The metanephric blastema and the formation of the nephron
The process of nephron formation is complicated and thus subdivided into various steps:
- The metanephric blastema surrounds each newly formed collecting duct. It condenses in order to form peritubular cell aggregates.
- Through induction signals (see: beginning of the chapter) , derived from the ureter anlage, the mesenchymal cells transform themselves and form vesicles.
- These vesicles grow longer and form an "S"-shaped tubule with three sections.
- Development of the distal section into the distal part (distal convoluted limb, thick ascending limb and thin ascending limb)
- Development of the middle section into the proximal part (thin descending limb, proximal straight and convoluted tubule)
- Development of the proximal section in the glomerular capsule. The epithelial vesicle secretes angiogenic factors. Thereby, over the course of the further development, endothelial cells are brought into the glomerular capsule. As soon as the afferent vessels come into close contact with the vesicular epithelium, it flattens and forms a cup with a bilaminar structure, Bowman's capsule (formation of the renal corpuscle: , ).
- At the same time as the formation of the renal corpuscle, the distal end of the epithelial vesicle fuses with the neighboring collecting duct. The metanephros thus becomes able to function and can filter the plasma from the glomeruli.
Through the proximal tubule the glomerular filtrate (primary urine) gets into the intermediate tubule, the distal tubule, connecting tubules and collecting duct. In these tubules the secondary urine arises through resorption and secretion processes. It then reaches the renal pelvis and, via the ureter, the bladder. During the pregnancy, the fetal urine is excreted into the amniotic cavity.
In this interactive diagram you can study the process that leads to nephron formation over the course of stages 13-23 (4th – 8th developmental weeks).
- The urine-forming system (filtration, resorption and secretion): distal, intermediate, and proximal tubules and the renal corpuscles (Bowman's capsule with capillaries) arise from the metanephric blastema of the metanephros.
- The urine excreting system - the ureter, the renal pelvis, the major and minor calices as well as the collecting ducts - arise from the ureteric bud anlage from the Wollfian duct