The drawing, at the electron microscopic level, shows the sectioned face on the right and a three-dimensional view of the basolateral surface of a cell with a partial cut face on the left. Here the interdigitating parts of the adjoining cell have been removed to show the basolateral interdigitations. Some of the interdigitating processes extend the full height of the cell. The processes are long in the basal region and create an elaborate extracellular compartment adjacent to the basal lamina. Apically, the microvilli (M) constitute the brush border. In some locations, the microvilli have been omitted, thereby revealing the convoluted character of the apical cell boundary (CB). (Based on Bulger RE. The shape of rat kidney tubular cells. Am J Anat 1965;116:253.) are present (see arrows, Figs. 20.18 and 20.19). These actin filaments may play a role in regulating the movement of fluid from the basolateral extracellular space across the tubule basal lamina toward the adjacent peritubular capillary.
Of the 180 L/day of ultrafiltrate entering the nephrons, approximately 120 L/day, or 65% of the ultrafiltrate, is reabsorbed by the proximal convoluted tubule. Two major proteins are responsible for fluid reabsorption in the proximal convoluted tubules:
FIGURE 20.18 • 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. 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. 32,000. 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. 20.16). 30,000.
FIGURE 20.19 • Electron micrograph of a proximal convoluted tubule cell. This section is almost tangential and slightly oblique to the base of a proximal convoluted tubule cell and the subjacent basal lamina and capillary. In the left part of the micrograph is the capillary endothelium (En). Characteristically, the endothelium possesses numerous fenestrations (EnF), and in this plane of section, the fenestrations are seen en face, displaying circular profiles. The plane of section also makes the basal lamina (BL) appear as a broad band of homogenous material. To the right of the basal lamina are the interdigitating basal processes of the proximal tubule cells. The long, straight processes contain longitudinally oriented actin filaments (arrows). In this plane of section, the basal extracellular space appears as a maze between the cellular processes. 32,000.