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Morphometric analysis of distinct microanatomy near the base of proximal tubule cells.

Models of cell shape in the rabbit S2 proximal renal tubule were derived from transmission electron micrographs and compared with scanning micrographs. Standard morphometric procedures were used to measure basolateral cell membrane surface density (SVt) relative to total epithelial volume in numerous zones of cell height. In the basal 20% region we also measured the volume fraction (F) of intercellular spaces and calculated new surface densities in reference only to the intercellular volume, SVi = SVt/F, or to the cellular volume, SVc = SVt/(1-F). Combined use of these surface densities then enabled us to calculate the diameter, length, and separation of effectively cylindrical microvilli at the cell base. Assuming that lateral cell membranes are radially oriented in the apical region but disposed on microvillus like structures of arbitrary orientation at the cell base, an improved cell model was developed that agreed with the scanning picture throughout the entire cell height. Basal microvillar elements contain approximately 60% of the total basolateral cell membrane surface area and possibly constitute a hydrostatic resistive region for absorbate flow. These features have interesting physiological implications.