Explore the fundamentals of the human reproductive system in this concise presentation, suitable for medical students and professionals alike. Covering anatomy, physiology, and Pregnancy, it offers essential knowledge for understanding reproductive health.

1. REPRODUCTIVE SYSTEM
Swetaba B. Besh
Assistant Professor
Department of Pharmacy, Pharmacology
Sumandeep Vidyapeeth Deemed to be University

2. • The ability to reproduce is one of the properties distinguishing living from non-
living matter. In mammals, including humans, the process is one of sexual
reproduction, in which the male and female organs differ anatomically and
physiologically, and the new individual develops from the fusion of two different sex
cells (gametes).
• The male gametes are called spermatozoa and the female gametes are called ova.
• The male and female reproductive organs can be grouped by function. The gonads
testes in males and ovaries in females produce gametes and secrete sex hormones.
• Various ducts then store and transport the gametes, and accessory sex glands produce
substances that protect the gametes and facilitate their movement.
• supporting structures, such as the penis in males and the uterus in females, assist the
delivery of gametes, and the uterus is also the site for the growth of the embryo and
fetus during pregnancy.
REPRODUCTIVE SYSTEM

3. Female reproductive system
• The organs of the female reproductive system
include the ovaries (female gonads); the uterine
(fallopian) tubes, or oviducts; the uterus; the
vagina; and external organs, which are
collectively called the vulva, or pudendum.
• The mammary glands are considered part of
both the integumentary system and the female
reproductive system.
• Gynecology is the specialized branch of
medicine concerned with the diagnosis and
treatment of diseases of the female reproductive
system
• The female reproductive organs, or genitalia,
include both external and internal organs.

5. External genitalia (vulva)
• The external genitalia are known
collectively as the vulva, and consist of the
mons pubis, labia majora and labia minora,
the clitoris, the vaginal orifice, the vestibule,
the hymen and the vestibular glands
(Bartholin’s glands).
• Anterior to the vaginal and urethral
openings is the mons pubis, an elevation of
adipose tissue covered by skin and coarse
pubic hair that cushions the pubic
symphysis.

6. Labia majora
• These are the two large folds forming the
boundary of the vulva.
• They are composed of skin, fibrous tissue and fat
and contain large numbers of sebaceous and
apocrine sweat glands.
• Anteriorly the folds join in front of the
symphysis pubis, and posteriorly they merge
with the skin of the perineum.
• The labia majora are covered by pubic hair and
contain an abundance of adipose tissue,
sebaceous (oil) glands, and apocrine
sudoriferous (sweat) glands.
• They are homologous to the scrotum.

7. Labia minora
• These are two smaller folds of skin between the labia majora, containing numerous sebaceous
and sweat glands.
• Unlike the labia majora, the labia minora are devoid of pubic hair and fat and have few
sudoriferous glands, but they do contain many sebaceous glands which produce antimicrobial
substances and provide some lubrication.
• The cleft between the labia minora is the vestibule. The vagina, urethra and ducts of the greater
vestibular glands open into the vestibule.
Clitoris
• The clitoris corresponds to the penis in the male and contains sensory nerve endings and erectile
tissue.
• The clitoris is a small cylindrical mass composed of two small erectile bodies, the corpora
cavernosa, and numerous nerves and blood vessels. The clitoris is located at the anterior junction of
the labia minora.
• The exposed portion of the clitoris is the glans clitoris. The clitoris is homologous to the glans
penis in males.

8. Vestibular glands
• The vestibular glands (Bartholin’s glands) are situated one on each side near
the vaginal opening. The region between the labia minora is the vestibule.
They secrete mucus that keeps the vulva moist.
• Within the vestibule are the hymen, the vaginal orifice, the external urethral
orifice, and the openings of the ducts of several glands.
• The vestibule is homologous to the intermediate urethra of males.
Perineum
• The perineum is a roughly triangular area extending from the base of the labia
minora to the anal canal. It consists of connective tissue, muscle and fat. It
gives attachment to the muscles of the pelvic floor

9. Blood supply, lymph drainage and nerve supply
• Arterial supply:This is by branches from the internal pudendal arteries that
branch from the internal iliac arteries and by external pudendal arteries that
branch from the femoral arteries.
• Venous drainage: This forms a large plexus which eventually drains into the
internal iliac veins.
• Lymph drainage: This is through the superficial inguinal nodes.
• Nerve supply: This is by branches from pudendal nerves.

10. Internal genitalia
• The internal organs of the
female reproductive system
lie in the pelvic cavity and
consist of the vagina, uterus,
two uterine tubes and two
ovaries.

11. vagina
• The vagina is a fibromuscular tube lined with stratified squamous epithelium opening
into the vestibule at its distal end, and with the uterine cervix protruding into its
proximal end.
• It is also termed as a Birth Canal. Leading from outside of the body to cervix of uterus.
• It runs obliquely upwards and backwards at an angle of about 45° between the bladder
in front and rectum and anus behind.
• In the adult, the anterior wall is about 7.5 cm long and the posterior wall about 9 cm
long. The difference is due to the angle of insertion of the cervix through the anterior
wall.
Hymen:
• The hymen is a thin layer of mucous membrane that partially occludes the opening of
the vagina. It is normally incomplete to allow for passage of menstrual flow and is
stretched or completely torn away by sexual intercourse, insertion of a tampon or
childbirth.

12. Structure of the vagina
• The vaginal wall has three layers: an outer covering of areolar tissue, a middle layer of smooth muscle and an
inner lining of stratified squamous epithelium that forms ridges or rugae.
• It has no secretory glands but the surface is kept moist by cervical secretions.
• Between puberty and the menopause, Lactobacillus acidophilus, bacteria that secrete lactic acid, are normally
present maintaining the pH between 4.9 and 3.5.
• The acidity inhibits the growth of most other micro-organisms that may enter the vagina from the perineum
or during sexual intercourse.
Blood supply, lymph drainage and nerve supply
• Arterial supply: An arterial plexus is formed round the vagina, derived from the uterine and vaginal arteries,
which are branches of the internal iliac arteries.
• Venous drainage: A venous plexus, situated in the muscular wall, drains into the internal iliac veins.
• Lymph drainage: This is through the deep and superficial iliac glands.
• Nerve supply: This consists of parasympathetic fibres from the sacral outflow, sympathetic fibres from the lumbar
outflow and somatic sensory fibres from the pudendal nerves.

13. Uterus (Womb)
• The uterus is a hollow muscular pear-shaped organ, flattened anteroposteriorly. It lies in the
pelvic cavity between the urinary bladder and the rectum.
• In most women, it leans forward (anteversion), and is bent forward (anteflexion) almost at
right angles to the vagina, so that its anterior wall rests partly against the bladder below,
forming the vesicouterine pouch between the two organs.When the body is upright, the uterus
lies in an almost horizontal position.
• It is about 7.5 cm long, 5 cm wide and its walls are about 2.5 cm thick. It weighs between 30
and 40 grams.
• The parts of the uterus are the fundus, body and cervix
Fundus: This is the dome-shaped part of the uterus above the openings of the uterine tubes.
Body: This is the main part. It is narrowest inferiorly at the internal os where it is continuous
with the cervix.
Cervix (‘neck’ of the uterus): This protrudes through the anterior wall of the vagina, opening
into it at the external os.

15. Structure
• The walls of the uterus are composed of three layers of tissue: perimetrium, myometrium and
endometrium.
• Perimetrium: This is peritoneum, which is distributed differently on the various surfaces of the
uterus.Anteriorly it lies over the fundus and the body where it is folded on to the upper surface
of the urinary bladder. This fold of peritoneum forms the vesicouterine pouch.
• Posteriorly the peritoneum covers the fundus, the body and the cervix, then it folds back on to
the rectum to form the rectouterine pouch.
• Laterally, only the fundus is covered because the peritoneum forms a double fold with the
uterine tubes in the upper free border. This double fold is the broad ligament, which, at its
lateral ends, attaches the uterus to the sides of the pelvis.
• Myometrium: This is the thickest layer of tissue in the uterine wall. It is a mass of smooth
muscle fibres interlaced with areolar tissue, blood vessels and nerves.
• Endometrium: This consists of columnar epithelium covering a layer of connective tissue
containing a large number of mucus-secreting tubular glands. It is richly supplied with blood by
spiral arteries, branches of the uterine artery.

17. • It is divided functionally into two layers:
• The functional layer is the upper layer and it thickens and becomes rich in
blood vessels in the first half of the menstrual cycle. If the ovum is not
fertilised and does not implant, this layer is shed during menstruation.
• The basal layer lies next to the myometrium, and is not lost during
menstruation. It is the layer from which the fresh functional layer is
regenerated during each cycle.
• The upper two-thirds of the cervical canal is lined with this mucous
membrane. Lower down, however, the mucosa changes, becoming
stratified squamous epithelium, which is continuous with the lining of the
vagina itself.

18. Blood supply, lymph drainage and nerve supply
• Arterial supply: This is by the uterine arteries, branches of the internal iliac
arteries. They pass up the lateral aspects of the uterus between the two layers
of the broad ligaments. They supply the uterus and uterine tubes and join with
the ovarian arteries to supply the ovaries.
• Venous drainage: The veins follow the same route as the arteries and
eventually drain into the internal iliac veins.
• Lymph drainage: Deep and superficial lymph vessels drain lymph from the
uterus and the uterine tubes to the aortic lymph nodes and groups of nodes
associated with the iliac blood vessels.
• Nerve supply: The nerves supplying the uterus and the uterine tubes consist
of parasympathetic fibres from the sacral outflow and sympathetic fibres from
the lumbar outflow.

19. Uterine/ Fallopian tubes
• The uterine (Fallopian) tubes are about 10 cm
long and extend from the sides of the uterus
between the body and the fundus.
• They lie in the upper free border of the broad
ligament and their trumpet-shaped lateral ends
penetrate the posterior wall, opening into the
peritoneal cavity close to the ovaries.
• The end of each tube has fingerlike projections
called fimbriae.
• The longest of these is the ovarian fimbria, which
is in close association with the ovary.

20. Structure
• The uterine tubes are covered with peritoneum (broad ligament), have a middle
layer of smooth muscle and are lined with ciliated epithelium.
• Blood and nerve supply and lymphatic drainage are as for the uterus.
Functions
• The uterine tubes propel the ovum from the ovary to the uterus by peristalsis and
ciliary movement.
• The secretions of the uterine tube nourish both ovum and spermatozoa.
• Fertilisation of the ovum usually takes place in the uterine tube, and the zygote is
propelled into the uterus for implantation.

21. Ovaries
• The ovaries are the female gonads (glands
producing sex hormones and the ova), and
they lie in a shallow fossa on the lateral
walls of the pelvis.
• They are 2.5–3.5 cm long, 2 cm wide and 1
cm thick.
• Each is attached to the upper part of the
uterus by the ovarian ligament and to the
back of the broad ligament by a broad band
of tissue, the mesovarium.
• Blood vessels and nerves pass to the ovary
through the mesovarium

22. Structure
• The ovaries have two layers of tissue.
Medulla: This lies in the centre and consists of fibrous tissue, blood vessels and nerves.
Cortex:
• This surrounds the medulla. It has a framework of connective tissue, or stroma, covered by
germinal epithelium.
• It contains ovarian follicles in various stages of maturity, each of which contains an ovum.
• Before puberty the ovaries are inactive but the stroma already contains immature (primordial)
follicles, which the female has from birth.
• During the childbearing years, about every 28 days, one or more ovarian follicle (Graafian
follicle) matures, ruptures and releases its ovum into the fallopian tube, also known as the
uterine tube or oviduct. This is called ovulation and it occurs during most menstrual cycles.
• Following ovulation, the ruptured follicle develops into the corpus luteum (meaning ‘yellow
body’), which in turn will leave a small permanent scar of fibrous tissue called the corpus
albicans (meaning ‘white body’) on the surface of the ovary.

23. Blood supply, lymph drainage and nerve supply
• Arterial supply: This is by the ovarian arteries, which branch from
the abdominal aorta just below the renal arteries.
• Venous drainage: This is into a plexus of veins behind the uterus
from which the ovarian veins arise. The right ovarian vein opens into
the inferior vena cava and the left into the left renal vein.
• Lymph drainage: This is to the lateral aortic and preaor_x0002_tic
lymph nodes. The lymph vessels follow the same route as the arteries.
• Nerve supply: The ovaries are supplied by parasympathetic nerves
from the sacral outflow and sympathetic nerves from the lumbar
outflow

24. Functions
• The ovary is the organ in which the female gametes are stored and
develop prior to ovulation.
• Their maturation is controlled by the hypothalamus and the
anterior pituitary gland, which releases gonadotrophins (follicle
stimulating hormone, FSH, and luteinising hormone, LH), both of
which act on the ovary.
• In addition, the ovary has endocrine functions, and releases
hormones essential to the physiological changes during the
reproductive cycle.

25. FUNCTIONS OF FEMALE PRODUCTIVE SYSTEM
Formation of ova
Reception of spermatozoa
Provision of suitable environments for fertilisation and fetal
development
Parturition (childbirth)
Lactation, the production of breast milk, which provides
complete nourishment for the baby in its early life.

26. Male Reproductive System
• The organs of the male reproductive system include the testes, a system of
ducts (epididymis, ductus deferens, ejaculatory ducts, and urethra), accessory
sex glands (seminal vesicles, prostate, and bulbourethral glands), and several
supporting structures, including the scrotum and the penis.
• The testes (male gonads) produce sperm and secrete hormones.
• The duct system transports and stores sperm, assists in their maturation, and
conveys them to the exterior.
• Semen contains sperm plus the secretions provided by the accessory sex
glands.
• The supporting structures have various functions. The penis delivers sperm
into the female reproductive tract and the scrotum supports the testes.

28. Testes
• The testes are the male reproductive glands and are the
equivalent of the ovaries in the female.
• They are about 4.5 cm long, 2.5 cm wide and 3 cm thick and
are suspended in the scrotum by the spermatic cords. They
are surrounded by three layers of tissue.
Scrotum
• The scrotum is a pouch of pigmented skin, fibrous and
connective tissue and smooth muscle.
• It is divided into two compartments, each of which contains
one testis, one epididymis and the testicular end of a
spermatic cord.
• It lies below the symphysis pubis, in front of the upper parts
of the thighs and behind the penis.

29. • The normal temperature of testes in the scrotum is about 3 degrees lower than the internal
temperature
• When the body is chilled the smooth muscles contracts and bring the testes closer to the pelvic cavity.
• The scrotum remains connected with the abdomen or pelvic cavity by the inguinal canal.
• The spermatic cord formed from the spermatic artery, vein and nerve bound together with connective
tissue passes into the testes through inguinal canal

31. Structure of testis
• Seminiferous tubules-Each testes has 200-300 lobules, and within each
lobules there are 1 to 4 highly coiled loops composed of germinal epithelial
cells(male germ cells or spermatogonia), called seminiferous tubules.
• Sertoli cells- support germ cells and provide nutrition. Secrete ABP that
concentrate testosterone inthe seminiferous tubule.
• Leydig cells or interstitial cells(endocrine portion of the testis) present in
between the seminiferous tubules in connective tissue which secrete
androgens (testosterone)
• Male accessory ducts-
• Rete testes- all the seminiferous tubules opens into a network called rete
testes.
• Vas efferens- from rete testes, 8 to 15 tubules called vas efferens arise.Vas
efferens join together andform the head of epididymis and then converge to
form the duct of epididymis.

32. • Epididymis- duct of epididymis is an enormously convoluted tubule with a length of
about 4 meter. It begin at head, where it receives vas efferens. It stores and mature the
sperms and also secretes a fluid to nourish the sperms
• Vas deferens- epididymis leaves the scrotum as the deferent duct (vas deferens) and
enters the abdominal cavity through the spermatic cord (inguinal canal). The vas
deferens loops over the urinary bladder where it is joined by duct from the seminal
vesicle to form the ejaculatory duct. Vasa deferentia carry sperms.
• Rete testis, vasa efferentia, epididymis and vasa deferentia are called male sex
accessory ducts. These ducts tore and transport the sperms from the testis to the outside
urethra.
• Ejaculatory duct- ejaculatory ducts are two short tubes (2cm long) each formed by the
union of the duct from a seminal vesicle and a vas deferens. They pass through the
prostate gland and join the prostatic part of urethra.

34. Tunica vaginalis:
• This is a double membrane, forming the outer covering of the testes, and is a
downgrowth of the abdominal and pelvic peritoneum. During early fetal life, the
testes develop in the lumbar region of the abdominal cavity just below the
kidneys.
• They then descend into the scrotum, taking with them coverings of peritoneum,
blood and lymph vessels, nerves and the deferent duct.
• The peritoneum eventually surrounds the testes in the scrotum, and becomes
detached from the abdominal peritoneum. Descent of the testes into the scrotum
should be complete by the 8th month of fetal life.
Tunica albuginea:
• This is a fibrous covering beneath the tunica vaginalis. Ingrowths form septa,
dividing the glandular structure of the testes into lobules.
Tunica vasculosa:
• This consists of a network of capillaries supported by delicate connective tissue.

35. Functions
• Spermatozoa (sperm) are produced in the seminiferous tubules of the testes, and mature as
they pass through the long and convoluted epididymis, where they are stored.
• FSH from the anterior pituitary stimulates sperm production. A mature sperm has a head, a
body, and a long whip-like tail used for motility.
• The head is almost completely filled by the nucleus, containing its DNA. It also contains the
enzymes required to penetrate the outer layers of the ovum to reach, and fuse with, its
nucleus.
• The body of the sperm is packed with mitochondria, to fuel the propelling action of the tail
that powers the sperm along the female reproductive tract.

36. Spermatic cords
• The spermatic cords suspend the testes in the scrotum.
• Each cord contains a testicular artery, testicular veins, lymphatics, a deferent
duct and testicular nerves, which come together to form the cord from their
various origins in the abdomen.
• The cord, which is covered in a sheath of smooth muscle and connective and
fibrous tissues, extends through the inguinal canal and is attached to the testis
on the posterior wall.

37. Blood supply, lymph drainage and nerve supply
• Arterial supply: The testicular artery branches from the abdominal aorta, just
below the renal arteries.
• Venous drainage: The testicular vein passes into the abdominal cavity. The
left vein opens into the left renal vein and the right into the inferior vena cava.
• Lymph drainage: This is through lymph nodes around the aorta.
• Nerve supply: This is provided by branches from the 10th and 11th thoracic
nerves.

38. Urethra
• The male urethra provides a common pathway for the flow of
urine and semen.
• It is about 19–20 cm long and consists of three parts. The
prostatic urethra originates at the urethral orifice of the
bladder and passes through the prostate gland.
• There are two urethral sphincters-
• a) Internal sphincter (under involuntary control)- it consists
of smooth muscle fibres situated at the neck of the bladder
above the prostate gland.
• b) External sphincter (under voluntary control)- consists of
skeletal muscle fibres surrounding the membranous part.

39. It consists of three parts:
1) Prostatic urethra- it originates at the urethral orifice of
the bladder the bladder and passes through the prostate
gland
2) Membranous urethra- shortest and narrowest extends
from the prostate gland part and to the bulb of the penis,
after passing through the perineal membrane.
3) Penile or spongiosa urethra- lies withing the corpus
spongiosum of the penis and terminates at the external
urethral orifice (urethral meatus) in the glans penis.

40. Penis:
• The penis has a root and a shaft. The root anchors the penis in
the perineum and the shaft (body) is the externally visible,
moveable portion of the organ.
• It is formed by three cylindrical masses of erectile tissue and
smooth muscle.
• The erectile tissue is supported by fibrous tissue and covered
with skin and has a rich blood supply.
• The two lateral columns are called the corpora cavernosaand
the column between them, containing the urethra, is the corpus
spongiosum.

41. • At its tip it is expanded into a triangular structure known as the
glans penis. Just above the glans the skin is folded upon itself
and forms a movable double layer, the foreskin or prepuce.
• Arterial blood is supplied by deep, dorsal and bulbar arteries of
the penis, which are branches from the internal pudendal
arteries.
• A series of veins drain blood to the internal pudendal and
internal iliac veins.
• The penis is supplied by autonomic and somatic nerves.
• Parasympathetic stimulation leads to filling of the spongy
erectile tissue with blood, caused by arteriolar dilation and
venoconstriction, which increases blood flow into the penis
and obstructs outflow.

42. Physiology of Menstruation
• This is a series of events, occurring regularly in females every 26 to 30
days throughout the childbearing period between menarche and
menopause.
• The cycle consists of a series of changes taking place concurrently in
the ovaries and uterine lining, stimulated by changes in blood
concentrations of hormones.
• Hormones secreted during the cycle are regulated by negative feedback
mechanisms.

44. • The hypothalamus secretes luteinising hormone releasing hormone
(LHRH), which stimulates the anterior pituitary to secrete:
1. Follicle stimulating hormone (FSH), which promotes the maturation of
ovarian follicles and the secretion of oestrogen, leading to ovulation. FSH is
therefore predominantly active in the first half of the cycle. Its secretion is
suppressed once ovulation has taken place, to prevent other follicles
maturing during the current cycle
2. Luteinising hormone (LH), which triggers ovulation, stimulates the
development of the corpus luteum and the secretion of progesterone.
• The hypothalamus responds to changes in the blood levels of oestrogen and
progesterone.
• It is stimulated by high levels of oestrogen alone (as happens in the first half
of the cycle) but suppressed by oestrogen and progesterone together (as
happens in the second half of the cycle).

47. • The average length of the cycle is about 28 days.
• By convention the days of the cycle are numbered from the beginning of
the menstrual phase, which usually lasts about 4 days.
• This is followed by the proliferative phase(approximately 10 days), then
by the secretory phase (about 14 days).
• Matrix Metalloproteinases (MMPs): MMPs are a group of enzymes
involved in tissue remodeling. In the context of ovulation, MMPs play a
crucial role in weakening and breaking down the follicular wall (ovarian
wall) to allow the mature egg to be released. Specifically, MMP-2 and
MMP-9 are known to be involved in this process.

49. 1. Menstrual Phase
• When the ovum is not fertilised, the corpus luteum starts to degenerate.
• Progesterone and oestrogen levels therefore fall, and the functional layer of the
endometrium, which is dependent on high levels of these ovarian hormones, is
shed in menstruation.
• The menstrual flow consists of the secretions from endometrial glands, endometrial
cells, blood from the degenerating capillaries and the unfertilised ovum.
• During the menstrual phase, levels of oestrogen and progesterone are very low
because the corpus luteum that had been active during the second half of the previous
cycle has degenerated.
• This means the hypothalamus and anterior pituitary can resume their cyclical activity,
and levels of FSH begin to rise, initiating a new cycle.

50. 2. Proliferative phase
• At this stage an ovarian follicle, stimulated by FSH, is growing towards
maturity and is producing oestrogen, which stimulates proliferation of
the functional layer of the endometrium in preparation for the
reception of a fertilised ovum.
• The endometrium thickens, becoming very vascular and rich in mucus-
secreting glands.
• Rising levels of oestrogen are responsible for triggering a surge of LH
approximately mid-cycle.
• This LH surge triggers ovulation, marking the end of the proliferative
phase.

51. 3. Secretory phase
• After ovulation, LH from the anterior pituitary stimulates development
of the corpus luteum from the ruptured follicle, which produces
progesterone, some oestrogen, and inhibin.
• Under the influence of progesterone, the endometrium becomes
oedematous and the secretory glands produce increased amounts of
watery mucus.
• This assists the passage of the spermatozoa through the uterus to the
uterine tubes where the ovum is usually fertilised.
• There is a similar increase in secretion of watery mucus by the glands
of the uterine tubes and by cervical glands that lubricate the vagina.

52. • The ovum may survive in a fertilisable form for a very short time after
ovulation, probably as little as 8 hours.
• The spermatozoa, deposited in the vagina during intercourse, may be
capable of fertilising the ovum for only about 24 hours although they can
survive for several days.
• This means that the period in each cycle during which fertilisation can
occur is relatively short. Observable changes in the woman’s body occur
around the time of ovulation.
• Cervical mucus, normally thick and dry, becomes thin, elastic and watery,
and body temperature rises by about 1°C immediately following ovulation.
• Some women experience abdominal discomfort in the middle of the cycle,
thought to correspond to rupture of the follicle and release of its contents
into the abdominal cavity.

53. • After ovulation, the combination of progesterone, oestrogen and inhibin
from the corpus luteum suppresses the hypothalamus and anterior
pituitary, so FSH and LH levels fall.
• Low FSH levels in the second half of the cycle prevent further follicular
development in case a pregnancy results from the current cycle.
• If the ovum is not fertilised, falling LH levels leads to degeneration and
death of the corpus luteum, which is dependent on LH for survival.
• The resultant steady decline in circulating oestrogen, progesterone and
inhibin leads to degeneration of the uterine lining and menstruation,
with the initiation of a new cycle.

54. • If the ovum is fertilised there is no breakdown of the endometrium and
no menstruation.
• The fertilised ovum (zygote) travels through the uterine tube to the
uterus where it becomes embedded in the wall and produces human
chorionic gonadotrophin (hCG), which is similar to anterior pituitary
luteinising hormone.
• This hormone keeps the corpus luteum intact, enabling it to continue
secreting progesterone and oestrogen for the first 3–4 months of the
pregnancy, inhibiting the maturation of further ovarian follicles.
• During that time the placenta develops and produces oestrogen,
progesterone and gonadotrophins.

56. 1.Mitosis: Mitosis is a process of cell division in which a single cell divides into two identical
daughter cells. It occurs in somatic cells and is responsible for growth, repair, and asexual
reproduction in organisms. Mitosis involves the division of the nucleus and the distribution of
genetic material (chromosomes) into the daughter cells, resulting in each daughter cell having
the same number and type of chromosomes as the parent cell.
2.Meiosis I: Meiosis I is the first stage of meiosis, a specialized type of cell division that occurs
in germ cells (cells that give rise to gametes - sperm and eggs). Meiosis I involves the
reduction of chromosome number from diploid to haploid and the reshuffling of genetic
material through processes like crossing over. It results in the formation of two haploid
daughter cells, each containing a unique combination of genetic material.
3.Meiosis II: Meiosis II is the second stage of meiosis, following meiosis I. It is similar to
mitosis in that it involves the division of sister chromatids. However, the starting cells in
meiosis II are haploid (having one set of chromosomes) rather than diploid. Meiosis II results
in the formation of four haploid daughter cells, each with a single set of chromosomes, and
contributes to genetic diversity among gametes.

57. Spermatogenesis
Anatomy of Sperm Cell:
• Each day about 300 million sperm complete the process
of spermatogenesis.
• A sperm is about 60 μm long and contains several
structures that are highly adapted for reaching and
penetrating a secondary oocyte.
• The major parts of a sperm are the head and the tail.
• The flattened, pointed head of the sperm is about 4–5 μm
long. It contains a nucleus with 23 highly condensed
chromosomes.
• Covering the anterior two-thirds of the nucleus is the
acrosome, a caplike vesicle filled with enzymes that help
a sperm to penetrate a secondary oocyte to bring about
fertilization.

58. • The tail of a sperm is subdivided into four parts: neck,
middle piece, principal piece, and end piece.
• The neck is the constricted region just behind the
head that contains centrioles. The centrioles form the
microtubules that comprise the remainder of the tail.
• The middle piece contains mitochondria arranged in
a spiral, which provide the energy (ATP) for
locomotion of sperm to the site of fertilization and for
sperm metabolism.
• The principal piece is the longest portion of the tail,
and the end piece is the terminal, tapering portion of
the tail.
• Once ejaculated, most sperm do not survive more
than 48 hours within the female reproductive tract.

59. Cross section of seminiferous tubule:

60. Process of Spermatogenesis
• In humans, spermatogenesis takes 65–75 days. It begins with the
spermatogonia, which contain the diploid (2n) number of chromosomes.
• Spermatogonia are types of stem cells; when they undergo mitosis, some
spermatogonia remain near the basement membrane of the seminiferous tubule
in an undifferentiated state to serve as a reservoir of cells for future cell
division and subsequent sperm production (Type A cells).
• The rest of the spermatogonia lose contact with the basement membrane,
squeeze through the tight junctions of the blood–testis barrier, undergo
developmental changes, and differentiate into primary spermatocytes (Type
B cells).
• Primary spermatocytes, like spermatogonia, are diploid (2n); that is, they have
46 chromosomes.

61. • Shortly after it forms, each primary spermatocyte replicates its DNA and then
meiosis begins.
• In meiosis I, homologous pairs of chromosomes line up at the metaphase plate, and
crossingover occurs. Then, the meiotic spindle pulls one (duplicated) chromosome of
each pair to an opposite pole of the dividing cell.
• The two cells formed by meiosis I are called secondary spermatocytes. Each
secondary spermatocyte has 23 chromosomes, the haploid number (n).
• Each chromosome within a secondary spermatocyte, however, is made up of two
chromatids (two copies of the DNA) still attached by a centromere. No replication of
DNA occurs in the secondary spermatocytes.
• In meiosis II, the chromosomes line up in single file along the metaphase plate, and
the two chromatids of each chromosome separate.
• The four haploid cells resulting from meiosis II are called spermatids.
• A single primary spermatocyte therefore produces four spermatids via two rounds of
cell division (meiosis I and meiosis II).

62. • The final stage of spermatogenesis, spermiogenesis, is the development of
haploid spermatids into sperm.
• No cell division occurs in spermiogenesis; each spermatid becomes a single
sperm cell.
• During this process, spherical spermatids transform into elongated, slender
sperm.
• An acrosome forms atop the nucleus, which condenses and elongates, a flagellum
develops, and mitochondria multiply.
• Sertoli cells dispose of the excess cytoplasm that sloughs off .
• Finally, sperm are released from their connections to sertoli cells, an event known
as spermiation.
• Sperm then enter the lumen of the seminiferous tubule. Fluid secreted by
sustentacular cells pushes sperm along their way, toward the ducts of the testes.

64. Oogenesis
• The formation of gametes in the ovaries is termed oogenesis.
• In contrast to spermatogenesis, which begins in males at puberty, oogenesis begins in
females before they are even born.
• Oogenesis occurs in essentially the same manner as spermatogenesis; meiosis takes
place and the resulting germ cells undergo maturation.
• During early fetal development, primordial (primitive) germ cells migrate from the yolk
sac to the ovaries. There, germ cells differentiate within the ovaries into oogonia.
• Oogonia are diploid (2n) stem cells that divide mitotically to produce millions of germ
cells. Even before birth, most of these germ cells degenerate in a process known as
atresia.
• A few, however, develop into larger cells called primary oocytes that enter prophase of
meiosis I during fetal development but do not complete that phase until aft er puberty.

65. • During this arrested stage of development, each primary oocyte is surrounded by a single
layer of flat follicular cells, and the entire structure is called a primordial follicle.
• The ovarian cortex surrounding the primordial follicles consists of collagen fibers and
fibroblast-like stromal cells.
• At birth, approximately 200,000 to 2,000,000 primary oocytes remain in each ovary.
• Of these, about 40,000 are still present at puberty, and around 400 will mature and ovulate
during a woman’s reproductive lifetime.
• The remainder of the primary oocytes undergo atresia.
• Each month after puberty until menopause, gonadotropins (FSH and LH) secreted by the
anterior pituitary further stimulate the development of several primordial follicles, although
only one will typically reach the maturity needed for ovulation.
• A few primordial follicles start to grow, developing into primary follicles.
• Each primary follicle consists of a primary oocyte that is surrounded in a later stage of
development by several layers of cuboidal and lowcolumnar cells called granulosa cells.
• The outermost granulosa cells rest on a basement membrane.

66. • As the primary follicle grows, it forms a clear glycoprotein layer called the zona
pellucida between the primary oocyte and the granulosa cells.
• In addition, stromal cells surrounding the basement membrane begin to form an
organized layer called the theca folliculi With continuing maturation, a primary
follicle develops into a secondary follicle.
• In a secondary follicle, the theca differentiates into two layers: (1) the theca interna, a
highly vascularized internal layer of cuboidal secretory cells that secrete estrogens,
and (2) the theca externa, an outer layer of stromal cells and collagen fibers.
• In addition, the granulosa cells begin to secrete follicular fluid, which builds up in a
cavity called the antrum in the center of the secondary follicle. The innermost layer of
granulosa cells becomes firmly attached to the zona pellucida and is now called the
corona radiata.
• The secondary follicle eventually becomes larger, turning into a mature (graafian)
follicle. While in this follicle, and just before ovulation, the diploid primary oocyte
completes meiosis I, producing two haploid (n) cells of unequal size each with 23
chromosomes.

68. • The smaller cell produced by meiosis I, called the first polar body, is essentially a
packet of discarded nuclear material.
• The larger cell, known as the secondary oocyte, receives most of the cytoplasm.
• Once a secondary oocyte is formed, it begins meiosis II but then stops in metaphase.
• The mature (graafian) follicle soon ruptures and releases its secondary oocyte, a process
known as ovulation.
• At ovulation, the secondary oocyte is expelled into the pelvic cavity together with the
first polar body and corona radiata.
• Normally these cells are swept into the uterine tube. If fertilization does not occur, the
cells degenerate. If sperm are present in the uterine tube and one penetrates the
secondary oocyte, however, meiosis II resumes.
• The secondary oocyte splits into two haploid cells, again of unequal size. The larger cell
is the ovum, or mature egg; the smaller one is the second polar body.
• The nuclei of the sperm cell and the ovum then unite, forming a diploid zygote.

70. Physiology of Fertilization
• Fertilization is commonly known as conception. Once the fertilized gamete
(ovum) implants itself in the uterine lining, pregnancy begins.
• The fusion of male and female gametes ( sperm and ovum, respectively)
usually occurs following the act of sexual intercourse.
• Fertilization is the natural life process, which is carried out by the fusion of
both male and female gametes, which results in the formation of a zygote. In
humans, the process of fertilization takes place in the fallopian tube.
• However, artificial insemination and in vitro fertilization have made
achieving pregnancy possible without engaging in sexual intercourse.
• However, artificial insemination and in vitro fertilization have made
achieving pregnancy possible without engaging in sexual intercourse.
• The process of fertilization occurs in several steps and the interruption of any
of them can lead to failure.

71. • After intercourse, semen is ejaculated into the female reproductive tract. Following
ejaculation, the composition of semen begins its work.
• Fructose and citrate within the semen provide nutrition for sperm cells.
• Fibrinogen, another component of semen, aids in clotting at the female reproductive wall to
prevent leakage.
• After some time, fibrinolytic enzyme is released from semen, which helps in thinning the
semen, facilitating its upward travel.
• Additionally, prostaglandins in semen initiate contractions of the uterus.
• Simultaneously, oxytocin is released from the female reproductive tract, further contracting
the uterine wall.
• After ovulation takes place, there is a rupture of the Graafian follicle and the release of the
secondary oocyte from the ovary.
• Following ovulation, the fimbriae, finger-like projections at the end of the fallopian tube,
catch the released ovum.
• The ovum then passes from the fimbriae to the infundibulum, which is the opening of the
fallopian tube.
• With the help of cilia, tiny hair-like structures lining the fallopian tube, the ovum is
propelled towards the ampulla region of the fallopian tube.

73. • The structure of a sperm cell begins with its outer
plasma membrane.
• This membrane encloses a cap-like structure termed as
the acrosomal membrane.
• The acrosomal membrane has two layers: the outer
acrosomal membrane, which directly contacts the
plasma membrane, and the inner acrosomal
membrane, which directly contacts the nucleus.
• The acrosomal membrane contains proteolytic or
acrosomal enzymes, which play a crucial role in
penetrating the egg cell during fertilization.
• The cytoplasm of a sperm cell is very small compared
to other cells.
• The nucleus of the sperm cell is haploid in nature,
meaning it contains half the number of chromosomes
found in somatic cells, with 23 chromosomes carrying
the genetic material of the male.

74. • Outside of the egg cell, many follicular cells are
attached to each other with hyaluronic acid.
• The combination of these follicular cells and
hyaluronic acid is known as the cumulus matrix.
• The egg has a membrane known as the zona
pellucida, which contains a protein binding site for
sperm cells.
• After binding to the zona pellucida, sperm cells
penetrate it to fertilize the egg.
• Following the zona pellucida, there is the plasma
membrane of the egg cell.
• Inside the egg, there is a large cytoplasm
surrounding the nucleus.
• The nucleus of the egg cell is also haploid,
containing 23 chromosomes, and is the last
structure within the egg cell.

75. • During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a
process called capacitation, or priming. The fluids improve the motility of the spermatozoa.
• They also deplete cholesterol molecules embedded in the membrane of the head of the sperm, thinning
the membrane in such a way that will help facilitate the release of the lysosomal (digestive) enzymes
needed for the sperm to penetrate the oocyte’s exterior once contact is made.
• Sperm must undergo the process of capacitation in order to have the “capacity” to fertilize an oocyte.
• If they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte’s
thick outer layer of cells.
• Motile sperm cells reach the cumulus matrix surrounding the egg cell in order to fertilize it.
• Upon reaching the cumulus matrix, the sperm releases an enzyme called sperm lysin, which breaks
down the cumulus matrix.
• Additionally, hyaluronidase enzyme is released, which dissolves the hyaluronic acid holding the
follicular cells together.
• Now, from the numerous sperm cells present, one sperm cell's head attaches to the protein binding site
on the zona pellucida of the egg cell.
• Once attached, the sperm cell can penetrate the zona pellucida and enter the egg cell, initiating
fertilization.

77. • ZP3 serves as the primary binding site on the zona pellucida for fertilization.
• Interaction with the ZP3 protein binding site stimulates the sperm cell, leading
to the release of calcium ions (Ca++) within the sperm cell.
• The outer acrosomal membrane of the sperm attaches to the plasma membrane of
the egg cell, and acrosomal enzymes start digesting the zona pellucida layer near
the ZP3 binding site.
• Subsequently, the inner acrosomal membrane of the sperm is also exposed to the
plasma membrane of the egg cell.
• Immediately, sodium channels open in the egg cell membrane, allowing sodium
ions to enter from the extracellular space into the egg cell.
• Simultaneously, calcium ions start to release from the endoplasmic reticulum of
the egg cell.
• This rapid influx of positively charged ions into the egg cell is known as fast
blockage or electrical blockage, preventing polyspermy.
• Polyspermy is the entry of multiple sperm cells into the egg, and this process
prevents further sperm cell entry into the egg cell.

78. • Cortical cells surrounding the egg release enzymes that completely degrade the zona pellucida layer. As
a result, all the protein binding sites of the zona pellucida become non-functional.
• With the protein binding sites rendered non-functional, any remaining sperm cells are unable to bind to
these sites, and their entry is completely blocked.
• This process is known as slow blockage, which further ensures that no additional sperm can enter the
egg cell.
• The nucleus of the sperm enters the cytoplasm of the egg cell, triggering a high influx of calcium ions
(Ca++) into the egg cell.
• This influx of calcium ions leads to the completion of meiosis II in the egg cell.
• After meiosis II is completed, the egg cell divides, forming a polar body and the egg cell itself. The
polar body then degenerates and degrades.
• Meanwhile, the sperm cell and the egg cell fuse with each other, forming a zygote.
• These processes collectively known as karyogamy, where two haploid cells, the sperm and the egg,
fuse together, forming one diploid cell, the zygote.
• This process is widely recognized as fertilization, marking the beginning of embryonic development.

79. Implantation
• Once fertilization happens, the cell starts to divide and multiply within
24 hours in the fallopian tube.
• This detached multi-celled structure is called a zygote. Later, after 3-4
days it travels to the uterus and now we call it as an embryo.
• The embryo develops and undergoes various stages and gets attached to
the endometrial layer of the uterus.
• This process of attachment is known as implantation.

80. Physiology of Twins or Multiple Birth
Maternal Twins:
• Maternal twins occur when a single fertilized egg (zygote) splits into two embryos.
• This splitting typically occurs within the first two weeks after fertilization.
• Maternal twins share the same genetic material and are identical in terms of DNA. They are also known
as identical twins.
• Maternal twins may share a placenta and amniotic sac or have separate placentas and sacs, depending on
when the zygote splits.
• The occurrence of maternal twins is not influenced by genetic predisposition or family history but
happens randomly.
Fraternal Twins:
• Fraternal twins occur when two separate eggs are fertilized by two separate sperm cells.
• These twins are genetically similar to any other siblings, sharing approximately 50% of their DNA.
• Fraternal twins can have different genders and may or may not resemble each other closely.
• They each have their own placenta and amniotic sac, as they develop independently in the uterus.
• The likelihood of fraternal twins is influenced by genetic factors, specifically the mother's genetic
predisposition to releasing multiple eggs during ovulation.
• Factors such as maternal age, ethnicity, and family history can also affect the likelihood of conceiving
fraternal twins.

82. Physiology of Pregnancy
• Pregnancy and the associated changes are a normal physiological process in
response to the development of the fetus.
• These changes happen in response to many factors; hormonal changes,
increase in the total blood volume, weight gain, and increase in foetus size as
the pregnancy progresses.
• The full gestation period is 39-40 weeks, and a pre-term birth is classed as
delivery before 37 weeks gestation.
• It can be subdivided into distinct gestational periods.
• The first 2 weeks of prenatal development are referred to as the pre-
embryonic stage.
• A developing human is referred to as an embryo during weeks 3–8, and a
fetus from the ninth week of gestation until birth.

83. Pre-Implantation Devlopment
Cleavage and Blastulation
• Following fertilization, the zygote and its associated membranes, together referred to as the
conceptus, continue to be projected toward the uterus by peristalsis and beating cilia.
• During its journey to the uterus, the zygote undergoes five or six rapid mitotic cell divisions.
• Although each cleavage results in more cells, it does not increase the total volume of the conceptus.
• Each daughter cell produced by cleavage is called a
• Approximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. The cells that had
been loosely grouped are now compacted and look more like a solid mass. The name given to this
structure is the
• Once inside the uterus, the conceptus floats freely for several more days. It continues to divide,
creating a ball of approximately 100 cells, and consuming nutritive endometrial secretions called
uterine milk while the uterine lining thickens.
• The ball of now tightly bound cells starts to secrete fluid and organize themselves around a fluid-
filled cavity, the blastocoel.

84. Cleavage and Blastulation

85. • The cells that form the outer shell are called trophoblasts and iner eells called Embryoblast.
• These cells will develop into the chorionic sac and the fetal portion of the placenta.
• As the blastocyst forms, the trophoblast excretes enzymes that begin to degrade the zona
pellucida. In a process called the conceptus breaks free of the zona pellucida in
preparation for implantation.
Implantation
• At the end of the first week, the blastocyst comes in contact with the uterine wall and adheres
to it, embedding itself in the uterine lining via the trophoblast cells. Implantation can be
accompanied by minor bleeding.
• The blastocyst typically implants in the fundus of the uterus or on the posterior wall.
• A significant percentage (50–75 percent) of blastocysts fail to implant; when this occurs, the
blastocyst is shed with the endometrium during menses.

86. • When implantation succeeds and the blastocyst adheres to the endometrium, the superficial
cells of the trophoblast fuse with each other, forming the , a
multinucleated body that digests endometrial cells to firmly secure the blastocyst to the
uterine wall.
• In response, the uterine mucosa rebuilds itself and envelops the blastocyst.
• The trophoblast secretes human chorionic gonadotropin (hCG), a hormone that directs the
corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to
suppress menses.
• These functions of hCG are necessary for creating an environment suitable for the
developing embryo.
• As a result of this increased production, hCG accumulates in the maternal bloodstream and is
excreted in the urine.
• Implantation is complete by the middle of the second week.
• Just a few days after implantation, the trophoblast has secreted enough hCG for an at-home
urine pregnancy test to give a positive result.

88. Embryogenesis
• As the third week of development begins, the two-layered disc of cells becomes a three-
layered disc through the process of , during which the cells transition from
totipotency to multipotency.
• The embryo, which takes the shape of an oval-shaped disc, forms an indentation called the
along the .
• A node at the caudal or “tail” end of the primitive streak emits growth factors that direct
cells to multiply and migrate.
• Cells migrate toward and through the primitive streak and then move laterally to create two
new layers of cells.
• The first layer is the , a sheet of cells that displaces the hypoblast and lies
adjacent to the yolk sac.
• The second layer of cells fills in as the middle layer, or
• The cells of the epiblast that remain (not having migrated through the primitive streak)
become the

89. • Each of these germ layers will develop into
specific structures in the embryo.
• Whereas the ectoderm and endoderm form tightly
connected epithelial sheets, the mesodermal cells
are less organized and exist as a loosely connected
cell community.
• The ectoderm gives rise to cell lineages that
differentiate to become the c
.
• Mesodermal cells ultimately become the
• The endoderm goes on to form the

91. Embryonic Membranes Devlopment
• During the second week of development, with the embryo implanted in the uterus, cells
within the blastocyst start to organize into layers.
• Some grow to form the extra-embryonic membranes needed to support and protect the
growing embryo: the amnion, the yolk sac, the allantois, and the chorion.
• At the beginning of the second week, the cells of the inner cell mass form into a two-
layered disc of embryonic cells, and a space the amniotic cavity opens up between it and
the trophoblast.
• Cells from the upper layer of the disc (the epiblast) extend around the amniotic cavity,
creating a membranous sac that forms into the amnion by the end of the second week.
• The amnion fills with amniotic fluid and eventually grows to surround the embryo.
• Early in development, amniotic fluid consists almost entirely of a filtrate of maternal
plasma, but as the kidneys of the fetus begin to function at approximately the eighth
week, they add urine to the volume of amniotic fluid.

92. • On the ventral side of the embryonic disc, opposite the amnion,
extend into the blastocyst cavity and form a
• The yolk sac supplies some nutrients absorbed from the trophoblast and also provides
primitive blood circulation to the developing embryo for the second and third week of
development.
• When the placenta takes over nourishing the embryo at approximately week 4, the yolk sac
has been greatly reduced in size and its main function is to serve as the source of blood cells
and germ cells.
• During week 3, a develops into the , a
primitive excretory duct of the embryo that will become part of the urinary bladder.
• Together, the stalks of the yolk sac and allantois establish the outer structure of the
umbilical cord.
• The last of the extra-embryonic membranes is the which is the one membrane that
surrounds all others.

94. Organogenesis from Ectoderm
Neuralation
• Following gastrulation, rudiments of the central nervous system develop from the
ectoderm in the process of Neurulation.
• Specialized neuroectodermal tissues along the length of the embryo thicken into the
neural plate.
• During the fourth week, tissues on either side of the plate fold upward into a neural
fold.
• The two folds converge to form the neural tube. The tube lies atop a rod-shaped,
mesoderm-derived notochord, which eventually becomes the
• Block-like structures called somites form on either side of the tube, eventually
differentiating into the axial skeleton, skeletal muscle, and dermis.
• During the fourth and fifth weeks, the anterior neural tube dilates and subdivides to
form vesicles that will become the brain structures.

95. • The embryo, which begins as a flat sheet of
cells, begins to acquire a cylindrical shape
through the process of embryonic folding.
• The embryo folds laterally and again at
either end, forming a C-shape with distinct
head and tail ends.
• The embryo envelops a portion of the yolk
sac, which protrudes with the umbilical
cord from what will become the abdomen.
• The folding essentially creates a tube,
called the primitive gut, that is lined by the
endoderm.
• The amniotic sac, which was sitting on top
of the flat embryo, envelops the embryo as
it folds.

97. • Like the central nervous system, the heart also begins its development in the embryo as a
tube-like structure, connected via capillaries to the chorionic villi.
• Cells of the primitive tube shaped heart are capable of
.
• The heart begins beating in the beginning of the fourth week, although it does not
actually pump embryonic blood until a week later, when the oversized liver has begun
producing red blood cells. (This is a temporary responsibility of the embryonic liver that the
bone marrow will assume during fetal development.)
• During weeks 4–5, the eye pits form, limb buds become apparent, and the rudiments of the
pulmonary system are formed.
• During the sixth week, uncontrolled fetal limb movements begin to occur.
• The gastrointestinal system develops too rapidly for the embryonic abdomen to
accommodate it, and the intestines temporarily loop into the umbilical cord.
• Paddle-shaped hands and feet develop fingers and toes by the process of apoptosis
(programmed cell death), which causes the tissues between the fingers to disintegrate.

98. • By week 7, the facial structure is more complex and includes nostrils, outer ears, and lenses.
• By the eighth week, the head is nearly as large as the rest of the embryo’s body, and all major
brain structures are in place.
• The external genitalia are apparent, but at this point, male and female embryos are
indistinguishable.
• Bone begins to replace cartilage in the embryonic skeleton through the process of
ossification.
• During weeks 9–12 of fetal development, the brain continues to expand, the body elongates,
and ossification continues.
• Fetal movements are frequent during this period, but are jerky and not well-controlled.
• The bone marrow begins to take over the process of erythrocyte production—a task that the
liver performed during the embryonic period. The liver now secretes bile.
• The fetus circulates amniotic fluid by swallowing it and producing urine.
• The eyes are well-developed by this stage, but the eyelids are fused shut. The fingers and toes
begin to develop nails.

99. • Weeks 13–16 are marked by sensory organ development. The eyes move closer together;
blinking motions begin, although the eyes remain sealed shut.
• The lips exhibit sucking motions. The ears move upward and lie flatter against the head.
The scalp begins to grow hair.
• The excretory system is also developing: the kidneys are well-formed, and meconium, or
fetal feces, begins to accumulate in the intestines.
• Meconium consists of ingested amniotic fluid, cellular debris, mucus, and bile.
• During approximately weeks 16–20, as the fetus grows and limb movements become more
powerful, the mother may begin to feel quickening, or fetal movements.
• However, space restrictions limit these movements and typically force the growing fetus
into the “fetal position,” with the arms crossed and the legs bent at the knees.
• Sebaceous glands coat the skin with a waxy, protective substance called vernix caseosa that
protects and moisturizes the skin and may provide lubrication during childbirth.

100. • Developmental weeks 21–30 are characterized by rapid weight gain, which is important for
maintaining a stable body temperature after birth.
• The bone marrow completely takes over erythrocyte synthesis, and the axons of the spinal cord
begin to be myelinated, or coated in the electrically insulating glial cell sheaths that are
necessary for efficient nervous system functioning. (The process of myelination is not
completed until adolescence.) During this period, the fetus grows eyelashes.
• The eyelids are no longer fused and can be opened and closed. The lungs begin producing
surfactant, a substance that reduces surface tension in the lungs and assists proper lung
expansion after birth.
• Inadequate surfactant production in premature newborns may result in respiratory distress
syndrome, and as a result, the newborn may require surfactant replacement therapy,
supplemental oxygen, or maintenance in a continuous positive airway pressure (CPAP)
chamber during their first days or weeks of life.
• In male fetuses, the testes descend into the scrotum near the end of this period.

101. • The fetus continues to lay down subcutaneous fat from week 31 until birth.
• The added fat fills out the hypodermis, and the skin transitions from red and
wrinkled to soft and pink.
• Lanugo is shed, and the nails grow to the tips of the fingers and toes.
• Once born, the newborn is no longer confined to the fetal position, so
subsequent measurements are made from head-to-toe instead of from crown-
to-rump.
• At birth, the average length is approximately 51 cm (20 in).

104. Embryogenesis from Mesoderm

105. 1.Intermediate Mesoderm:
1.Urogenital System Development: The intermediate mesoderm plays a crucial role in the
development of the urogenital system. It gives rise to structures such as the pronephros,
mesonephros, and metanephros, which are sequential kidney structures. Additionally, the
intermediate mesoderm contributes to the development of the gonads (testes or ovaries) and
their associated ducts, including the Wolffian and Müllerian ducts.
2.Paraxial Mesoderm:
1.Somitogenesis: The paraxial mesoderm undergoes segmentation to form somites, which are
blocks of mesodermal tissue. Somitogenesis occurs in a cranial-to-caudal progression along
the embryo's axis. Somites give rise to various structures, including the axial skeleton (such
as vertebrae, ribs, and part of the skull), skeletal muscles, and dermis of the skin.
2.Axial Skeleton Development: The sclerotome, derived from somites, forms the
cartilaginous precursor of the axial skeleton. The somites also contribute to the formation of
the intervertebral discs and other connective tissues within the vertebral column.

106. 1.Lateral Mesoderm:
1.Somatic Mesoderm: The somatic mesoderm, located laterally adjacent to the
ectoderm, contributes to the development of structures outside the body cavity. It gives
rise to the parietal layer of the serous membranes lining the body cavities, including the
pleura (lining the thoracic cavity), pericardium (lining the heart cavity), and peritoneum
(lining the abdominal cavity).
2.Splanchnic Mesoderm: The splanchnic mesoderm, located laterally adjacent to the
endoderm, contributes to the development of structures within the body cavity. It gives
rise to the visceral layer of the serous membranes lining the internal organs, including
the visceral peritoneum covering abdominal organs such as the liver, spleen, and
intestines.
3.Coelom Formation: Both the somatic and splanchnic mesoderm contribute to the
formation of the coelom, a fluid-filled body cavity. The coelom eventually divides into
the pleural, pericardial, and peritoneal cavities, which house the lungs, heart, and
abdominal organs, respectively.

108. Embryogenesis from Endoderm
1.Formation of the Gut Tube: During early embryonic development, the endoderm undergoes folding to
form a tube-like structure called the gut tube. The gut tube extends from the anterior (near the future
mouth) to the posterior (near the future anus) regions of the embryo. This tube gives rise to the
gastrointestinal tract, which includes the esophagus, stomach, small intestine, and large intestine.
2.Organ Bud Formation: From the gut tube, various outgrowths called organ buds develop. These organ
buds eventually differentiate into specific organs of the digestive system. For example:
1.Liver: Liver bud formation occurs as an outgrowth from the foregut region of the gut tube. The
liver bud develops into the liver and associated structures such as the gallbladder and bile ducts.
2.Pancreas: Pancreatic buds form from the foregut endoderm and give rise to the pancreas, which
plays a vital role in digestion and glucose metabolism.
3.Lungs: Lung buds emerge as outgrowths from the foregut endoderm. They undergo branching
morphogenesis to form the intricate network of airways and alveoli within the lungs.
4.Thyroid and Parathyroid Glands: These glands develop from endodermal tissue located near the
base of the tongue, known as the thyroid diverticulum.

109. 3. Differentiation of Epithelial Layers: The endoderm gives rise to epithelial linings
of various organs and structures throughout the body. These epithelial layers play
essential roles in absorption, secretion, and protection. For example, the inner lining
of the gastrointestinal tract and respiratory tract is derived from endoderm.
4.Formation of Endodermal Derivatives in the Digestive Tract: Within the
gastrointestinal tract, endodermal derivatives include:
1.Villi and Microvilli: These finger-like projections increase the surface area for nutrient absorption
in the small intestine.
2.Goblet Cells: Goblet cells secrete mucus, which helps lubricate and protect the epithelial lining of
the digestive tract.
3.Endocrine Cells: Endocrine cells within the gut mucosa produce hormones that regulate various
physiological processes, such as digestion and appetite.

111. Development of the Placenta
• During the first several weeks of development, the cells of the endometrium referred to as decidual
cells nourish the nascent embryo. During prenatal weeks 4–12, the developing placenta gradually takes
over the role of feeding the embryo, and the decidual cells are no longer needed.
• The mature placenta is composed of tissues derived from the embryo, as well as maternal tissues of the
endometrium.
• The placenta connects to the conceptus via the umbilical cord, The umbilical cord carries
deoxygenated blood and wastes through two umbilical arteries from the fetus to the placenta. Nutrients
and oxygen are carried from the mother to the fetus through the single umbilical vein.
• The umbilical cord is surrounded by the amnion, and the spaces within the cord around the blood
vessels are filled with Wharton’s jelly, a mucous connective tissue.
• The maternal portion of the placenta develops from the , the
• To form the embryonic portion of the placenta, the syncytiotrophoblast and the underlying cells of
the trophoblast (cytotrophoblast cells) begin to proliferate along with a layer of extraembryonic
mesoderm cells.

114. • These form the chorionic membrane, which envelops the
entire conceptus as the chorion.
• The chorionic membrane forms finger-like structures called
chorionic villi that burrow into the endometrium like tree
roots, making up the fetal portion of the placenta.
• Meanwhile, fetal mesenchymal cells derived from the
mesoderm fill the villi and differentiate into blood vessels,
including the three umbilical blood vessels that connect the
embryo to the developing placenta.
• placentation is complete by weeks 14–16. As a fully
developed organ, the placenta provides nutrition and
excretion, respiration, and endocrine function.
• It receives blood from the fetus through the umbilical
arteries. Capillaries in the chorionic villi filter fetal wastes
out of the blood and return clean, oxygenated blood to the
fetus through the umbilical vein.

115. • Nutrients and oxygen are transferred from maternal blood surrounding the villi through the
capillaries and into the fetal bloodstream.
• Some substances move across the placenta by simple diffusion. Oxygen, carbon dioxide, and any
other lipid-soluble substances take this route. Other substances move across by facilitated
diffusion. This includes water-soluble glucose.
• The fetus has a high demand for amino acids and iron, and those substances are moved across the
placenta by active transport.
• Maternal and fetal blood does not commingle because blood cells cannot move across the
placenta.
• Although blood cells are not exchanged, the chorionic villi provide ample surface area for the
two-way exchange of substances between maternal and fetal blood.
• The rate of exchange increases throughout gestation as the villi become thinner and increasingly
branched.
• The placenta is permeable to lipid-soluble fetotoxic substances: alcohol, nicotine, barbiturates,
antibiotics, certain pathogens, and many other substances that can be dangerous or fatal to the
developing embryo or fetus.

116. Physiology of Parturition
• Labor is the process by which the fetus is expelled from the uterus through the
vagina, also referred to as giving birth. A synonym for labor is parturition.
• The onset of labor is determined by complex interactions of several placental and
fetal hormones. Because progesterone inhibits uterine contractions, labor cannot
take place until the effects of progesterone are diminished.
• Toward the end of gestation, the levels of estrogens in the mother’s blood rise
sharply, producing changes that overcome the inhibiting effects of progesterone.
• The rise in estrogens results from increasing secretion by the placenta of
corticotropinreleasing hormone, which stimulates the anterior pituitary gland of
the fetus to secrete ACTH (adrenocorticotropic hormone).
• In turn, ACTH stimulates the fetal adrenal gland to secrete cortisol and
dehydroepiandrosterone (DHEA), the major adrenal androgen. The placenta then
converts DHEA into an estrogen.

117. • High levels of estrogens cause the number of receptors for oxytocin on uterine muscle
fibers to increase, and cause uterine muscle fibers to form gap junctions with one
another.
• Oxytocin released by the posterior pituitary stimulates uterine contractions, and relaxin
from the placenta assists by increasing the flexibility of the pubic symphysis and helping
dilate the uterine cervix.
• Estrogen also stimulates the placenta to release prostaglandins, which induce production
of enzymes that digest collagen fibers in the cervix, causing it to soften.
• Control of labor contractions during parturition occurs via a positive feedback cycle.
• Contractions of the uterine myometrium force the baby’s head or body into the cervix,
distending (stretching) the cervix.
• Stretch receptors in the cervix send nerve impulses to neurosecretory cells in the
hypothalamus, causing them to release oxytocin into blood capillaries of the posterior
pituitary gland.
• Oxytocin then is carried by the blood to the uterus, where it stimulates the myometrium to
contract more forcefully.

118. • As the contractions intensify, the baby’s body stretches the cervix still more, and the
resulting nerve impulses stimulate the secretion of yet more oxytocin.
• With birth of the infant, the positive feedback cycle is broken because cervical distension
suddenly lessens.
• Uterine contractions occur in waves that start at the top of the uterus and move downward,
eventually expelling the fetus.
• True labor begins when uterine contractions occur at regular intervals, usually producing
pain. As the interval between contractions shortens, the contractions intensify. Another
symptom of true labor in some women is localization of pain in the back that is intensified
by walking.
• The most reliable indicator of true labor is dilation of the cervix and the “show,” a discharge
of a blood-containing mucus into the cervical canal.
• In false labor, pain is felt in the abdomen at irregular intervals, but it does not intensify and
walking does not alter it significantly. There is no “show” and no cervical dilation.

119. True labor/ Parturition can be divided into three stages:
1 Stage of dilation:
• The time from the onset of labor to the complete dilation of the cervix is the stage
of dilation. This stage, which typically lasts 6–12 hours, features regular
contractions of the uterus, usually a rupturing of the amniotic sac, and complete
dilation (to 10 cm) of the cervix. If the amniotic sac does not rupture spontaneously,
it is ruptured intentionally.
2 Stage of expulsion:
• The time (10 minutes to several hours) from complete cervical dilation to delivery
of the baby is the stage of expulsion.
3 Placental stage:
• The time (5–30 minutes or more) after delivery until the placenta or “afterbirth” is
expelled by powerful uterine contractions is the placental stage. These contractions
also constrict blood vessels that were torn during delivery, reducing the likelihood
of hemorrhage.

122. • As a rule, labor lasts longer with first babies, typically about 14 hours.
• For women who have previously given birth, the average duration of
labor is about 8 hours although the time varies enormously among
births.
• About 7% of pregnant women do not deliver by 2 weeks after their due
date.
• Such cases carry an increased risk of brain damage to the fetus, and
even fetal death, due to inadequate supplies of oxygen and nutrients
from an aging placenta.
• Post-term deliveries may be facilitated by inducing labor, initiated by
administration of oxytocin (Pitocin®), or by surgical delivery (cesarean
section)

123. Introduction to genetics
Chromosomes
• Nearly every body cell contains, within its nucleus, an identical copy of the entire complement
of the individual’s genetic material.
• Two important exceptions are red blood cells (which have no nucleus) and the gametes or sex
cells. In a resting cell, the chromatin (genetic material, is diffuse and hard to see under the
microscope, but when the cell prepares to divide, it is collected into highly visible, compact,
sausage-shaped structures called chromosomes.
• Each chromosome is one of a pair, one inherited from the mother and one from the father, so
the human cell has 46 chromosomes that can be arranged as 23 pairs.
• A cell with 23 pairs of chromosomes is termed diploid. Gametes (spermatozoa and ova) with
only half of the normal complement, i.e. 23 chromosomes instead of 46, are described as
haploid. Chromosomes belonging to the same pair are called homologous chromosomes.
• The complete set of chromosomes from a cell is its karyotype

124. • Each pair of chromosomes is numbered, the largest pair being no. 1. The first 22
pairs are collectively known as autosomes, and the chromosomes of each pair
contain the same amount of genetic material.
• The chromosomes of pair 23 are called the sex chromosomes because they
determine the individual’s gender. Unlike autosomes, these two chromosomes are
not necessarily the same size; the Y chromosome is much shorter than the X
and is carried only by males.
• A child inheriting two X chromosomes (XX), one from each parent, is female,
and a child inheriting an X from his mother and a Y from his father (XY) is male.
• Each end of the chromosome is capped with a length of DNA called a telomere,
which seals the chromosome and is structurally essential.
• During replication, the telomere is shortened, which would damage the
chromosome, and so it is repaired with an enzyme called telomerase.

125. Genes
• Along the length of the chromosomes are the genes. Each gene contains
information in code that allows the cell to make (almost always) a
specific protein, the so-called gene product.
• Each gene codes for one specific protein, and research puts the number
of genes in the human genome at between 25,000 and 30,000.
• Genes normally exist in pairs, because the gene on one chromosome is
matched at the equivalent site (locus) on the other chromosome of the
pair.

126. DNA
• Genes are composed mainly of very long strands of DNA; the total length of
DNA in each cell is about a metre.
• Because this is packaged into chromosomes, which are micrometres (10−6 m)
long, this means that the DNA must be tightly wrapped up to condense it into
such a small space.
• DNA is a double-stranded molecule, made up of two chains of nucleotides.
Nucleotides consist of three subunits:
• a sugar
• a phosphate group
• a base.
• The DNA molecule is sometimes likened to a twisted ladder, with the uprights
formed by alternating chains of sugar and phosphate units. In DNA, the sugar is
deoxyribose, thus DNA.

127. • The bases are linked to the sugars, and each base binds to another base on the
other sugar/phosphate chain, forming the rungs of the ladder.
• The two chains are twisted around one another, giving a double helix (twisted
ladder) arrangement.
• The double helix itself is further twisted and wrapped in a highly organised
way around structural proteins called histones, which are important in
maintaining the heavily coiled three-dimensional shape of the DNA.
• The term given to the DNA–histone material is chromatin.
• The chromatin is supercoiled and packaged into the chromosomes shortly
before the cell divides

128. Protein Synthesis
• DNA holds the cell’s essential biological information, written within the base code
in the centre of the double helix.
• The products of this information are almost always proteins. Proteins are essential to
all aspects of body function, forming the major structural elements of the body as
well as the enzymes essential for all biochemical processes within it.
• The building blocks of human proteins are about 20 different amino acids.
• As the cell’s DNA is too big to leave the nucleus, an intermediary molecule is
needed to carry the genetic instructions from the nucleus to the cytoplasm, where
proteins are made.
• This is called messenger (m)RNA.

129. Messenger ribonucleic acid (mRNA)
• mRNA is a single-stranded chain of nucleotides synthesised in the nucleus from
the appropriate gene, whenever the cell requires to make the protein for which
that gene codes.
• There are three main differences between the structures of RNA and DNA:
1. it is single instead of double stranded
2. it contains the sugar ribose instead of deoxyribose
3. it uses the base uracil instead of thymine.
• Using the DNA as a template, a piece of mRNA is made from the gene to be
used. This process is called transcription.
• The mRNA then leaves the nucleus through the nuclear pores and carries its
information to the ribosomes in the cytoplasm.

130. Transcription
• Because the code is buried within the
DNA molecule, the first step is to
open up the helix to expose the bases.
• Only the gene to be transcribed is
opened; the remainder of the
chromosome remains coiled.
• Opening up the helix exposes both
base strands, but the enzyme that
makes the mRNA uses only one of
them, so the mRNA molecule is
single, not double stranded.

131. • As the enzyme moves along the opened
DNA strand, reading its code, it adds the
complementary base to the mRNA.
• Therefore, if the DNA base is cytosine,
guanine is added to the mRNA molecule
(and vice versa); if it is thymine, adenine is
added; if it is adenine, uracil is added
(remember there is no thymine in RNA, but
uracil instead).
• When the enzyme gets to a ‘stop’ signal, it
terminates synthesis of the mRNA
molecule, and the mRNA is released.
• The DNA is zipped up again by other
enzymes, and the mRNA then leaves the
nucleus.

132. Translation
• Translation is synthesis of the final protein using the information carried on mRNA. It
takes place on free ribosomes in the cytoplasm and those attached to rough
endoplasmic reticulum.
• First, the mRNA attaches to the ribosome. The ribosome then ‘reads’ the base
sequence of the mRNA.
• Because proteins are built from up to 20 different amino acids, it is not possible to use
the four bases individually in a simple one-to-one code.
• To give enough options, the base code in RNA is read in triplets, giving a possible 64
base combinations, which allows a coded instruction for each amino acid as well as
other codes, e.g. stop and start instructions.
• Each of these specific triplet sequences is called a codon; for example, the base
sequence ACA (adenine, cytosine, adenine) codes for the amino acid cysteine.

133. • The first codon is a start codon, which initiates
protein synthesis.
• The ribosome slides along the mRNA, reading
the codons and adding the appropriate amino
acids to the growing protein molecule as it
goes.
• The ribosome continues assembling the new
protein molecule until it arrives at a stop
codon, at which point it terminates synthesis
and releases the new protein.
• Some new proteins are used within the cell
itself, and others are exported, e.g. insulin
synthesised by pancreatic β-islet cells is
released into the bloodstream.

134. Gene expression
• Although all nucleated cells (except gametes) have an identical set of
genes, each cell type uses only those genes related directly to its own
particular function.
• For example, the only cell type containing haemoglobin is the red blood
cell, although all body cells carry the haemoglobin gene.
• This selective gene expression is controlled by various regulatory
substances, and the genes not needed by the cell are kept switched off.