Steps in the morphogenesis of a polarized epithelium : I. uncoupling the roles of cell-cell and cell-substratum contact in establishing plasma membrane polarity in multicellular epithelial (MDCK) cysts

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Abstract
The development of cell polarity in Madin-Darby canine kidney (MDCK) cells has been analyzed under conditions in which cells are induced to form multicellular epithelial cysts in stages that mimic the ontogeny of epithelial tissues and organs in vivo. The morphogenesis of MDCK cysts in suspension culture or in a collagen gel proceeds in distinct stages involving the initial aggregation of cells followed by development of a closed monolayer of polarized epithelial cells that surrounds a central lumen. The polarity of cells was determined at each stage by analyzing the distributions of marker proteins of the apical (gpl35) and basal-lateral (Na+,K+-ATPase) domains of the plasma membrane, the tight junction (ZO-1) and proteins involved in cell-cell (uvomorulin) and cell-substratum contact (type IV collagen). We show that cells have a distinctive and opposite polarity in cysts formed in suspension culture compared to those formed in collagen gels. In suspension culture, the basal-lateral membrane faces the central lumen and the apical membrane faces the outside, whereas in collagen gel, the basal-lateral membrane faces the outside collagen and the apical membrane faces the central lumen. Detailed analysis of the distributions of marker proteins during the morphogenesis of these three-dimensional structures indicates that: (1) cell-cell contact is sufficient to trigger the segregation of marker proteins of the apical and basal-lateral membrane domains to distinct regions of the membrane; (2) cell-cell contact induces association of the tight junction protein ZO-1 with the contact zone between cells; (3) localization of the tight junction protein ZO-1 to the apex of the lateral membrane and the establishment of the epithelial axis, however, requires the formation of a basal lamina and cell-substratum contact; (4) in suspension culture, MDCK cysts secrete and establish a basal lamina in the central lumen. These results show that cell-cell and cell-substratum contact have distinct roles in the morphogenesis of polarized epithelia. We suggest that the mechanisms involved in triggering cell polarity may be common to different polarized epithelia in vivo.