Development of the Urinary System

The ureteric bud penetrates the metanephric mass, which condenses around the diverticulum to form the metanephrogenic cap (Fig III-3-19). The bud ddates to form the renal pelvis, which subsequently splits into the cranial and caudal major calyces. Each major calyx buds into the metanephric tissue to form the minor calyces. One to 3 million collecting tubules develop from the minor calyces, thus forming the renal pyramids.

Penetration of collecting tubules into the metanephric mass induces cells of (he tissue cap to form nephrons, or excretory units.

• The proximal nephron forms Bowman's capsule, whereas the distal nephron connects to a collecting tubule.

■ Lengthening of the excretory tubule gives rise to the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.

The kidneys develop in the pelvis but appear to ascend into the abdomen as a result of fetal growth of the lumbar and sacral regions. With their ascent, the ureters elongate, and the kidneys become vascularized by lateral splanchnic arteries, which arise from the abdominal aorta.

Cloacal membrane

Urogenital sinus

Urorectal septum

Mesonephros

Rectum

Para-

mesonephric duct

Allantois bladder

Kidney

Mesonephros

Mesonephric duct

Ureter

Urogenital Anal membrane membrane

Rectum

Cloacal membrane

Urogenital sinus

Urorectal septum

Mesonephros

Rectum

Mesonephric duct Metanephric blastema

Para-

mesonephric duct

Allantois bladder

Kidney

Mesonephros

Mesonephric duct

Ureter

Urogenital Anal membrane membrane

Rectum

End of Week 5

End of Week 8

Figure 111-3-19. Development of the Urinary System

EMBRYOLOGY OF BLADDER AND URETHRA

The urorectal septum divides the cloaca into the anorectal canal and the urogenital sinus by week 7.

* The upper and largest part of the urogenital sinus becomes the urinary bladder, which is initially continuous with the allantois. As the lumen of the allantois becomes obliterated, a fibrous cord, the urachus, connects the apex of the bladder to the umbilicus. In the adult, this structure becomes the median umbilical ligament.

* The mucosa of the trigone of the bladder is formed by the incorporation of the caudal mesonephric ducts into the dorsal bladder wall. This mesodermal tissue is eventually replaced by endodermal epithelium so that the entire lining of the bladder is of endodermal origin.

* The smooth muscle of the bladder is derived from splanchnic mesoderm.

The male urethra is anatomically divided into three portions: prostatic, membranous, and spongy (penile).

* The prostatic urethra, membranous urethra, and proximal penile urethra develop from the narrow portion of the urogenital sinus below the urinary bladder.

* The distal spongy urethra is derived from the ectodermal cells of the glans penis.

The female urethra is derived from two sources. The upper two thirds develop from the mesonephric ducts, and the lower portion is derived from the urogenital sinus.

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