Ultrastructure of the tubular nephron of the garter snake Thamnophis sirtalis.

Ultrastructural features of the tubular nephron of the garter snake, with special reference to modifications for conservation of water, were studied using transmission electron microscopy, freeze-fracture and tracer experiments. Although a nephric loop (loop of Henle) is lacking, the tubules appear to be structurally well adapted for efficient ion and water reabsorption. The most prominent features are well developed microvilli in the proximal tubule and elaborate latteral folds, particularly in the distal tubule and collecting ducts. The latter structures are highly interdigitated, creating complex intercellular channels, perhaps facilitating transepithelial fluid transport. Only the proximal tubule actively absorbs and degrades protein tracers from the lumen. The cells of the collecting duct secrete mucus which may precipitate and bind urate salts in the lumen. This may be significant in the excretion of these salts, a process which combines maximal removal of salts and nitrogenous wastes with minimal loss of water.