Ultrastructure of the nephron of freshwater turtles, Pseudemys scripta elegans and Mauremys caspica.

The nephrons of the freshwater turtles Pseudemys scripta elegans and Mauremys caspica consist of renal corpuscle, neck segment, proximal tubule, intermediate segment, distal tubule and collecting duct. The renal corpuscle has large and scarce capillaries with clear and dark fenestrated endothelial cells containing some rod-shaped bodies, a thin filtration barrier and a well-developed mesangium, the cells of which show secretory, phagocytic and contractile features, and in M. caspica a cilium. The podocytes with a well-developed Golgi apparatus seem to be active secretory cells. Numerous dense bodies similar to lysosomes, but not previously reported in vertebrates, are conspicuous in podocytes of M. caspica. The proximal tubule displays a well-developed brush border with long and densely-packed microvilli and no basal labyrinth; mitochondria are scattered throughout the cytoplasm. Several dense and clear vesicles related to the prominent endocytotic apparatus can be seen. Wavy filament bundles, not previously reported in vertebrate kidneys, can be observed in proximal tubule cells of M. caspica. Three regions can be distinguished in the well-developed intermediate segment as well as in the distal tubule; the latter has a few short microvilli or a smooth luminal surface and lateral interdigitated processes. The collecting duct, the cells of which contain numerous mucous droplets, is similar in both sexes; there is no sexual segment.