Figure 1914

Drawing of proximal convoluted tubule cells. 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 interdigita-tions. 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. AmJAnat 1965;116:253.)

• Basal striations, consisting of elongate mitochondria concentrated in the basal processes and oriented vertically to the basal surface (see Fig. 19.15)

In well-fixed histologic preparations, the basal striations and the apical brush border help to distinguish the cells of

the proximal convoluted tubule from those of the other tubules.

At the very base of the proximal convoluted tubule cell, in the interdigitating processes, bundles of 6-nm microfilaments are present (see arrows, Figs. 19.15 and 19.16). 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.

The proximal convoluted tubule reabsorbs about 150 L of fluid per day or about 80% of the ultrafiltrate. Two ma-

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