INDIANA UNIVERSITY
SCHOOL OF MEDICINE
The urogenital system develops from:
• the intermediate mesoderm (fig. 12-1B),
• the mesodermal epithelium (mesothelium) of the peritoneal cavity,
• and the endoderm of the urogenital sinus (fig. 12-20A).
The intermediate mesoderm used to lie lateral to the somites, then moved away from the somites during the lateral fold. It forms the urogenital ridge (fig. 12-1F) which is comprised of:
• a nephrogenic cord or ridge (fig. 12-2A)
• and a gonadal or genital ridge (fig. 12-29C).
3 successive sets of kidneys develop:
• The nonfunctional, rudimentary pronephroi develop early in week 4. But they degenerate, leaving behind the pronephric ducts which run to the cloaca (fig. 12-2). These ducts will remain for other kidneys.
• The mesonephroi develop later during week 4, serving as temporary excretory organs.
• The functional metanephroi or permanent kidneys develop early in week 5. They are functional by week 11-13 and excrete urine into the amniotic fluid. This excretion continues during fetal life and the fetus swallows this urine mixed in the amniotic fluid. It is then absorbed in the stomach and duodenum to the blood for transport to the placenta and disposal.
o If renal agenesis or urethral obstruction occurs, oligohydramnios results.
o If esophageal or duodenal atresia occurs, then polyhydramnios results.
The metanephros develops mesodermally from the metanephric diverticulum or ureteric bud which is a dorsal outgrowth from the mesonephric duct near the cloaca (fig. 12-6).
• Its stalk gives rise to the ureter (fig. 12-6C),
• its cranial end to the renal pelvis,
• its first 4 generations of tubules to the major calyces,
• its second 4 generations to the minor calyces (fig. 12-6D)
• and the remaining generations of tubules to the collecting tubules (fig. 12-6E).
The metanephric diverticulum or ureteric bud penetrates the metanephric mesoderm in the caudal part of the nephrogenic cord and stimulates the formation of the metanephric mass or cap (fig. 12-9).
The metanephric mesoderm gives rise to the nephrons (glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle and distal convoluted tubule; fig. 12-7). The cortex of the kidney in the newborn contains mostly undifferentiated mesenchyme; the nephrons continue to develop several months after birth.
Ascension of the kidneys (fig. 12-10): The kidneys are first located in the pelvis ventral to the sacrum but gradually ascend to the abdomen. They reach the adult position by week 9 having touched the suprarenal glands (fig. 12-10). This is due to the disproportionate growth between the lumbar and sacral regions: the sacral region grows faster than the lumbar region.
The kidneys rotate 90 degrees from anterior to medial.
During their ascension, the blood supply changes continuously so that an adult may have 2 to 4 renal arteries (fig. 12-11).
The suprarenal glands ( fig. 12-27):
• The cortex forms from the mesoderm,
• the medulla from neural crest cells (receiving preganglionic sympathetic fibers from the celiac plexus).
The urinary bladder develops from the urogenital sinus and the surrounding splanchnic mesenchyme (fig. 12-20). The urogenital sinus is comprised of 3 regions:
• The cranial or vesical region which will form the bladder and which is attached to the allantois. After birth, the allantois degenerates and becomes the urachus forming the median umbilical ligament. The transitional epithelium of the bladder develops from endoderm of the urogenital sinus.
• The middle or pelvic region.
• and the caudal or phallic region.
The female urethra and almost all of the male urethra have the same origin.
The glans penis in the male develops from the ectodermal glandular plate (figs. 12-24, 12-25)
Developmental abnormalities of the kidney and excretory passages are common:
• Incomplete division of the metanephric diverticulum or ureteric bud results in double ureter (fig. 12-12B-D) and supernumerary kidney (fig. 12-12F).
• Failure of the kidney to "ascend" from its embryonic position in the pelvis results in an ectopic kidney that is abnormally rotated (fig. 12-12B).
• Various congenital cystic conditions of the kidneys may result from failure of nephrons derived from the metanephric mesoderm to connect with collecting tubules derived from the metanephric diverticulum.
