The Renal Corpuscle

The renal corpuscle consists of a compact tuft of interconnected capillary loops, the glomerulus (pl. glomeruli) or glomerular capillaries, surrounded by a balloonlike
hollow capsule: Bowman’s capsule. Blood enters and leaves Bowman’s capsule through arterioles that penetrate the surface of the capsule at the vascular pole. A fluid-filled space (the urinary space or Bowman’s space) exists within the capsule, and it is into this space that fluid filters. Opposite the vascular pole, Bowman’s capsule has an opening that leads into the first portion of the tubule.

Diagram of a longitudinal section through a glomerulus and its juxtaglomerular (JG) apparatus. The JG apparatus consists of the granular cells (GC), which secrete renin, the macula densa (MD), and the extraglomerular mesangial cells (EGM). E, endothelium of the capillaries; EA, efferent arteriole; AA, afferent arteriole; PE, parietal (outer) epithelium of Bowman’s space; PO, podocytes of Bowman’s capsule; GBM, glomerular basement membrane; US, “urinary” (Bowman’s) space. (Reproduced with permission from Kriz W et al. In: Davidson AM, ed. Proceedings of the 10th International Congress on Nephrology, Vol 1. London: Balliere Tindall; 1987.)

The filtration barrier in the renal corpuscle through which all filtered substances must pass consists of 3 layers: the capillary endothelium of the glomerular capillaries, a rather thick basement membrane, and a single-celled layer of epithelial cells. The first layer, the endothelial cells of the capillaries, is perforated by many large fenestrae (“windows”), like a slice of Swiss cheese, and

A, Anatomy of the glomerulus. B, Cross-section of glomerular membranes. US, “urinary” (Bowman’s) space; E, epithelial foot processes; GBM, lomerular basement membranes; End, capillary endothelium; Cap, lumen of capillary. (Courtesy HG Rennke. Originally published in Fed Proc 1977;36:2019; reprinted with permission.) C, Scanning electron micrograph of podocytes covering glomerular capillary loops; the view is from inside Bowman’s space. The large mass is a cell body. Note the remarkable interdigitation of the foot processes from adjacent podocytes and the slits between them. (Courtesy of C. Tisher.)

is freely permeable to everything in the blood except red blood cells and platelets. The middle layer, the capillary basement membrane, is not a membrane in the sense of a lipid bilayer membrane but is a gel-like acellular meshwork of glycoproteins and proteoglycans, with a structure like a kitchen sponge. The third layer consists of epithelial cells that rest on the basement membrane and face Bowman’s space. These cells are called podocytes. They are quite different from the relatively simple, flattened epithelial cells that line the outside of Bowman’s capsule. The podocytes have an unusual octopus-like structure. Small “fingers,” called pedicels (or foot processes), extend from each arm of the podocyte and are embedded in the basement membrane. Pedicels from a given podocyte interdigitate with the pedicels from adjacent podocytes. Spaces between adjacent pedicels constitute the path through which the filtrate, once through the endothelial cells and basement membrane, travels to enter Bowman’s space. The foot processes are coated by a thick layer of extracellular material, which partially occludes the slits, and extremely thin processes called slit diaphragms bridge the slits between the pedicels. Slit diaphragms are widened versions of the tight junctions and adhering junctions that link all contiguous epithelial cells together. These are like miniature ladders. The pedicels form the sides of the ladder, and the slit diaphragms are the rungs.
The functional significance of this anatomic arrangement is that it permits the filtration of large volumes of fluid from the capillaries into Bowman’s space but restricts filtration of large plasma proteins such as albumin.
Another cell type—the mesangial cell—is found in the central part of the glomerulus between and within capillary loops. Glomerular mesangial cells act as phagocytes and remove trapped material from the basement membrane. They also contain large numbers of myofilaments and can contract in response to a variety of stimuli in a manner similar to vascular smooth muscle cells. The role of such contraction in influencing filtration by the renal corpuscles is discussed.