Abc

Figure 14.12 Divisions of the cloaca into the urogenital sinus and anorectal canal. The mesonephric duct is gradually absorbed into the wall of the urogenital sinus, and the ureters enter separately. A. At the end of the fifth week. B. 7 weeks. C. 8 weeks.

(see Chapter 13). The urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus. The tip of the septum will form the perineal body (Fig. 14.12C). Three portions of the urogenital sinus can be distinguished: The upper and largest part is the urinary bladder (Fig. 14.13A). Initially the bladder is continuous with the allantois, but when the lumen of the allantois is obliterated, a thick fibrous cord, the urachus, remains and connects the apex of the bladder with the umbilicus (Fig. 14.13B). In the adult, it is known as the median umbilical ligament. The next part is a rather narrow canal, the pelvic part of the urogenital sinus, which in the male gives rise to the prostatic and membranous parts of the urethra. The last part is the phallic part of the urogenital sinus. It is flattened from side to side, and as the genital tubercle grows, this part of the sinus will be pulled ventrally (Fig. 14.13A). (Development of the phallic part of the urogenital sinus differs greatly between the two sexes: see Genital System; p. 337.)

During differentiation of the cloaca, the caudal portions of the mesonephric ducts are absorbed into the wall of the urinary bladder (Fig. 14.14). Consequently, the ureters, initially outgrowths from the mesonephric ducts, enter the bladder separately (Fig. 14.14B). As a result of ascent of the kidneys, the orifices of the ureters move farther cranially; those of the mesonephric ducts move close together to enter the prostatic urethra and in the male become the ejaculatory ducts (Fig. 14.14, C and D). Since both the mesonephric ducts and ureters originate in the mesoderm, the mucosa of the bladder formed by incorporation of the ducts (the trigone of the bladder) is also mesodermal. With time the mesodermal lining of the trigone is replaced by endodermal epithelium, so that finally the inside of the bladder is completely lined with endodermal epithelium.

Allantois

Pelvic part of urogenital sinus

Urinary bladder

Urachus

Urinary bladder

Allantois

Pelvic part of urogenital sinus

Allantois Urogenital Sinus

Definitive urogenital sinus

Anorectal canal

Definitive urogenital sinus

Anorectal canal

Urachus

Prostate gland

Pelvis Part Urogenital Sinus

Penile urethra

Prostatic and membranous urethra

Prostate gland

Penile urethra

Prostatic and membranous urethra

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