilation: is the first stage of labor it begins when there is
objective evidence of progressive dilation of the cervix
 Expulsion: is the second stage of labor it begins when the
cervix is fully dilated and ends with delivery of the baby
 The placental stage: is the third stage of labor it begins as
soon as the baby is born and ends when the placenta and
membranes are expelled
 Recovery: is the fourth stage of labor it begins as soon as the
placenta and fetal membranes are expelled, this stages lasts
about 2 hrs and prevents excessive uterine bleeding

112. FETAL MEMBRANES
 Fetal membranes include all the extra-embryonic structure, which are
derived from the primitive blastomeres
 The fetal membranes are include the following structures
 Chorion
 Amnion
 Umbilical cord
 York sac

113. THE CHORION
 The chorion is the name given to the trophoblast after the
extraembryonic mesoderm is formed from its inner surface
 Before implantation , the outer wall of the blastocyst is simply called
the TROPOBLAST
 As the blastocyst begins to be implanted, the trophoblast becomes
differentiated in to two layers
 Outer SYNCYTIO-TROPHOBLAST
 Inner CYTO-TROPHOBLAS
 As development proceeds, the cells continue to develop from the
inner surface of the cyto-trophoblast to form a very loose tissue
called the EXTRA-EMBRYONIC MESODERM
 After the extra-embryonic mesoderm is completely formed the
trophoblast becomes called the CHORION and the blastocyst
becomes called the CHORIONIC VESICLE

114. THE AMNION
 The amnion is a membrane which is continues with the ectoderm of
the embryo and bounds the amniotic cavity
 The amniotic cavity appears during implantation of the blastocyst as
small cleft between the ectoderm and the trophoplast
 Very early pregnancy the ectodermal cells are attached to the
trophoblast
 At about the 8th day small intercellular cleft appears between the
ectoderm and trophoblast
 As the amniotic cavity enlarges , a layer of large flattened cells called
the AMNIOBLAST develops from the inner surface of the trophoblast
and form a small space called the ROOF of the amniotic cavity, while
the FLOOR of the cavity is formed by the ectodermal germ layer of
the embtyonic disc
 As the amniotic cavity increases in size, the layer of amnioblast loses
its contact with the inner surface of the trophoblast and it becomes
known as the AMNION

115. THE AMNION

116. FUNCTIONS OF THE AMNIOTIC FLUID
1. Early In Pregnancy
 It serves as protective watery cushion, which absorbs push that
may hurt the embryo
 It prevents the adhesion of the embryo to the amnion
 It is bad conductor of the heat and therefore , keeps the
temperature of the embryo nearly constant
 It allows the embryo to move freely
 It provides a space where urine and mechonium can accumulate
2- Late In Pregnancy:
 It protects the fetus from strong muscular contractions of the
uterus in the early stage of labor
 The fetus begins to swallow its own amniotic fluid , this helps the
embryo to suckle

117. 3- At the End of Pregnancy
 It bulge in front of the embryo and form a bag of watery , which helps
to dilate the canal of cervix
 At birth when the amniotic sac ruptures , the amniotic fluid washes
the vagina just before the delivery of the fetus.

118. ABNORMALITIES OF THE AMNIOTIC FLUID
The amniotic fluid is about 1.5 litters, it may be pathologically increased
or decreased
1. If the volume becomes more than 2 litres, the condition is called
POLYHYDRAMNION
2. If the volume is less than 0.5 litres, the condition is called
OLIGOHYDRAMNIOS, which may lead to adhesion between the
embryo and the amnion

119. THE UMBILICAL CORD
 The umbilical cord is the pathway which connects the placenta with
the ventral aspect of the embryo
 It is tortuous cord which measures from 50-60cm long and about
1cm in diameter
 It has a smooth surface because it is covered by a layer of amnion
 The umbilical cord contains three umbilical vessels( one vein and two
arteries) embedded in a gelatinous material called WHARTON,S
JELLY
 The umbilical cord passes through three stages of development
 Primitive umbilical ring
 Primitive umbilical cord
 Fully developed umbilical cord
 As the embryo grows, the embryonic disc bulges in to the amniotic
cavity
 As result the junction between the amniotic cavity and the embryonic
disc is carried in to the ventral aspect of the embryo and is called
PRIMITIVE UMBILICAL RING

120. CONTINUE
 By the 5th week, the primitive umbilical ring constricts changing the primitive
umbilical ring in to tubular sheath of amnion called the PRIMITIVE
UMBILICAL CORD
ABNORMALITIES OF THE UMBILICAL CORD
 Very long umbilical cord: it is very dangerous it may wind around the neck of
the embryo
 Very short umbilical cord: it may cause premature separation of the placenta
 EXOMPHALOS: this is condition in which the physiological umbilical hernia
is not completely reduced, the umbilical cord is distended and looks like a
sac, which contains part of the midgut
 Double or triple umbilical cord
 An umbilical cord with only one umbilical artery

121. EXOMPHALOS

122. THE YOLK SAC
 The york sac starts as a cavity which develops on the ventral aspect
of the embryonic disc; its roof is formed by the endodermal germ
layer of the embryonic disc
 While its wall is formed of a layer of flat cells to which a layer of loose
extraembryonic mesoderm is later added
 There is primary, a secondary and definitive yolk sac
 The primary yolk sac also called the exocoelomic cavity is formed
when the endoderm from the embryonic disc grows down on the
inner surface of the trophoblast and forms a thin membrane called
HEUSER,S MEMBRANE
 Together the endoderm and the heuser,s membrane form the wall of
the primary york sac
 The primary yolk sac then separates it self from the trophoblast by
the development of the extra-embryonic coelom (coelomic cavity)

123. 2- Secondary Yolk Sac: later in the development the terminal end of the
primary yolk sac becomes cut of and the remaining part becomes the
secondary yolk sac
 The outer surface of the secondary yolk sac is covered by a layer
of mesoderm called SPLANCHNIC LAYER of the extra-embryonic
mesoderm
 A network of vessels appears in this mesoderm and becomes
connected with the vessels of the embryo by means of the
VITELLINE VESSELS
 The secondary yolk sac forms the following structures
 The gut inside the embryo
 The yolk sac stalk
 Definitive yolk sac
 The gut enters the embryo and divides to form the foregut, midgut
and hidgut.

124. FUNCTIONS OF THE YOLK SAC
 It has a role in the transfer of nutrients to the embryo during the 2nd
and 3rd weeks, when the uteroplacental circulation is established
 Blood development first occurs in the well-vascularized
extraembryonic mesoderm covering the wall of the yolk sac
beginning in the 3rd week
 During the 4th week the endoderm of the yolk sac is incorporated in
to the embryo as the primordial gut
 Primordial germ cells appear in the endodermal lining of the wall of
the yolk sac in the 3rd week and subsequently migrate to the
developing sex glands

125. The yolk stalk forms part of the primitive umbilical ; later it becomes
obliterated and the gut becomes completely separated from the
definitive yolk sac
The definitive yolk sac itself does not grow except very slowly and
never reaches a diameter more than 0.5 cm, it then shrinks and
changes to a small solid structure