Figure 197

Structure of the renal corpuscle, a. This schematic diagram shows the organization of the renal corpuscle and the structures associated with it at the vascular and urinary poles. Mesangial cells are associated with the capillary endothelium of the glomerulus and the glomerular basement membrane. The macula densa cells of the distal tubule are shown intimately associated with the juxtaglomerular cells of the afferent arteriole and the extraglomerular mesangial cells. (Modified from Kriz W, Sakai T. Morphological aspects of glomerular function. In: Nephrology: Proceedings of the Tenth International Congress of Nephrology. London: Bailliere-Tindall, 1987.) b. Photomicrograph of a H&E-stained specimen showing a renal corpuscle. The macula densa is seen in close proximity to the vascular pole. X160.

foot processes (pedicels) of podocytes extraglomerular mesangial cells parietal layer of Bowman's capsule proximal convoluted tubule basal lamina a afferent arteriole juxtaglomerular efferent arteriole glomerular capillaries podocyte basal lamina urinary space illaries (Fig. 19.10). The podocytes arise during embryonic development from one of the blind ends of the developing nephron through invagination of the end of the tubule to form a double-layered epithelial cup. The inner cell layer, i.e., the visceral cell layer, lies in apposition to a capillary network, the glomerulus, which forms at this site. The outer layer of these cells, the parietal layer, forms the squamous cells of Bowman's capsule. The cup eventually closes to form the spherical structure containing the glomerulus. As they differentiate, the podocytes extend processes around the capillaries and develop numerous secondary processes called pedicels or foot processes. The foot processes interdigitate with foot processes of neighboring podocytes, a feature that can be clearly demonstrated with the scanning electron microscope (SEM) (Figs. 19.11, a and b). The elongated spaces between the inter digi ta ting foot processes, called filtration slits, are about 25 nra wide and allow the ul-trafiltrate from the blood to enter Bowman's space. The foot processes contain numerous actin filaments that are thought to help regulate the size and patency of the filtration slits. An additional factor that may influence the passage of substances through the filtration slits is the presence of a thin membrane similar to the diaphragm of capillary fenestrations. This membrane, the filtration slit membrane, spans the slits (Fig. 19.12, inset). The filtration apparatus may thus be described as a semipermeable barrier having two discontinuous cellular layers applied to either side of a continuous extracellular layer, the basal lamina.

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