Fig. 3. (A) Longitudinal section of the Xenopus kidney stained with haematoxylin and eosin. A medial band of glomeruli (^*) separates the ventromedial zone from the dorsolateral zone of the Xenopus kidney. Proximal tubules and collecting ducts are contained within the dorsolateral zone, while distal tubules are located within the ventromedial zone. (B) Na⁺,K⁺-ATPase immunostaining (green) within a longitudinal section of the Xenopus kidney. Within the dorsolateral zone, only collecting ducts show Na⁺,K⁺-ATPase immunoreactivity. Within the ventromedial zone, distal tubules (both early and late) show immunofluorescence labelling for Na⁺,K⁺-ATPase. (C–F) Dual-immunofluorescence labelling of occludin and Na⁺,K⁺-ATPase within cross-sections of the (C) proximal tubule, (D) early distal tubule, (E) late distal tubule and (F) collecting duct of the Xenopus nephron. Occludin (red) and Na⁺,K⁺-ATPase (green) were differentially expressed in the nephron. Occludin was undetectable in proximal tubules, but immunolocalized to apical membranes of renal epithelial cells of distal tubules (early and late) and the collecting duct. Na⁺,K⁺-ATPase weakly immunolocalized basally in proximal tubules and exhibited robust basolateral localization in the distal and collecting segments. A control section, probed with secondary antibody only, is shown in G. L, lumen. Scale bars for A and B, 300 µm; scale bars for C–G, 20 µm.