Intermediate Mesoderm Development - Anatomy & Physiology


Introduction

The intermediate mesoderm exists as a strip of tissue between the lateral plate mesoderm and somites. It gives rise to the urinary system and some parts of the reproductive system. Kidney development includes three forms:

Pronephros
Mesonephros
Metanephros

Mammals develop all three, and continue to use the metanephros in adult life. More primitive animals have only the first one or two.

Pronephros

  • The earliest kidney. Develops in the anterior of the animal (adjacent to somites 1 - 10). Eliminated in later development.
  1. At intervals along the intermediate mesoderm, dorsal evaginations occur.
  2. Evaginations extend caudally and eventually fuse with more caudal evaginations.
    • The caudal - most evagination fuses with the cloaca.
  3. Forms a continuous excretory tube to the cloaca.
  4. Capillaries develop at the medial aspect of the intermediate mesoderm so that waste can diffuse from the blood to the excretory channel.
  5. Connections between the intermediate mesoderm and somites as well between the intermediate mesoderm and lateral plate mesoderm break down.

Mesonephros

  • Development occurs in an anterior to posterior direction; the mesonephros kidney is found at the thoracic to lumbar level.
  1. The medial portion of the intermediate mesoderm forms a cup in the region of the capillaries.
    • This is an early Bowman's capsule and improves waste removal.
  2. The excretory channel pinches off from all surrounding tissue (lateral plate mesoderm) to become entirely closed.
    • Excretory duct now known as the Wolffian Duct.
  • Amphibians and fish use the mesonephric kidney.

Metanephros

  • Formation of the metanephros employs recipricol signalling.
  1. At the level of the sacrum, a block of intermediate mesoderm breaks off from the Wolffian duct. It is called a metanephric blastema.
  2. The metanephric blastema signals to the Wolffian duct.
    • Wolffian duct is induced to grow, and a bud of Wolffian duct moves towards the metanephric blastema.
    • The bud is called the ureteric duct.
  3. The ureteric duct contacts the blastema, and divides into two branches.
  4. One of the branches induces adjacent mesenchyme to undergo a mesenchymal to epithelial transition.
    • It subsequently develops into a nephron.
  5. The remaining branch splits into two, and repeats the process.
  6. The primary branch forms the ureter; the remainder leads to the compact kidney of mammals.


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