92. The Urinary or Excretory System: The Mesonephros

  1. The mesonephros (midkidney) or wolffian body* differentiates in week 4 caudal to the rudimentary pronephros and may function as an interim kidney until the permanent kidney is established. The mesonephros regresses in week 8
    1. AS IN THE DEVELOPMENT OF THE PRONEPHROS, the mesonephric nephrotomes pass through the following stages, beginning in week 4
      1. Nephrotomal (mesonephric) vesicle formation
      2. The vesicle grows into an S-shaped mesonephric tubule
      3. The tubules lengthen rapidly
      4. There is an enlargement of the vesicle's internal or medial end to form a glomerular chamber opposite an arterial loop from the aorta
      5. Arrangement of the glomerular chamber around the arterial capillary cluster (glomerulus)
        1. The chamber forms a double-layer cup, the glomerular or Bowman's capsule
        2. The capsule plus the glomerulus forms the mesonephric or renal corpuscle
      6. The external end of the tubule opens into the mesonephric or Wolffian duct (which may or may not be preexistent as the pronephric duct)
    2. THE CHARACTERISTIC SEGMENTATION of the mesonephric nephrotomes is evident at the cranial portion of the mesonephric mass. The caudal portion regresses before its differentiation is complete.
  2. The anatomic relations of the mesonephros (Wolffian body)
    1. THE MESONEPHROS appears, in cross-section, to be a mass projecting into the peritoneal cavity
      1. It is attached by the urogenital mesentery and on its anteroexternal edge one sees the urogenital cord, containing the mesonephric (wolffian) and m?llerian ducts
      2. On its anterointernal side, one sees the gonadal primordium which is attached by the gonadal mesentery
    2. POSTERIORLY, THE MESONEPHROS (wolffian body) is attached to the posterior (dorsal) body wall by the mesonephric (wolffian body) mesentery which is wide and close to the root of the mesentery near the aorta
      1. The coelomic epithelium on the inside surface of this mesentery gives rise to the adrenal cortex
  3. Regression of the mesonephros (wolffian body)
    1. THE MESONEPHROS begins to regress toward the end of month 2
      1. No more than 40 mesonephric tubules are present in each kidney at any one time
      2. By the beginning of the fetal period, most of the mesonephros has degenerated and disappeared except for its duct and a few tubules near the gonads, which persist and are annexed by them as genital ducts in the male or form vestigial remnants in the female
      3. *The gonadal primordium develops at the internal, anterior side of the mesonephros (wolffian body).

the urinary or excretory system:  the mesonephros: image #1