Electron photomicrograph illustrating a portion of the ultrastructure of the glomerular capillary wall. The normal width of the lamina rara externa (LRE) plus the lamina densa (LD) plus the lamina rara interna (LRI) equals about 250 to 300 nm. The spaces between the foot processes (FP), having diameters of 20 to 60 nm, are called filtration slit pores. It is believed they are the path by which filtered fluid reaches the urinary space (U). The endothelial cells on the luminal aspect of the basement membrane (BM) are fenestrated, having diameters from 70 to 100 nm (see Fig. 1-9). The BM (LRE plus LD plus LRI) is composed of Type IV collagen and negativity charged proteoglycans (heparan sulfate). L—lumen. (From Churg and coworkers [1]; with permission.)

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