Three-dimensional Representation and Quantification of Endosomes in the Rat Kidney Proximal Tubule Cell

Abstract

The organization of the endosomal compartment in the rat kidney proximal tubule cell was studied by computer-aided three-dimensional reconstruction based on tracing of serial thin sections. Besides the well-documented endocytic elements such as small vesicles, tubules and large vacuoles, the endosomal compartment contained a prominent structural complex consisting of a central vacuolar element (with diameters up to 2000 nm) and associated systems of radiating tubular profiles. The large endosomes examined had from two to twenty-four tubules as straight or bending extensions directed randomly from the central vacuole. The tubules were 70-90 nm in diameter and varied with the length up to 1500 nm. Quantitative evaluation from examples of the large endosomes with more than ten tubular extensions, showed that 25-40% of the membrane resided in the tubular portion in contrast to 5-10% of the volume. These results indicate the high efficiency of the role of the endosomal tubules as a mediator of the membrane recycling, because the tubules simultaneously allow retention of most of the content in the central vacuole during the recycling process.