Development of urogenital system II

1. Development of Urogenital System- II
Dr. Prabhakar Yadav
Assistant Professor
Department of Human Anatomy
B.P. Koirala Institute of Health Sciences

2. Genital system: Consists of-
(a) Pair of gonads (testes or ovaries)
(b) Duct system of gonads
(c) external genital organs.
Genital system develops from 3 sources:
● Intermediate mesoderm
● Celomic epithelium covering the intermediate mesoderm
● Part of cloaca

3. Genetic sex ( XY or XX) is established at time of fertilization
4th week of IUL - Development of genital system begins
Indifferent stage of gonad- Up to 6 Wk. of IUL gonads are structurally similar
7th week of IUL- Gonads acquire morphological characteristics of Male or Female
12th week -Some male & female characteristics of external genitalia can be recognized.
20th week- Phenotypical differentiation is complete.
Phenotypical differentiation is determined by SRY ( Sex determining region on Y) gene located on short arm of Y
chromosome (Yp11).

4. SRY gene encodes for protein- Testis determining factor (TDF).
Presence of TDF leads to development of male genital organs
As indifferent gonad develops into testis, Leydig & Sertoli cells
produce testosterone & Mullerian inhibiting factor (MIF) which
results in phenotypically Male embryo.
In Absence of TDF, testosterone & MIF, indifferent gonad will
develop into ovary & embryo will be phenotypically female.

5. Gonads develop from 3 sources:
1. Intermediate mesoderm
2. Celomic epithelium covering intermediate mesoderm
3. Primordial germ cells.
Indifferent Stage: development of testis or ovary is similar
(gonads develop in intermediate mesoderm— medial to middle part of
mesonephros.)
4th week- genital ridge appears on medial side of mesonephric ridge.

6. Primordial germ cells: originate in epiblast, migrate through primitive streak
3rd week - resides in the wall of yolk sac close to allantois.
4th week- Migrate along wall of hindgut & dorsal mesentery of hindgut
5th week- reach the genital ridges

7. 6th week- invade the genital ridges
Cells of celomic epithelium proliferate & penetrate underlying
mesenchyme to form cords- primitive sex cords.
Primordial germ cells: have an inductive influence on development of
gonads into testis or ovary.
If the primordial germ cells fail to reach the genital ridges, gonads do not
develop.

8. Indifferent gonad: In both male & female embryos, primitive
sex cords are connected to celomic epithelium & it is
impossible to differentiate between male & female gonads.
Indifferent gonad:
Outer part – cortex
Central part -Medulla.
Definitive stage:
Development of testis or ovary depends upon chromosome
complement of primordial germ cells.
If sex chromosome complement is XY -develop into testis
If sex chromosome complement is XY - develop into ovary

9. Development of Testis
If sex chromosome complement is XY
• Under the influence of SRY gene that encodes for TDF,
------Primitive sex cords increase in length & extend into
medulla of indifferent gonad- testis or medullary cords.
In hilum, testis cords break up & anastomose with each
other to form rete testis
Testis cords then become horseshoe shaped & get
canalized to form seminiferous tubules.

10. 4th month: communication is established between testis cord & rete testis.
The seminiferous tubules are lined by two types of cells:
(a) sex cord cells (cells derived from surface celomic epithelium)- form sustentacular cells of Sertoli.
(b) primordial germ cells (derived from wall of the yolk sac)-form spermatogonia
Further development:
Mesoderm migrates beneath the celomicepithelium, forms dense layer of fibrous connective tissue tunica
albuginea -separates sex cords from celomic epithelium permanently.

11. Interstitial cells of Leydig:
• Derived from mesoderm.
• Lie between testis cords
• By 8th week -Leydig cells- produce- testosterone &
testis - influence sexual differentiation
of genital ducts & external genitalia.
 Until puberty- Testis cords remain solid
 Puberty- acquire lumen- form- seminiferous tubules.

12. • Canalized seminiferous tubules join rete testis tubules,
• Rete testis tubules enter ductuli efferentes- remaining mesonephric tubules .
• Ductuli efferentes / Efferent ductules link the rete testis & mesonephric (wolffian) duct, which becomes ductus
deferens.

13. Development of Ovary
If sex chromosome complement is XY
Primitive ( medullary )sex cords containing primordial germ cells extend into medulla
They dissociate into irregular cell clusters- - rete ovarii
rete ovarii - occupy medullary part of ovary close to mesonephric tubules.
Later rete ovarii disappear & are replaced by a vascular stroma that forms ovarian medulla .

14. Celomic epithelium of female gonad, continues to proliferate and in 7th week forms second generation of sex cords-
do not extend into medulla- cortical cords.
3rd month- cortical cords get fragmented & form isolated cell clusters.
Each cell cluster consists of a primordial germ cell in the center surrounded by a layer of celomic epithelial cells.
Primordial germ cells form: oogonia
celomic epithelial cells form: follicular cells.

15. Primordial germ cells form: oogonia
celomic epithelial cells form: follicular cells.
Resulting structure is: primordial follicle - remain confined in cortex of ovary.
• Primordial follicles are formed during fetal life.
• No new primordial follicles are formed after birth.
• Further development of primordial follicles takes place after puberty.

16. Mesoderm does not form tunica albuginea around ovary.
Therefore ovarian follicles are not separated from surface epithelium of ovary.

17. Development of Genital Ducts
Genital ducts of male:
vasa efferentia, duct of epididymis, vas deferens & ejaculatory duct
Genital ducts of female: Fallopian tube, uterus & vagina.
Indifferent Stage
Both male & female embryos have two pairs of genital ducts:
Mesonephric (wolffian) ducts &
Paramesonephric (müllerian) ducts.
Paramesonephric duct arises as a longitudinal invagination of
epithelium on the anterolateral surface of urogenital ridge
Cranially, opens into abdominal cavity with a funnel-like structure.
Caudally, it first runs lateral to the mesonephric duct, then crosses
mesonephric duct ventrally to grow caudomedially

18. In the midline, it comes in contact with paramesonephric duct
from opposite side.
Two ducts are initially separated by a septum but later fuse to
form uterovaginal canal.
Caudal end of uterovaginal canal projects into posterior wall of
phallic part of the definitive urogenital sinus, where it causes a
small swelling, -paramesonephric or müllerian tubercle.
Mesonephric ducts open into urogenital sinus on either side of
the müllerian tubercle.

19. Genital Ducts in the Female
Paramesonephric ducts. Initially, 3 parts can be recognized:
(1) cranial vertical portion that opens into abdominal cavity
(2) horizontal part that crosses mesonephric duct
(3) caudal vertical part that fuses with its partner from opposite side
First two parts develop into uterine tube
caudal parts fuse to form uterus/ Upper part of vagina.
Uterus divide into Body/corpus & cervix .
Surrounding mesoderm forms -myometrium & perimetrium.

20. After the ducts fuse in midline,
• Broad transverse pelvic fold is established- which extends from lateral sides of fused paramesonephric ducts
toward wall of pelvis -broad ligament of uterus.
• Uterine tube lies in its upper border & ovary lies on its posterior surface.
• Uterus & broad ligaments divide pelvic cavity into uterorectal pouch & uterovesical pouch.

21. Genital Ducts in Male
• As mesonephros regresses, few mesonephric tubules- epigenital tubules, establish contact with cords of rete testis & form
efferent ductules .
• Mesonephric tubules along caudal pole of testis- paragenital tubules, do not join the cords of the rete testis; paradidymis.
• Most of the mesonephric ducts persist & form main genital ducts.
• Most cranial portion of mesonephric ducts - appendix epididymis

22. • Mesonephric ducts elongate & become highly convoluted, forming epididymis.
• From the tail of epididymis to outbudding of seminal vesicle, mesonephric ducts obtain a thick muscular coat &
form ductus deferens.
• Mesonephric ducts beyond seminal vesicles: ejaculatory duct.
Under the influence of antimullerian hormone (AMH) produced by sertoli cells, paramesonephric ducts in the male
degenerate except for a small portion at their cranial ends : appendix testis

23. uterovaginal canal comes in contact with dorsal wall of
phallic part of definitive urogenital sinus
Formation of two sinovaginal bulbs
Endodermal cells of sinovaginal bulbs proliferate
form vaginal plate.
cells from tip of the uterovaginal canal (mesodermal) also
proliferate and add to vaginal plate.
vaginal plate is interposed between the uterovaginal canal
& urogenital sinus.
Upper end of vaginal plate expands & surrounds caudal end
of uterovaginal canal.
By fifth month vaginal plate is canalized to form lumen e
wing-like expansion of vagina around cervix forms : fornices
of vagina.
vagina remains separated from phallic part by : hymen
which consists of a thin layer of vaginal
Both the surfaces of the hymen are lined by endoderm.

24. Female retain some remnants excretory tubules, where they form epoophoron & paroophoron .
Mesonephric duct -may form Gartner’s cyst

25. Development of prostate: Develops from five epithelial buds (one anterior, two posterior & two lateral)
Epithelial buds arise from epithelium of prostatic urethra (caudal part of vesicourethral canal & pelvic part of definitive
urogenital sinus) and grow in the surrounding mesoderm.
Buds arising from mesodermal posterior wall of urethra above the openings of ejaculatory ducts form inner glandular zone
Buds arising from rest of the prostatic urethra (endodermal in origin) form outer glandular zone of the prostate.
Surrounding mesoderm forms dense fibrous stroma, smooth muscle of prostate gland & prostatic capsule.

26. EXTERNAL GENITALIA
Indifferent Stage

27. External Genitalia in the Male
under the influence of androgen
most distal portion of the urethra is formed during 4th month, ectodermal
external urethral meatus

28. External Genitalia in the Female:
Estrogens stimulate development
Genital tubercle forms clitoris
urethral folds:labia minora.
Genital swellings :labia majora.
Urogenital groove is open and forms: vestibule