Fig. 4. Immunofluorescence staining of (A–C) Na⁺,K⁺-ATPase (green) and (D–F) occludin (red) in cross-sections of goldfish proximal tubule (A,D,G), distal tubule (B,E,H) and collecting duct (C,F,I). Nuclei are stained with DAPI (blue) in A–C. G, H and I are merged images of A and D, B and E, and C and F, respectively. Immunostaining for Na⁺,K⁺-ATPase is restricted primarily to the basal membrane of renal epithelial cells in the proximal region of the nephron (A) and to the basolateral membrane of renal epithelial cells in distal and collecting segments (B,C). No occludin immunostaining is observed in proximal segments of the nephron (D). Strong and moderate occludin immunostaining is concentrated at the apical membrane of renal epithelial cells lining the lumen of the distal tubule and collecting duct, respectively (E,F). All scale bars are 20 µm.