The renal corpuscle is spherical and has an average diameter of 200 m. It consists of the glomerular capillary tuft and the surrounding visceral and parietal epithelial layers of Bowman’s capsule (Fig. 20.8). The filtration apparatus, also called glomerular fltration barrier, enclosed by the parietal layer of Bowman’s capsule, consists of three different components:  Endothelium of the glomerular capillaries, which possesses numerous fenestrations (Fig. 20.9). 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-1) water channels that allow the fast movement of water through the epithelium. Secretory products of endothelial cells, such as nitric oxide (NO) or prostaglandins (PGE2), play an important role in the pathogenesis of several thrombotic glomerular diseases.
 Glomerular basement membrane (GBM), a thick (300 to 370 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 6). The GMB is composed of a network consisting of type IV collagen (mainly 3, 4, and 5 chains), laminin, nidogen, entactin, together with proteoglycans such as agrin and perlecan, as well as multiadhvesive glycoproteins (see page 138). GBM can also be visualized employing immunofluorescence techniques using antibodies directed to a specific  chain of type IV collagen (Fig 20.10). Mutation in the gene encoding for the 5 chain of type IV collagen gives rise to the Alport’s syndrome of Bowman’s capsule glomerular basement membrane endothelial cells with fenestrations foot process of podocytepodocyte subpodocyte spaceendothelial blood
FIGURE 20.8 • Schematic dia gram of fltration barrier. The arrow indicates movement of plasma fluid across the glomerular filtration barrier, forming the glomerular ultrafiltrate (primary urine) that accumulates in the urinary space of the Bowman’s capsule. Note the layers of the filtration barrier that include, fenestrated glomerular endothelial cells, glomerular basement membrane, and podocytes with filtration slit diaphragms spanned between their foot processes. In addition, the endothelial surface layer of glycoproteins and subpodocyte spaces are shown on this diagram.
(hereditary glomerulonephritis), manifesting by hematuria (presence of the red blood cells in the urine); proteinuria (presence of significant amount of protein in the urine); and progressive renal failure. In Alport’s syndrome, the GBM becomes irregularly thickened with laminated lamina densa and fails to serve as an effective filtration barrier.