The structure of the glomerulus

By Supriyadi Pro -
The term ‘glomerulus’ is here used in its original sense to mean the small knot or ball of capillaries which forms the most noticeable feature of the Malpighian body or renal corpuscle. The afferent arteriole divides at the vascular pole into a number of branches which supply the lobules of the glomerulus.

These branches run fairly superficially towards the opposite pole before breaking up into a series of anastomosing capillary loops which return towards the vascular pole, becoming more deeply placed as they do so.

They unite to form the efferent arteriole which usually emerges from the central region of the glomerulus (Fig. 1.18) so that the most distal part of the capillary system, where equilibrium of the filtration pressures is reached, is deeply situated (Spinelli 1974).

Picture: A juxtamedullary glomerulus (human)
Fig.1.18: A juxtamedullary glomerulus (human) to show how the afferent arteriole gives rise to surface branches while the efferent arteriole emerges from the centre of the glomerulus, Microfil injection.

Three types of cells are found in the glomerulus, their nuclei being recognizable by light microscopy. These are the endothelial cells which line the capillaries, the epithelial cells (podocytes) which embrace the capillaries on their external aspect and the mesangial cells which occupy the central region of the glomerulus, filling up the spaces between the capillary loops.

In addition, the capillary basement membrane and the mesangial matrix are important components of the glomerulus. The actual barrier or filtration membrane between the blood and the urinary space consists of the capillary endothelium, the basement membrane and the contents of the spaces between the foot processes of the epithelial cells (Fig. 1.20/Read: The epithelial cells).

The relative importance of each of these components in the filtration mechanism has recently been reviewed by Farquhar (1975). lining are extremely thin except on the mesangial side of the capillary, where the cytoplasm is thicker to accommodate the nucleus and other organelles. On the rather more extensive free side of the capillary, the endothelium is fenestrated.

The fenestrations are, circular or oval but they are not as regular in size and shape as those of the peritubular capillaries or the ascending vasa recta. They are 50-100 nm in diameter and most investigators agree that they are not closed by a diaphragm, so that the basement membrane is in direct contact with the blood in the region of the fenestrations.

The fenestrations are, of course, far too large to be equated with the ‘pores’ which prevent the passage of large protein molecules through the filtration membrane.

Read the full article: The Anatomy of the Renal Circulation

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