20.2 Development of the renal anlage


The kidneys develop along a cranio-caudal gradient. Typically, the development passes through three stages:

  • Pronephros
  • Mesonephros
  • Metanephros

In principle, for all three developmental stages with similar designations, the same embryonic anlage - the intermediate mesoderm - is involved.
The term "holonephros" can thus be employed to designate all the parts of the urinary system that arise from it. The first two developmental stages have a transitory character and the definitive kidneys actually develop from the metanephros stage.

Color code
for the
urinary system

Urine excreting system:

Urine forming system:

Pronephros (forekidney): transitory structure

The pronephros develops during the 4th week, beginning in the cranial part of the nephrogenic cord and it atrophies during the 5th week. Three characteristic pronephros structures can be distinguished:

  • The pronephros duct 10 in the neck region
  • The pronephros tubules 11
  • The external or coelom glomeruli 13that have been shown to exist in humans (1)

Beginning with the 4th week, conforming to the cranio-caudal gradient, the pronephros in the neck region divides into independent masses of cells, the nephrotomes 11.
Each nephrotome develops into an epithelialized pronephros glomerulus. Laterally, they form the pronephros tubules that can partly bind with the coelom. Via the fusion of these tubules between two nephrotomes the hollow pronephros duct arises that is the anlage of the pronephric collecting duct. In humans, this pronephros system corresponds more to a primitive and transient structure that is functionally of no importance. According to the classical view, the pronephros duct stops in the caudal region at the level of the 13th -14th somite and then goes over into the mesonephric duct (Wolffian duct).
Fig. 5 - Early pronephros development
(stages 10-12)

Nephrogenic cord
Mesonephric duct (Wolff)
Intestinal tube
Atrophying nephrotomes
Yolk sac (umbilical vesicle)
Outflow of the mesonephric duct into the cloaca

Fig. 5
Schematic sagittal section through a
4-week-old embryo. One can recognize the segmented part of the nephrogenic cord, the pronephros and the unsegmented part, the mesonephros. In this stage
12 the mesonephric duct already discharges into the cloaca.

Meer info

The origin of the mesonephric duct (Wolffian duct):
At present it is not sure whether the mesonephric duct (Wolffian duct) arises from the joining up of these pronephros tubules or from the first mesonephric vesicles. (2)

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