Chapter 16 - Urogenital System
1. Development of the Urinary System
The urinary system develops in three stages from the intermediate mesoderm along the posterior
abdominal wall. It includes the kidneys, ureters, bladder, and urethra.
Kidney Systems: Sequential Development
Three overlapping kidney systems form in a cranial-to-caudal sequence:
1. Pronephros (Rudimentary Kidney):
o Timing: Early 4th week.
o Location: Cervical region.
o Structure:
§ Composed of 7–10 pairs of nephrotomes.
§ Primitive and nonfunctional in humans.
o Fate: Entire system degenerates by the end of the 4th week, but its duct persists as
the pronephric duct, contributing to later systems.
2. Mesonephros (Interim Kidney):
o Timing: Forms late in the 4th week and functions between the 6th–10th weeks.
o Location: Thoracic and upper lumbar regions (T1–L3).
o Structure:
§ Mesonephric tubules:
§ Develop S-shaped loops with Bowman’s capsule surrounding
glomeruli.
§ Mesonephric duct: (or wolffian duct)
§ Longitudinal duct that connects to the cloaca.
o Function:
§ Produces dilute urine temporarily during development.
o Fate:
§ Males: Some tubules persist to form parts of the reproductive system (e.g.,
epididymis, ductus deferens, seminal vesicles).
§ Females: The system degenerates almost completely, leaving vestigial
remnants (e.g., Gartner’s ducts).
3. Metanephros (Definitive Kidney):
o Timing: Forms in the 5th week, functional by the 10th week.
o Location: Pelvic region, later migrates cranially.
o Structure:
, § Ureteric bud:
§ Grows from the mesonephric duct.
§ Gives rise to the collecting system (ureters, renal pelvis, calyces,
and collecting ducts).
§ Metanephric mesoderm (blastema):
§ Condensed mesoderm forms the nephron structures: glomeruli,
proximal/distal tubules, and loop of Henle.
o Nephron Development:
§ Nephron formation begins with epithelialization of the mesenchymal cells
induced by the ureteric bud.
§ Functional nephrons form by the 10th–12th week.
Formation of the Collecting System
• Ureteric Bud Branching:
o Sequential branching forms:
§ Major calyces from the first few generations of branches.
§ Minor calyces and collecting ducts from subsequent branching.
o Branching morphogenesis is tightly regulated by molecular signals (e.g., GDNF, HGF,
and WNT pathways).
Developmental Movement: Kidney Ascent
• Initially located in the pelvic region.
• Between the 6th–9th weeks, kidneys ascend to the lumbar region, with rotation:
o Hilum faces ventrally initially, then rotates medially.
• Blood supply transitions from common iliac arteries to the abdominal aorta.
Molecular Regulation of Kidney Development
• WT1 (Wilms Tumor 1):
o Expressed in metanephric mesenchyme, ensuring responsiveness to ureteric bud
signals.
• Signals from Mesenchyme:
o GDNF and HGF:
§ Stimulate ureteric bud growth.
o FGF2 and BMP7:
§ Prevent apoptosis of mesenchymal cells.
• Signals from Ureteric Bud:
1. Development of the Urinary System
The urinary system develops in three stages from the intermediate mesoderm along the posterior
abdominal wall. It includes the kidneys, ureters, bladder, and urethra.
Kidney Systems: Sequential Development
Three overlapping kidney systems form in a cranial-to-caudal sequence:
1. Pronephros (Rudimentary Kidney):
o Timing: Early 4th week.
o Location: Cervical region.
o Structure:
§ Composed of 7–10 pairs of nephrotomes.
§ Primitive and nonfunctional in humans.
o Fate: Entire system degenerates by the end of the 4th week, but its duct persists as
the pronephric duct, contributing to later systems.
2. Mesonephros (Interim Kidney):
o Timing: Forms late in the 4th week and functions between the 6th–10th weeks.
o Location: Thoracic and upper lumbar regions (T1–L3).
o Structure:
§ Mesonephric tubules:
§ Develop S-shaped loops with Bowman’s capsule surrounding
glomeruli.
§ Mesonephric duct: (or wolffian duct)
§ Longitudinal duct that connects to the cloaca.
o Function:
§ Produces dilute urine temporarily during development.
o Fate:
§ Males: Some tubules persist to form parts of the reproductive system (e.g.,
epididymis, ductus deferens, seminal vesicles).
§ Females: The system degenerates almost completely, leaving vestigial
remnants (e.g., Gartner’s ducts).
3. Metanephros (Definitive Kidney):
o Timing: Forms in the 5th week, functional by the 10th week.
o Location: Pelvic region, later migrates cranially.
o Structure:
, § Ureteric bud:
§ Grows from the mesonephric duct.
§ Gives rise to the collecting system (ureters, renal pelvis, calyces,
and collecting ducts).
§ Metanephric mesoderm (blastema):
§ Condensed mesoderm forms the nephron structures: glomeruli,
proximal/distal tubules, and loop of Henle.
o Nephron Development:
§ Nephron formation begins with epithelialization of the mesenchymal cells
induced by the ureteric bud.
§ Functional nephrons form by the 10th–12th week.
Formation of the Collecting System
• Ureteric Bud Branching:
o Sequential branching forms:
§ Major calyces from the first few generations of branches.
§ Minor calyces and collecting ducts from subsequent branching.
o Branching morphogenesis is tightly regulated by molecular signals (e.g., GDNF, HGF,
and WNT pathways).
Developmental Movement: Kidney Ascent
• Initially located in the pelvic region.
• Between the 6th–9th weeks, kidneys ascend to the lumbar region, with rotation:
o Hilum faces ventrally initially, then rotates medially.
• Blood supply transitions from common iliac arteries to the abdominal aorta.
Molecular Regulation of Kidney Development
• WT1 (Wilms Tumor 1):
o Expressed in metanephric mesenchyme, ensuring responsiveness to ureteric bud
signals.
• Signals from Mesenchyme:
o GDNF and HGF:
§ Stimulate ureteric bud growth.
o FGF2 and BMP7:
§ Prevent apoptosis of mesenchymal cells.
• Signals from Ureteric Bud:
