Module
21
Icon module 21
Genital System

Male: differentiation of the urogenital sinus

In studies of the urinary system we have seen that the cloaca is subdivided by the uro-rectal septum into the rectum (dorsal) and the primary urogenital sinus (ventral).

The primitive urogenital sinus is in contact with the allantois. The upper part forms the future urinary bladder, while the middle and lower part forms the definitive urogenital sinus and, from it, the pelvic and phallic parts of the urethra (stage 17).

Fig. 39 - Development of the urogenital sinus
stage 13 (ca. 32nd day)
media/module21/u3yf1_devuretre.gif

1
Allantois
2
Cloaca
3
Urinary bladder

Fig. 40 - Development of the urogenital sinus
stage 23 (ca. 56th day)
media/module21/u3yf2_devuretre.gif

4
Phallic part of the definitive urogenital sinus
5
Rectum
6
Pelvic part of the definitive urogenital sinus

Legend
Fig. 39

The cloaca subdivides. Observe that the metanephros in this stage still lies quite caudally.

Fig. 40

The uro-rectal septum has subdivided the cloaca into the primary urogenital sinus (ventrally) and the rectum (dorsally).
Out of the primary urogenital sinus arise the urinary bladder and the definitive urogenital sinus.

The pelvic part of the definitive urogenital sinus forms the prostatic and membranous parts of the urethra, the phallic part of the definitive urogenital sinus, the spongy part of the urethra and the urethral vestibule.
Around the 4th month an epithelial cord grows on the glans penis - into the depths - and binds it with the spongy part of the urethra.

Fig. 41 - Development of the urethra
ca. 12th week
media/module21/u3yf3_devuretre.gif

  1. Urachus
  2. Rectum
  3. Urinary bladder
  4. Ureter
  5. Urethra, prostatic part
    (orange zone)
  6. Membranous part of the urethra
  7. Spongy part of the urethra
  8. Seminal vesicle
  9. Bulbourethral gland (Cowper's)
  10. Deferent duct

Legend
Fig. 41

The spongy part of the urethra comes to a dead end in the outer section of the penis. Only in the 4th month does an epithelial lamella form from the tip of the penis to the blind end of the urethra. From this arises the urethra within the glans.

Summary:
From the mesonephric duct (Wolff) arise:

  • Epididymis
  • Deferent duct
  • Seminal vesicle
  • Ejaculatory duct

From the endoderm of the urogenital sinus arise:

  • Urinary bladder
  • Out of the pelvic part of the definitive urogenital sinus: the prostatic and membranous parts of the urethra, the prostate and the bulbourethral gland (Cowper's).
  • Out of the phallic part of the definitive urogenital sinus: the spongy part of the urethra and the urethral glands (Littre's)

.