Mesonephros: transitory kidney

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 (stage 10) de la région dorsolombaire
Mesonephric duct (stage 11)
Glomerular capillary network (stage 13)

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 (stage 11). It releases itself from the nephrogenic cord and is finally localized under the ectoderm, which probably plays an inductive role in its formation.
Released from the nephrogenic cord, it develops in the caudal direction and canalizes itself at the same time (stage 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

media/module20/t3a2_evolmesonephro.gif — **Fig. 6 -** Development of the mesonephros

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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 epithelial transformation and forms a central lumen. Only the caudal part remains mesenchymal. Observe the ureter bud at its caudal end.

media/module20/t3b_devmesonephro.gif — **Fig. 7 -** Development of the mesonephros as the first excretory organ

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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.

Mesonephros: first excretory organ

Shortly after the differentiation of the mesonephric duct, through mesenchymal-epithelial transformation, the mesonephric vesicles (stage 11) arise out of the nephrogenic cord, which represents a mesoderm condensate.
Via the mesonephric tubules they connect up with the mesonephric duct (stage 14).
This differentiation takes place bilaterally in the area between the upper thoracic region (Th1) and the lumbar region (L3). A cranio-caudal gradient is also visible here. To the extent in which new mesonephric vesicles and tubules develop caudally, the cranial elements begin to atrophy so that never more than 30 pairs exist in the mesonephros. They form the excretory system that closely resembles the adult nephrons.

Medially, the mesonephric vesicle dead-ends in that it forms a funnel (Bowman's capsule). Each of these funnels surrounds a tuft of capillaries (glomerulus), which have also arisen in the nephrogenic tissue and come from lateral (visceral) branches of the dorsal aorta. They drain into the inferior cardinal vein.
The capsule with glomerulus together form a renal corpuscle. A renal corpuscle and its associated tubule are called a nephron and the functional unit an excretory mesonephric unit.

In the interactive diagram below you can study the development of the urine excretory mesonephric unit over the course of stages 11 to 17, which roughly corresponds to the 4th to 6th weeks of development.

Illustrations

Synoptic view of the illustrations presented in the interactive diagram.

media/module20/t3d1_excretmeso.gif — **Fig. excretricemesoB01 -** Excretory mesonephric unit

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During stage 11 (approximately 29 days) an arching of the intermediate mesoderm on each side of the spinal cord with a cranio-caudal gradient forms from the nephrogenic cord.

While this is just beginning to differentiate in the caudal region, it already begins to atrophy in the cranial one.

media/module20/t3d2_excretmeso.gif — **Fig. excretricemesoB02 -** Excretory mesonephric unit

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During stages 11-12 (29-30 days) the first polarized epithelial cells form out of the initially mesodermal condensate and differentiate themselves into mesonephros vesicles. They bind with the mesonephric duct (Wolffian duct).

media/module20/t3d3_excretmeso.gif — **Fig. excretricemesoB03 -** Excretory mesonephric unit

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During Stage 13 (ca. 32 days) these vesicles differentiate themselves to become mesonephric tubules and take up a connection with the mesonephric duct (Wolffian duct).

Thus the first excretory units are engendered that are very similar to the nephrons in the adult.

media/module20/t3d4_excretmeso.gif — **Fig. excretricemesoB04 -** Excretory mesonephric unit

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In stage 15 (ca. 36 days) the tubule ends, which terminate medially, form themselves into a funnel (Bowman’s capsule) that envelops a tuft of capillaries (glomerulus). This stems from arterial branches of the dorsal aorta and drains into the inferior cardinal vein.

media/module20/t3d5_excretmeso.gif — **Fig. excretricemesoB05 -** Excretory mesonephric unit

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In stage 17 (ca. 41 days), these excretory mesonephric units begin to function and from the 6th week they start to produce urine. After 10 weeks, they cease their function again and atrophy.

The production of urine begins in the mesonephros during the 6th week (stage 17). After the 10th week these nephrons become inactive and atrophy. While in the female all mesonephric tubules atrophy completely, in the male a few that lie caudally remain in order to develop into the testicular efferent ducts.