Three-dimensional characteristics of the mitochondria of the rat nephron

Abstract

We propose a new model for the mitochondrion of the proximal and distal nephron. The introduction of a new block-staining technique allowed us to observe mitochondria in thick sections (0.5 to 1.5 μm) with a standard transmission electron microscope (80 to 100 kV). Combined with stereomicroscopy and serial sections, the thick section offered a three-dimensional view of the chondriome. After glutaraldehyde fixation, rat kidney blocks were immersed in a double-lead and copper citrate solution, which stained mitochondria selectively. Mitochondria were observed to anastomose, branching was evident, and many lateral connections were seen. In cells of both the proximal and distal tubules, mitochondria were observed to extend into the cytoplasm in various directions. In the apical region of both distal and proximal tubules, mitochondria appeared to be small, not numerous, and rarely branched, which corresponds to their classical aspect. The block-staining technique thus confirms what is generally accepted. At the basal part of these cells, however, the spatial morphology of the chondriome appeared quite different from the classical description. In the distal tubule, the chondriome often had the appearance of an irregular lamina to which cords or pseudopods were attached that extended towards various cytoplasmic areas. The extent of branching in the cells of the proximal convoluted segment was more difficult to assess because of the interdigitations of neighboring cells. The chondriome seemed to be made of a few giant ramifying mitochondria and isolated small ones, thus suggesting the possibility of two populations of mitochondria in the nephron of the rat.