Gonadal development occurs slowly in females The X chromosmes bear genes for

ovarian development Autosomal gene also appears to play a

role in ovarian development Until 10th week: ovary cannot be

identified histologically

Gonadal cords do not become prominent. They extend into the

medulla and form rete ovarii, which is rudimentary. Normally, it degenerates

and disappears as well as the gonadal cords.

Cortical cords extend from the surface

epithelium of the developing ovary into

the underlying mesenchyme

Time: early fetal period

Epithelium: mesothelium-derived As the cortical cords increase in size, primordial germ cells are incorporated in them

Cords begin to break up

forming primordial follicles

Time: about 16 weeks

Formed at about 16 weeks Each consists of:

Oogonium: derived from a primordial germ cell

Surrounding it: a layer of flattened follicular cells

Derived from surface epithelium Active mitosis of oogonia occurs during

fetal life producing thousands of primordial follicles No oogonia form postatally. Many oogonia degenerate before birth About 2 million remain enlarge to become

primary oocytes before birth

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

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

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

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

The caudal fused portions of these ducts form the uterovaginal primordium It gives rise to

uterus and superior part of vagina The uterine tubes develop

from the unfused cranial part of the paramesonephric ducts

The endometrial stroma and myometrium are derived from splanchnic mesenchyme

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

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

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

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

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

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