Three-dimensional organization of the vacuolar apparatus involved in endocytosis and membrane recycling of rat kidney proximal tubule cells. An electron-microscopic study of serial sections.

The present study was performed to examine the three-dimensional structure of the vacuolar apparatus involved in endocytosis and especially the membrane recycling pathway of rat renal proximal tubule cells by electron microscopy of serial sections. In five series of consecutive ultrathin sections, endocytic invaginations (INV), small endocytic vacuoles (SEV; < 0.5 micron in diameter), large endocytic vacuoles (LEV; > 0.5 micron in diameter) and dense apical tubules (DAT) were followed. LEV connected to SEV and dense apical tubules were reconstructed from these sections, and the distribution, size, shape and spatial relationships were examined. The analysis of small coated profiles (< 0.5 micron in diameter) in the apical cytoplasm showed that the INV account for 21.3%, SEV connected to DAT account for 36.4% while SEV free in the cytoplasm account for 42.3%. The free SEV generally have a larger diameter than those connected to DAT. Of the DAT, 56.5% are free in the cytoplasm, 36.9% are seen connected to SEV and 6.5% are in continuity with LEV. The reconstructed images showed that LEV are spherical structures connected to DAT and SEV. A quantitative analysis showed that the smaller the surface area of LEV, the higher the relative number of DAT connected, and also shown was that the number of DAT attached to SEV is 2.03 times more than that of DAT connected to LEV. These results demonstrating a three-dimensional model for the vacuolar apparatus involved in the endocytosis and membrane recycling pathway of the proximal tubule cells suggest that DAT originate mainly from SEV but also from LEV, illustrating that membrane recycling occurs at any stage of the initial endocytic process, and implies that most membrane material return to the cell surface via a short, fast recycling route.