Introduction
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 module |
| Urine excreting system: yellow-beige-orange |
| Urine forming system pink-lilac-violet |
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 (stage 9) in the neck region
- The pronephros tubule (stage 11)
- The external or coelom glomeruli (stage 13) that have been shown to exist in humans
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 (stage 10).
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.
- 1
- Nephrogenic cord
- 2
- Mesonephric duct (Wolff)
- 1+2
- Mesonephros
- 3
- Intestinal tube
- 4
- Cloaca
- 5
- Atrophying nephrotomes
- 6
- Yolk sac (umbilical vesicle)
- 7
- Allantois
- 8
- Outflow of the mesonephric duct into the cloaca
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, the mesonephric duct already discharges into the cloaca.
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).
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.