The metanephric blastema and the formation of the nephron
Scanning electron microscopy (SEM) of a vascular cast (following corrosion of adherent tissue).
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: interactive diagram, overview over all the pictures).
- 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).
Synoptic view of the illustrations presented in the interactive diagram.
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
During the stages 13-14 (ca. 32-33 days) each collecting duct is surrounded by metanephrogenic blastema.
It becomes condensed and forms dense, peritubular cell aggregates that, via gene activation, differentiate themselves to become nephrons.
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
In stage 19 (ca. 46 days), due to the diffusible signal substances (WT-1HNF-1, etc.) of the ureter anlage, the mesenchymal cells transform into epithelial tissue and form vesicles that keep getting longer.
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
During stages 19-20 (ca. 46-49 days) these vesicles lengthen and form an S-shaped tubule that exhibits 3 sections.
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
From the end of stage 20 (ca. 49 days) the epithelium, which comes into contact with an afferent arteriole, becomes thinner and forms a funnel with a double-layered structure, Bowman’s capsule.
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
During stage 20-21 (ca. 49-51 days), at the same time in which the renal corpuscle forms, the other end of the original vesicle joins up with the distal part of the collecting duct.
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
At the end of stage 21 (ca. 51. days) the development of the tubular system has progressed so far that the glomerular capsule forms from the proximal segment (and with the glomerulus forms a functional renal corpuscle).
and the nephron
- 1
- Collecting duct
- 1a
- Collecting duct lumen
- 2
- Condensation of the metanephric blastema into
cell aggregates - 3
- Metanephric tissue cap
- 4
- Metanephric vesicle (cut section)
- 5
- Newly-created dichotomous division
- 6
- New branch of the collecting duct
- 7
- Metanephric vesicle that forms
a tubule - 8
- Bowman's capsule
- 9
- Afferent arteriole
- 10
- Proximal convoluted tubule
- 11
- Distal convoluted tubule
- 12
- Proximal straight tubule
- 13
- Distal straight tubule
In stage 23 (ca. 23 days) the development of the tubular system is completed.
The distal segment forms the distal tubuleand the ascending thin limb of the intermediate tubule while the middle segment forms the proximal tubule and the descending thin limb of the intermediate tubule.
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.
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