What is mesangium?

The glomerular mesangium, or simply mesangium, is a structure of cells and connective tissue that serves as a support for glomerular capillaries. From these capillaries, surrounded by Bowman’s capsule, blood is filtered to form urine. In addition to their support function, mesangial cells play a key role in regulating glomerular filtration.

Description and features

The mesangium is located between the capillaries of the glomerular interior, hence its name: from the Greek mesoentre or half, and angel, cup. On the one hand, it is limited by the endothelium of the capillaries and, on the other hand, by the basement membrane, which surrounds both the mesangium and the capillaries.

It is made up of a network of connective tissue, known as the mesangial matrix, and by specialized cells known as mesangial cells. Mesangial cells are those that secrete the mesangial matrix, composed of fibronectin, type IV collagen, perlecan and laminin. In addition to this matrix, mesangial cells emit pseudopods with actin and myosin filaments that anchor in the basement membrane.

Many mesangial cells are in direct contact with the capillary walls. In some areas, they are separated from the lumen (the inside of the vessel) only by a layer of endothelial cells.

Part of the mesangium exits the intraglomerular space through the vascular pole of the renal corpuscle. Mesangial cells in this zone are known as extraglomerular mesangial cells, Lacis cells, Goormaghtigh cells, or Polkissen cells, and are different from intraglomerular mesangial cells.

Extraglomerular mesangial cells are part of the juxtaglomerular apparatus near the macula densa cells of the distal convoluted tubule, which approaches the glomerulus near the vascular pole, and the juxtaglomerular cells associated with the afferent arteriole (the one that enters the intraglomerular space).

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Functions

The mesangium has several functions within the renal corpuscle, which is the structure where blood is filtered and urine begins to form. The first and most obvious is the bra’s function of the intraglomerular capillary network thanks to the structure created by the mesangial matrix and the pseudopods of the mesangial cells.

Mesangial cells are a highly specialized type of pericyte (contractile cells that surround capillaries) and play an active role in regulating glomerular filtration. These cells respond to various signals to contract and relax, mainly angiotensin II, thanks to which they regulate the flow in intraglomerular capillaries and with it glomerular filtration.

Intraglomerular mesangial cells also function phagocytically, being one of the few examples of phagocytes derived from smooth muscle cells rather than monocytes. Mesangial cells phagocytose components shed by the basement membrane and collaborate with neutrophils to remove other debris, including debris from other mesangial cells that have undergone apoptosis.

The function of extraglomerular mesangial cells is still not well understood, but it has been related to renal autoregulation and regulation of systemic blood pressure through the renin-angiotensin system. They have also been observed to secrete erythropoietin, so they may be involved in the regulation of erythropoiesis.

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