DEVELOPMENT OF FEMALE GENITAL SYSTEM

1. DEVELOPMENT OF FEMALE GENITAL SYSTEM By: Dr. Sharon D. Rasquinha

2. 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

3. 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

5. 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

6. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

14. 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

15. 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

17. 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

18. 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

20. 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

22. 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

24. 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

25. 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

27. 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

28. 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

30. 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

32. 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

34. 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

35. 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

37. 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

38. 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

40. 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

41. 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

43. 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