Filtration Apparatus of the Kidney

The renal corpuscle contains the filtration apparatus of the kidney

The renal corpuscle is spherical and has an average diameter of 200 /xm. It consists of the glomerular capillary tuft and the surrounding visceral and parietal epithelial layers of Bowman's capsule (Figs. 19.7 and 19.8). The filtration apparatus, enclosed by the parietal layer of Bowman's capsule, consists of three components:

• Endothelium of the glomerular capillaries, which possesses numerous fenestrations (Fig. 19.9). These fenestrations are larger (70 to 90 nm in diameter), more numerous, and more irregular in outline than fenestrations in other capillaries. Moreover, the diaphragm that spans the fenestrations in other capillaries is absent in the glomerular capillaries. Endothelial cells of glomerular capillaries possess a large number of aquaporin-1 (AQP-'l) water channels that allow the fast movement of water through the epithelium.

• Glomerular basement membrane (GBM), a thick (300 to 350 nm) basal lamina that is the joint product of the endothelium and the podocytes, the cells of the visceral layer of Bowman's capsule. Because of its thickness, it is prominent in histologic sections stained with the periodic acid-Schiff (PAS) procedure (see Fig. 1.2, page 7). The GBM is the principal component of the filtration barrier.

• Visceral layer of Bowman's capsule, which contains specialized cells called podocytes or visceral epithelial cells. These cells extend processes around the glomerular cap-

distal convoluted tubule smooth muscle foot processes (pedicels) of podocytes afferent arteriole juxtaglomerular glomerular capillaries podocyte

mesangial cells extraglomerular mesangial cells efferent arteriole urinary space

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