We examined epithelial cell surface polarity in subconfluent and confluent Madin-Darby canine kidney (MDCK) cells with monoclonal antibodies directed against plasma membrane glycoproteins of 35,000, 50,000, and 60,000 mol wt. The cell surface distribution of these glycoproteins was studied by immunofluorescence and immunoelectron microscopy. At the ultrastructural level, the electron-dense reaction product localizing all three glycoproteins was determined to be uniformly distributed over the apical and basal cell surfaces of subconfluent MDCK cells as well as on the lateral surfaces between contacted cells; however, after formation of a confluent monolayer, these glycoproteins could only be localized on the basal-lateral plasma membrane. The development of cell surface polarity was followed by assessing glycoprotein distribution with immunofluorescence microscopy at selected time intervals during growth of MDCK cells to form a confluent monolayer. These results were correlated with transepithelial electrical resistance measurements of tight junction permeability and it was determined by immunofluorescence that polarized distributions of cell surface glycoproteins were established just after electrical resistance could be detected, but before the development of maximal resistance. Our observations provide evidence that intact tight junctions are required for the establishment of the apical and basal-lateral plasma membrane domains and that development of epithelial cell surface polarity is a continuous process.

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