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20.3 The upper urinary tract: development of the metanephros
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The metanephric blastema and the formation of the nephron
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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.
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- 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
 21 . 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: interactive diagram, overview over all the pictures).
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- 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.
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More info
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Physiology of the kidneys
The fetal kidneys do not have to maintain the water and electrolyte households in the organism nor do they have to clean the blood from substances (mainly end products of cellular metabolism in the form of urea, uric acid and creatinine ) that are usually eliminated with the urine. These functions are performed by the placenta. The kidneys are, however, involved in the production of amniotic fluid. This is why an agenesis of the kidneys on both sides leads to an oligohydramnios (deficiency of amniotic fluid), which is described in greater detail in the chapter on pathology.
Naturally, besides their excretory function, the kidneys also have an endocrine one in that they secrete hormones (renin, erythropoetin, prostaglandin and kallikrein). Even though some of the endocrine functions only begin at birth, the embryonic kidneys nevertheless have an important task to fulfill, namely the production of erythropoetin. In addition, renin-positive cells can already be found in the mesonephros of 5-6 week old embryos (19).
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Summary:
- 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.
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Summary
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Overview of the differentiation of the intermediate mesoderm into the definitive kidneys.
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