Download 11.Dev. of Female Genital System

DEVELOPMENT OF FEMALE
GENITAL SYSTEM
By: Dr. Mujahid Khan
Development of Gonads
The gonads are derived from 3 sources:
 The
mesothelium (mesodermal epithelium)
lining the posterior abdominal wall
 The
underlying mesenchyme (embryonic
connective tissue)
 The
primordial germ cells
Indifferent Gonads
 The
initial stages of gonadal development
occur during the fifth week
A
thickened area of mesothelium develops
on the medial side of the mesonephros
 Proliferation
of this epithelium and the
underlying mesenchyme produces a bulge
on the medial side of the mesonephros
called gonadal ridge
Indifferent Gonads

Finger like epithelial cords or Gonadal cords
soon grow into the underlying mesenchyme

The indifferent gonad now consists of an
external cortex and an internal medulla

In embryos with an XX sex chromosome
complex, the cortex differentiates into an ovary
and the medulla regresses

In embryos with an XY sex chromosome
complex, the medulla differentiates into a testis
and the cortex regresses
Primordial Germ Cells

These large, spherical cells are visible early in the fourth
week among the endodermal cells of the yolk sac near
the allantois

During folding of the embryo, the dorsal part of the yolk
sac is incorporated into the embryo

With this the primordial germ cells migrate along the
dorsal mesentery of the hindgut to the gonadal ridges

During the sixth week the primordial germ cells enter the
underlying mesenchyme and are incorporated in the
gonadal cords
Sex Determination
 Chromosomal
and genetic sex is
determined at fertilization
 It
depends upon whether an X-bearing
sperm or a Y-bearing sperm fertilizes the
X-bearing ovum
 The
type of gonads develop is determined
by the sex chromosome complex of the
embryo (XX or XY)
Sex Determination

Before the seventh week, the gonads of the two
sexes are identical in appearance called
indifferent gonads

Development of the male phenotype requires a
Y chromosome

The SRY gene for a testes-determining factor
(TDF) has been localized in the sex-determining
region of the Y chromosome

Two X chromosomes are required for the
development of the female phenotype
Sex Determination

The Y chromosome has a testes-determining effect on
the medulla of the indifferent gonad

The absence of a Y chromosome results in the formation
of an ovary

Testosterone, produced by the fetal testes, determines
the maleness

Primary female sexual differentiation in the fetus does
not depend on hormones

It occurs even if the ovaries are absent
Development of Ovaries
 Gonadal
development occurs slowly in
female embryos
 The
X chromosomes bear genes for
ovarian development and an autosomal
gene also appears to play a role in ovarian
organogenesis
 The
ovary is not identifiable histologically
until about the 10th week
Development of Ovaries

Gonadal cords do not become prominent but
they extend into the medulla and form a
rudimentary rete ovarii

This structure and gonadal cords normally
degenerate and disappear

Cortical cords extend from the surface
epithelium of the developing ovary into the
underlying mesenchyme during the early fetal
period
Development of Ovaries
 As
the cortical cords increase in size,
primordial germ cells are incorporated in
them
 At
about 16 weeks these cords begin to
break up into isolated cell clusters called
primordial follicles
 Each
primordial follicle consists of an
oogonium, derived from primordial germ
cell
Development of Ovaries

Each oogonium is surrounded by a single layer of
flattened follicular cells derived from the surface
epithelium

Active mitosis of oogonia occurs during fetal life
producing thousands of primordial follicles

No oogonia form postnatally

Many oogonia degenerate before birth

About 2 million remain enlarge to become primary
oocytes before birth
Development of Ovaries

After birth the surface epithelium of the ovary flattens to
a single layer of cells continuous with the mesothelium of
the peritoneum at the hilum of the ovary

The surface epithelium of the ovary was formerly
inappropriately called the germinal epithelium

The surface epithelium becomes separated from the
follicles in the cortex by a thin fibrous capsule called
tunica albuginea

As the ovary separates from the regressing
mesonephros, it is suspended by a mesentery called
mesovarium
Development of Genital Ducts

Both male and female embryos have two pairs of genital
ducts

The mesonephric ducts (wolffian ducts) play an
important role in the development of the male
reproductive system

The paramesonephric ducts (mullerian ducts) have a
leading role in the development of the female
reproductive system

Till the end of sixth week, the genital system is in an
indifferent state, when both pairs of genital ducts are
present
Development of Genital Ducts

The mesonephric ducts, which drained urine
from the mesonephric kidneys play a major role
in the development of male reproductive system

The paramesonephric ducts play an essential
role in the development of the female
reproductive system

The funnel shaped cranial ends of these ducts
open into the peritoneal cavity

The paramesonephric ducts pass caudally,
parallel to the mesonephric ducts
Development of Genital Ducts

Both the paramesonephric ducts pass caudally
and reach the future pelvic region

Cross ventral to the mesonephric ducts

Fuse to form a Y-shaped uterovaginal
primordium in the midline

This tubular structure projects into the dorsal
wall of the urogenital sinus and produces an
elevation called sinus (muller) tubercle
Development of Female Genital
Ducts & Glands

In female embryos, the mesonephric ducts
regress because of the absence of testosterone

Paramesonephric ducts develop because of the
absence of mullerian inhibiting substance (MIS)

Female sexual development does not depend on
the presence of ovaries or hormones

The paramesonephric ducts form most of the
female genital tract
Development of Female Genital
Ducts & Glands

The uterine tubes develop from the unfused
cranial part of the paramesonephric ducts

The caudal fused portions of these ducts form
the uterovaginal primordium

It gives rise to uterus and superior part of vagina

The endometrial stroma and myometrium are
derived from splanchnic mesenchyme
Development of Female Genital
Ducts & Glands
 Fusion
of the paramesonephric ducts also
brings together a peritoneal fold that forms
the broad ligament
 Also
forms two peritoneal compartments,
the rectouterine pouch and the
vesicouterine pouch
Development of Vagina
 The
vaginal epithelium is derived from the
endoderm of the urogenital sinus
 The
fibromuscular wall of the vagina
develops from the surrounding
mesenchyme
 Contact
of the uterovaginal primordium
with the urogenital sinus forming the sinus
tubercle
Development of Vagina

Sinus tubercle induces the formation of paired
endodermal outgrowths called sinovaginal bulbs

The sinovaginal bulbs fuse to form a vaginal
plate

Later the central cells of the plate break down,
forming lumen of vagina

The peripheral cells of the plate form the vaginal
epithelium
Development of Vagina

Until the fetal life, the lumen of the vagina is
separated from the cavity of the urogenital sinus
by a hymen

The hymen is formed by invagination of the
posterior wall of the urogenital sinus, resulting
from expansion of the caudal end of the vagina

The hymen remains as a thin fold of mucous
membrane just within the vaginal orifice
Development of External Genitalia
 Up
to the seventh week of development
the external genitalia are similar in both
sexes
 Distinguishing
sexual characteristics begin
to appear during the ninth week
 External
genitalia are not fully
differentiated until the twelfth week
Development of External Genitalia

Early in the fourth week, proliferating
mesenchyme produces a genital tubercle in both
sexes at the cranial end of the cloacal
membrane

Labioscrotal swelling and urogenital folds soon
develop on each side of the cloacal membrane

The genital tubercle soon elongates to form a
primordial phallus
Development of External Genitalia
 When
the urorectal septum fuses with the
cloacal membrane, it divides it into a
dorsal anal membrane and a ventral
urogenital membrane
 The
urogenital membrane lies in the floor
of a median cleft, the urogenital groove,
which is bounded by urogenital folds
Development of External Genitalia
 The
anal and urogenital membranes
rupture a week later forming the anus and
urogenital orifice, respectively
 In
female fetus the urethra and vagina
open into a common cavity called
vestibule
Development of Female External
Genitalia

Estrogen produced by the placenta and fetal
ovaries appear to be involved in feminization of
indifferent external genitalia

Growth of the primordial phallus gradually
ceases and becomes clitoris

The clitoris is relatively large at 18 weeks

It develops like a penis but the urogenital folds
do not fuse, except posteriorly
Development of Female External
Genitalia
 Urogenital
folds fuses posteriorly to form
the frenulum of the labia minora
 The
unfused parts of the urogenital folds
form the labia minora

The labioscrotal folds fuse posteriorly to
form the posterior labial commisure
Development of Female External
Genitalia

The labioscrotal folds fuse anteriorly to form the
anterior labial commisure and mons pubis

Most parts of the labioscrotal folds remain
unfused and form two large folds of skin called
labia majora

Labia majora are homologous to the scrotum