Development of the Urinary System

Introduction

• Focus on the development of the urinary system.
• Key foundational structure: Intermediate Mesoderm.

Formation of Germ Layers

• Bilaminar Disc: Week 2
  • Epiblast (blue cells)
  • Hypoblast (green cells)
• Gastrulation: Formation of three germ layers
  • Endoderm (replaces hypoblast)
  • Mesoderm (new layer above endoderm)
  • Ectoderm (from epiblast)

Mesoderm Differentiation

• Three Components of Mesoderm:
  1. Paraxial Mesoderm
  2. Intermediate Mesoderm (focus of lecture)
  3. Lateral Plate Mesoderm
     • Somatic Layer (with ectoderm)
     • Splanchnic Layer (with endoderm)

Folding and Development

• Folding Process: Lateral folding and cranial-caudal folding.
• Urogenital Ridge: Formed from the intermediate mesoderm.
  • Nephrogenic Cord: Core structure for urinary system.

Development of Nephrogenic Cord

• Week 4: Development of Pronephros in the cervical region.
  • Includes duct and nephrotomes.
  • Degenerates by the end of Week 4.
• Mesonephros: Develops from the thoracic to lumbar region.
  • Primitive urinary system until Week 10.
  • Includes mesonephric duct and tubules.

Advanced Development

• Metanephros: Develops in the pelvic region.
  • Metanephric blastoma releases growth factors.
  • Stimulates formation of the ureteric bud from mesonephric duct.

Reciprocal Induction

• Ureteric bud and metanephric blastoma grow through reciprocal induction.
• Development of collecting system:
  • Ureteric Bud → Renal Pelvis → Major Calyx → Minor Calyx → Collecting Tubules

Formation of the Nephron

• Metanephric Mesoderm Cap: Becomes metanephric vesicle → Metanephric tubule.
• Differentiation into nephron components:
  • Bowman's Capsule
  • Proximal and Distal Convoluted Tubules
  • Loop of Henle

Kidney Development

• Kidneys ascend from the pelvic region to upper quadrants.
• Blood Supply Shift: From common iliacs to renal arteries.

Development of Bladder and Urethra

• Cloaca Division: Via urorectal septum
  • Urogenital Sinus: Becomes bladder and urethra
  • Hindgut: Becomes anal canal
  • Ureter connects to bladder
• Urachus Formation: Allantois becomes median umbilical ligament.

Final Notes

• Primitive urinary system is replaced by metanephros by Week 10.
• Understanding the interplay between different mesodermal structures and growth factors is crucial.

Remember, the development of the urinary system involves intricate interactions between embryonic structures and processes, making growth factors and mesodermal layers pivotal.