The basement membrane and The mesangial cells

By Supriyadi Pro -
The basement membrane - The endothelium of the capillaries rests on a prominent basement membrane to which, on its outer aspect, the foot processes are applied (Fig. 1.20/Read: The epithelial cells). It is difficult to give a figure for the thickness of the basement membrane since it varies according to the species. It also increases in thickness with age. Jorgensen (1967) estimated the thickness in the human kidney to be 257-418 nm but Latta et al. (1975) give a figure of between 100 and 150 nm.

The basement membrane consists of a central dense layer with inner and outer more translucent zones. With certain staining methods several systems of fibres can be Visualized (Latta 1970, Latta et al. 1975), but no ‘pores’ can be seen even in experimental animals in which tracer particles can be seen apparently making their way through the basement membrane.

On biochemical analysis, the basement membrane contains a collagen-like material containing 8-10 per cent carbohydrate, the components of which vary from species to species (Kefalides 1972).

The mesangial cells
The mesangial cells occupy the axial region of the glomerulus and they are embedded in an extensive matrix of a spongy material which resembles basement membrane. They are very irregular in shape with long processes which extend between the capillaries. The cytoplasm contains a prominent Golgi zone, granular endoplasmic reticulum, free ribosomes, glycogen granules and mitochondria.

It also contains large bundles of filaments which may be contractile since they have been shown to contain actomyosin (Becker 1972). Furthermore, Accinni et al. (1975) have demonstrated the presence of actin or actin-like protein not only in the mesangial cells but also in the mesangial matrix, the glomerular basement membrane and the podocytes.

It seems possible that contraction of filaments in the mesangium and in other components of the glomerulus may play a part in the control of blood flow through the glomerular capillaries and in this respect it is interesting that Pricam et al. (1974) and Boll et al. (1975) have shown that these cells, as well as the lacis cells, are joined by gap junctions. These are known to allow electrical and metabolic coupling between cells so that the whole mass of mesangial cells could act as a syncytium.

A further important function of the mesangial cells lies in their phagocytic properties. A number of workers have shown that when electronopaque tracers are administered to animals, many of the particles that succeed in passing through the filtration membrane end up in large vesicles within the mesangial cells.

Read the full article: The Anatomy of the Renal Circulation

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