Development of Urinary System

Development of the Urinary System

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

Pronephros

• Forms at 4 weeks, represented by 7-10 solid cell groups in the cervical region
• Non-functional and vestigial, regressing later on

Mesonephros

• 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

Metanephros (Definitive Kidney)

• 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 System

• 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

Excretory System

• 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)

Kidney Development

• 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

Abnormalities

• 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

Position of the Kidney

• 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