Cell biological features of an embryonic renal collecting duct (CD) epithelium kept for 13 days in a fluid gradient. In the gradient container at the luminal side IMDM + aldosterone (1 × 10-7 M) + 15 mmol/l NaCl, while at the basal side IMDM + aldosterone (1 × 10-7 M) was transported. Immunohistochemistry shows a positive label for tissue-specific (a) Na/K ATPase α5, (b) TROMA I, (c) cingulin and (d) COX 2 protein. In contrast, perfusion culture of epithelia with IMDM at the luminal and basal side does not develop these features. Basal lamina (asterisk), lumen (arrow).