Embryology Course, Second year, Faculty of Medicine, Damascus University

1. Genital System Development
www.marwanalhalabi.com
Marwan Alhalabi
Professor of Reproductive Medicine and Infertility,
Damascus University
Head of Assisted Reproduction Unit, Orient Hospital
President of Middle East Fertility Society
Past President of Syrian Society of Obstetricians and Gynecologists

2. The Urogenital System

3. Primordial Germ Cells

4. Neural tube
Dorsal aorta
Dorsal mesentery
Mesonephros
Gonadal ridges
Gut
Gonadal
Ridges

5. Gonadal Ridges

6. Indifferent Gonad

7. Indifferent Gonad

8. Indifferent Gonad

9. Indifferent Gonad

10. Genital System Development

11. Testis Development

12. Testis Development

13. Testis Development

14. Testis Development

15. Testis Development

16. Testis Development

17. Testis Development

18. Testis Development

19. Ovarian Development

20. Ovarian Development

21. Ovarian Development

22. Ovarian Development

23. Ovarian Development

24. Ovarian Development

25. Ovarian Development

26. Ovarian Development

27. Ovarian Development

28. GonadDevelopment

29. Genital Ducts

30. Wolffian ducts

31. Müllerian ducts

32. Reproductive Ducts

33. Reproductive Ducts

34. Male Reproductive Ducts

35. Male Reproductive Ducts

36. Male Reproductive Ducts

37. Male Reproductive Ducts

38. Male Accessory Glands

39. Male Accessory Glands

40. Female Reproductive Ducts

41. Female Reproductive Ducts

42. Female Reproductive Ducts

43. Female Reproductive Ducts

44. Female Reproductive Ducts

45. Female Reproductive Ducts

46. Female Reproductive Ducts

47. Uterus Development

48. Uterus Development

49. Vagina Development

50. Vagina Development

51. External
Genitalia

52. External Genitalia-Indifferent Stage

53. External Genitalia-Indifferent Stage

54. Indifferent Stage

55. Male External Genitalia

56. Male External Genitalia

57. Male External Genitalia

58. Male External Genitalia

59. Male External Genitalia

60. Male External Genitalia

61. Male External Genitalia

62. Male External Genitalia

63. Male External Genitalia

64. Male External Genitalia

65. Male External Genitalia

66. Blue = DHT
Brown = Testosterone
Male virilization

67. Female External Genitalia

68. Female External Genitalia

69. Female External Genitalia

71. External Genitalia

72. Descent of The Testis

73. Descent of The Testis

74. Descent of The Testis

75. Descent of The Testis

76. Descent of The Testis

77. Inguinal Canal Formation

78. Inguinal Canal Formation

80. Relocation of The Ovaries

81. Relocation of the Ovaries

82. 82
Thank you

84. Development of Gonads
• The gonads
(testes and
ovaries) are
derived from
three
sources:

85. Indifferent Gonads
• The initial stages of gonadal development occur during the
fifth week when 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—the gonadal ridge.
• Fingerlike epithelial cords—gonadal cords—soon grow
into the underlying mesenchyme.

86. Indifferent Gonads
• The indifferent gonad now consists of an external
cortex and an internal medulla.
• In embryos with an XX sex chromosome complex,
the cortex of the indifferent gonad 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.

88. Primordial Germ Cells
• Large, spherical sex cells are first recognizable at 24 days
after fertilization among the endodermal cells of the
umbilical vesicle near the origin of the allantois .
• During the sixth week, the primordial germ cells enter the
underlying mesenchyme and are incorporated in the
gonadal cords

89. Development Of Genital System
• Chromosomal sex of an embryo is determined at
fertilization by the kind of sperm (X or Y) that
fertilizes the oocyte.
• Male and female morphologic characteristics do
not begin to develop until the seventh week.

90. Development of Testes
• TDF (testis determining factor) induces the seminiferous cords to
condense and extend into the medulla of the indifferent gonad,
where they branch and anastomose to form the rete testis.
• Gradually the enlarging testis separates from the degenerating
mesonephros and is suspended by its own mesentery, the
mesorchium.
• The seminiferous cords develop into the seminiferous tubules, tubuli
recti, and rete testis.
• The seminiferous tubules are separated by mesenchyme that gives
rise to the interstitial cells (Leydig cells).
• By the eighth week, these cells begin to secrete androgenic
hormones—testosterone and androstenedione

91. Development of Testes
• In addition to testosterone (Steroli cells), the fetal testes produce a
glycoprotein, antimüllerian hormone (AMH) or müllerian-inhibiting
substance (MIS).
• AMH suppresses development of the paramesonephric ducts,
which form the uterus and uterine tubes.

92. Development of Testes
• The walls of the seminiferous tubules are composed of two
types of cells:
• Sertoli cells, supporting cells derived from the surface
epithelium of the testis
• Spermatogonia, primordial sperm cells derived from the
primordial germ cells

94. Development of Ovaries
• Gonadal development occurs slowly in female embryos.
• The ovary is not identifiable histologically until approximately
the 10th week.
• Cortical cords extend from the surface epithelium of the
developing ovary into the underlying mesenchyme during the
early fetal period.

95. Development of Ovaries
• As the cortical cords increase in size, primordial germ cells are
incorporated in them. At approximately 16 weeks, these cords
begin to break up into isolated cell clusters—primordial
follicles—each of which contains an oogonium, derived from a
primordial germ cell

97. Development of Genital Ducts
• During the fifth and sixth weeks, the genital system is an indifferent
state, and two pairs of genital ducts are present:
• The mesonephric ducts (Wolffian ducts) play an important part in the
development of the male reproductive system and
• The paramesonephric ducts (müllerian ducts) have a leading role in the
development of the female reproductive system.

98. Development of Male Genital
Ducts and Glands
• Testosterone stimulates the mesonephric ducts to form male
genital ducts, whereas MIS causes the paramesonephric ducts to
regress.
• Under the influence of testosterone, each mesonephric duct
becomes the epididymis (proximal), Efferent ductules and a
muscular ductus deference (Distal).

101. Development of Female Genital
Ducts and Glands
• The mesonephric ducts of female embryos regress because
of the absence of testosterone.
• The paramesonephric ducts develop because of the absence
of MIS. Female sexual development during the fetal period
does not depend on the presence of ovaries or hormones.
• Later, estrogens produced by the maternal ovaries and the
placenta stimulate development of the uterine tube, uterus,
and the superior part of the vagina.

103. Development of Female Genital
Ducts and Glands
• The paramesonephric ducts form most of the female genital tract.
• The uterine tubes develop from the unfused cranial parts of these
ducts .
• The caudal fused portions form the uterovaginal primordium, which
gives rise to the uterus and the superior part of the vagina.
• The endometrial stroma and myometrium are derived from
splanchnic mesenchyme.
• Auxiliary Genital Glands in Females: Outgrowths from the urethra
into the surrounding mesenchyme form the urethral glands and
paraurethral glands

104. Development of Vagina
The fibromuscular wall of the vagina develops from the surrounding mesenchyme.
Contact of the uterovaginal primordium with the urogenital sinus, forming the sinus tubercle ,
induces the formation of paired endodermal outgrowths—the sinovaginal bulbs.
They extend from the urogenital sinus to the caudal end of the uterovaginal primordium. The
sinovaginal bulbs fuse to form a vaginal plate.
Later the central cells of this plate break down, forming the lumen of the vagina.
The hymen usually ruptures during the perinatal period and remains as a thin fold of mucous
membrane just within the vaginal orifice.

107. Development of External
Genitalia
• Up to the seventh week, the external genitalia are similar in both
sexes.
• Distinguishing sexual characteristics begin to appear during the 9th
week, but the external genitalia are not fully differentiated until the
12th week.
• Early in the fourth week, proliferating mesenchyme produces a
genital tubercle (primordium of penis or clitoris) in both sexes at the
cranial end of the cloacal membrane.
• Labioscrotal swellings and urogenital folds soon develop on each
side of the cloacal membrane.

109. Development of Male External Genitalia
• Masculinization of the indifferent external genitalia is induced by testosterone .
• The urethral folds fuse with each other along the ventral surface of the penis to
form the spongy urethra.
• The surface ectoderm fuses in the median plane of the penis, forming the
penile raphe and enclosing the spongy urethra within the penis.
• During the 12th week, a circular ingrowth of ectoderm occurs at the periphery
of the glans penis.
• When this ingrowth breaks down, it forms the prepuce (foreskin).
• The corpora cavernosa and corpus spongiosum of the penis develop from
mesenchyme in the phallus (forms penis).
• The labioscrotal swellings grow toward each other and fuse to form the
scrotum. The line of fusion of these folds is clearly visible as the scrotal raphe.

110. Development of Female
External Genitalia
• The Primordial Phallus In The Female Fetus Gradually Becomes The Clitoris.
• The Clitoris Is Still Relatively Large At 18 Weeks. The Urethral Folds Do Not Fuse,
Except Posteriorly, Where They Join 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 Commissure
And Anteriorly To Form The Anterior Labial Commissure And Mons Pubis.
• Most Parts Of The Labioscrotal Folds Remain Unfused, But Develop Into Two Large
Folds Of Skin, The Labia Majora.

111. Development of Inguinal Canals
• The inguinal canals form pathways for the testes to descend from the
abdominal wall into the scrotum. Inguinal canals develop in both
sexes.
• As the mesonephros degenerates, a ligament—the gubernaculum—
develops on each side of the abdomen from the caudal pole of the
gonad.
• The gubernaculum attaches caudally to the internal surface of the
labioscrotal swellings (future halves of the scrotum or labia majora).

113. Relocation of Testes And Ovaries:
Testicular Descent
• Testicular descent is associated with:
• Enlargement of the testes and atrophy of the mesonephroi allowing
movement of the testes caudally along the posterior abdominal wall
• Atrophy of the paramesonephric ducts induced by the müllerian-
inhibiting substance (MIS), enabling the testes to move
transabdominally to the deep inguinal rings.
• Enlargement of the processus vaginalis guiding the testis through the
inguinal canal into the scrotum

116. Ovarian Descent
• The ovaries also descend from the lumbar region of the posterior
abdominal wall and relocate to the lateral wall of the pelvis.
• The gubernaculum is attached to the uterus near the attachment of
the uterine tube. The cranial part of the gubernaculum becomes the
ovarian ligament, and the caudal part forms the round ligament of
the uterus.
• The relatively small processus vaginalis in the female usually
obliterates and disappears long before birth

117. Acknowledgement