Histological and enzymatic studies on the nephron of the lungfish,Lepidosiren paradoxa

Nephron of South American lungfish was examined historically and enzyme-histochemically. Cells of the first and second proximal segments exhibited poor interdigitation forming narrow intercellular spaces, whereas the distal segment consisted of deeply interdigitated cells with wide intercellular spaces. Activities of aerobic enzymes (malate, isocitrate, NADH, and β-hydroxybutyrate dehydrogenases), Na-K-ATPase, and carbonic anhydrase were mostly detected in the distal segment. In contrast, hexokinase activity was mostly seen in the 1st and 2nd proximal segments. In the collecting tubule, two types of cells were distinguished by their histological and enzyme-histochemical features. One type showed deep interdigitation and intense carbonic anhydrase activity. The other did not have heavy interdigitation and carbonic anhydrase activity. However, both cell type exhibited intense activities of aerobic enzymes. These structures and enzyme distributions in the lungfish nephron indicate that the lungfish is more specialized in nephron than teleosteans and elasmobranchs, though, slightly similar to the latter.