1. DEVELOPMENT OF FEMALE GENITAL SYSTEM

3. 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

8. 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

10. 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

11. 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

13. 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)

14. 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

15. 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

17. 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 10 th week

18. 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

20. 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

21. 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

23. 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

24. 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

26. 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

28. 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

30. 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

31. 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

33. 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

34. Genital duct development in female Paramesonephric ducts develop into the main genital ducts Initially 3 parts can be recognized in each duct:  Cranial vertical portion that opens into the abdominal cavity....develop into uterine tube  Horizontal part that crosses the mesonephric duct...develop into uterine tube  Caudal vertical part that fuses with its partner from the opposite side...fuse to form uterine canal

35. Development of Vagina After the solid tip of the mesonephric ucts reaches the urogenital sinus; two solid evaginations (sinovaginal bulbs) grow out from the pelvic part of the sinus. Sinovaginal bulbs proliferate and form a solid vaginal plate. Proliferation continues at the cranial end of the plate; increasing the distance between the uterus and the urogenital sinus. By the 5th month vaginal outgrowth is entirely canalized. Vaginal fornices (wing-like expansions of the vagina around the end of uterus) are of paramesonephric origin

36. Development of Vagina Thus the vagina has two origines: upper portion derived from the uterine canal lower portion derived from the urogenital sinus Lumen of the vagina remains separated from that of the urogenital sinus by a thin tissue plate; the hymen Hymen consists of epithelial lining of the sinus and a thin layer of vaginal cells. It usualy develops an opening during perinatal life

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

46. 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