Fig. 3. Cross sections of the glandular structures in the rectal part of the intestine in elephant fish. Adjacent sections were stained with anti-Na⁺,K⁺-ATPase serum (A,C,D) and with periodic acid–Schiff (PAS; B). (C,D) Magnified views of the section in A, showing the transition from the secretory epithelium (stained with anti-Na⁺,K⁺-ATPase serum) to the mucus cells along the single tubule (arrows). Immmunoreactive signals for Na⁺,K⁺-ATPase were localized on the basolateral membranes but not on the apical membranes of epithelial cells. Scale bars, 500 µm (A–C); 100 µm (D).