Development of Urinary System

Study Notes

During intrauterine life, three overlapping kidney systems are formed in a cranial to caudal sequence: pronephros, mesonephros, and metanephros

Derived from intermediate mesoderm in the upper thoracic to upper lumbar segments
Forms an S-shaped loop, acquiring a tuft of capillaries (glomerulus) and Bowman's capsule, creating a renal corpuscle
Tubules enter the collecting duct (mesonephric duct or Wolffian duct)
By the end of the 2nd month, most tubules disappear; in males, a few caudal tubules and the mesonephric duct persist, while in females, it disappears completely

Appears at 5 weeks, developing from metanephric mesoderm
Consists of the excretory system and collecting system, derived from different sources of intermediate mesoderm
The duct system differs from that of the other kidney systems

Collecting ducts develop from the ureteric bud, an outgrowth of the mesonephric duct
The bud penetrates the metanephric tissue, forming the primitive renal pelvis and major calyces
Further development forms minor calyces and collecting tubules, converging on the minor calyx to form the renal pyramid
The ureteric bud gives rise to:
- Ureter
- Renal pelvis
- Major and minor calyces
- Approximately 1-3 million collecting tubules

Newly formed collecting tubules are covered by metanephric tissue cap cells, which form small vesicles (renal vesicles or S-shaped tubules)
Capillaries grow in the S-shaped tubules, giving rise to glomeruli
Glomeruli and tubules together form nephrons (excretory units)
Bowman's capsule forms at the proximal end, with an open connection to the collecting tubules at the distal end
Longitudinal extension of the excretory tubules forms:
- Proximal convoluted tubules
- Loop of Henle
- Distal convoluted tubules
Nephrons are formed until birth (approximately 1 million in each kidney)

Urine production begins at 12 weeks
At birth, kidneys have a lobulated appearance, which disappears during infancy as a result of further growth of nephrons
There is no increase in nephron numbers after birth; kidney size increases mainly due to elongation of proximal convoluted tubules and an increase in interstitial tissue

Wilms tumor: a cancer of the kidneys that usually affects children by age 5 years, but may also occur in the fetus
Renal agenesis: severe malformations requiring dialysis and transplantation in the 1st year of life

Initially, kidneys are in the pelvic region, later shifting to a more cranial position in the abdomen due to diminution of body curvature and growth in the lumbar and sacral regions
The relocating kidney is revascularized by a series of arterial sprouts from the dorsal aorta, replacing the original renal artery in the sacral region
Abnormal location of the kidneys can occur, where one kidney remains in the pelvis, known as a pelvic kidney