Electron micrograph of a proximal tubule cell. The apical surface of the cell shows the closely packed microvilli (Mv) that collectively are recognized as the brush border in the light microscope. Many vesicles (V) are evident in the apical cytoplasm. Also present in the apical region of the cell are lysosomes (L). The nucleus has not been included in the plane of section. Extensive numbers of longitudinally oriented mitochondria (M) are present in the cell within the interdigitating processes. The mitochondria are responsible for the appearance of the basal striations seen in the light microscope, particularly if the extracellular space is enlarged. The electron micrograph also reveals a basal lamina and a small amount of connective tissue and the fenestrated endothelium (En) of an adjacent peritubular capillary, x 15,000. Upper inset. This higher magnification of the microvilli shows the small endocytotic vesicles that have pinched off from the plasma membrane at the base of the microvilli, x32,000. Lower inset. A higher magnification of the basal portion of the interdigitating processes (IP) below the reach of the mitochondria. The extreme basal aspect of these processes reveals a dense material (arrows) that represents bundles of actin filaments (see Fig. 19.16). x 15,000.
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