Morphology of the kidney in larvae of Bufo viridis (Amphibia, Anura, Bufonidae)

This study deals primarily with the morphology and ultrastructure of the pr onephros in the green toad Bufo viridis during prometamorphosis when the pr onephros and the developing mesonephros function simultaneously. Furthermor e, the mesonephros was studied during pro- and postmetamorphosis with empha sis on the distal segments of the nephron. The paired kidneys consist of tw o cranial pronephroi immediately behind the gill region and two more caudal elongated mesonephroi. Each pronephros consists of a single convoluted tub ule which opens into the coelom via three nephrostomes. This tubule is divi ded into three ciliated tubules, three proximal tubule branches, a common p roximal tubule and a distal tubule, which in turn continues into the nephri c duct. No intermediate segment is present. The length of the pronephric tu bule is 12 mm, including the three branches of the ciliated tubules and pro ximal tubules. Primary urine is formed upon filtration from an external glo merulus, which is a convoluted capillary lined by podocytes, a specializati on of the coelomic epithelium. From the coelom the filtrate is swept into t he ciliated tubules. In the collecting duct system of the developing mesone phric nephron epithelial cells with conspicuous, apical osmiophilic granule s appear in larvae of 9-10 mm. Heterocellularity of mixed intercalated (mit ochondria rich) cells and principal cells is observed in the collecting duc t system and nephric duct from a larval body length of 14 mm. As the prolif eration of mitochondria-rich cells proceeds, the osmiophilic granules disap pear and are completely absent from the adult amphibian mesonephros. (C) 20 00 Wiley-Liss, Inc.