20.2 Development of the renal anlage



Mesonephros: transitory kidney

Quiz

Quiz 02

The mesonephros differentiates itself during the 4th week and after the 8th week it degenerates. It replaces the pronephros and develops from three structures:

  • Nephrogenic cord 10 in the dorso-lumbal region
  • Mesonephric duct 11
  • Glomerular capillary network 13

Quiz

Quiz 17

The mesonephric duct forms on the dorsal side of the nephrogenic cord at the level of the 9th somite. Initially it consists of a solid mesenchymal cord of cells 11. It releases itself from the nephrogenic cord and is finally localized under the ectoderm, which probably plays an inductive role in its formation (3).
Released from the nephrogenic cord, it develops in the caudal direction and canalizes itself at the same time 12, in order to finally end in the cloaca. As soon as it is canalized one calls it the mesonephric duct (Wolffian duct). At the site where the mesonephric duct (Wolffian duct) discharges into the cloaca, the rear wall of the bladder forms.		 Fig. 6 - Development of the
mesonephros		  Legend

1
2
1+2
3
4
5
6
7
8

9		 Nephrogenic cord
Mesonephric duct
Mesonephros
Intestine
Cloaca
Atrophied nephrotome
Yolk sac (umbilical vesicle)
Allantois
Outflow of the mesonephric duct
into the cloaca
Ureter bud (anlage)

Fig. 6
Sagittal section of a 5-week-old embryo. The pronephros atrophies while the mesonephric duct grows caudally and fuses with the cloaca wall. In this stage, it goes through a mesenchymal- epithelial transformation and forms a central lumen. Only the caudal part remains mesenchymal. Observe the ureter bud at its caudal end.


Fig. 7 - Development of the mesonephros as the first excretory organ  Legend

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2
3
4
5
6
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8
9
10
11		 Neural tube
Notochord
Aorta dorsalis
Dorsal mesentery
Intestinal tube
Ectoderm
Somite
Inferior cardinal vein
Mesonephric duct (Wolffian duct)
Mesonephric tubule
Urogenital ridge

Fig. 7
Transverse section along A (see previous figure).
The urogenital ridge projects into the lumen of the coelom. With the S-shaped mesonephric tubules the mesonephric duct (Wolffian duct) forms a transitory precursor of the adult excretory system. The medial end of the mesonephric tubule is closed and forms a funnel (Bowman’s capsule) that surrounds a tuft of capillaries (the glomerulus). The capillaries come from lateral branches of the dorsal aorta and drain into the inferior cardinal vein.
This functional unit is also termed the excretory unit of the mesonephros.


More info

The mesonephric duct (Wolffian duct)
Animal experiments have shown that the growth and the caudal extension of the mesonephric duct depend on the presence of the extracellular molecule fibronectin (an integrin). The differentiation of the solid cord into a duct depends on the secretion of the BMP4 (bone morphogenetic protein 4) by the ectoderm (4).

Quiz

Quiz 03


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