Factors affecting the differentiation of epithelial transport and responsiveness to hormones.

Under standard culture conditions, epithelial cells grow with their basal surface attached to the culture dish and their apical surface facing the medium. Morphological and functional markers are located in the appropriate plasma membrane, and transepithelial transport occurs in a variety of cultured epithelia. As a result of the polarity of the cells and the presence of tight junctions between cells, on standard tissue culture dishes there is restricted access of growth medium to the basolateral surface of the epithelium, which is the surface at which nutrient exchange normally occurs. Greater differentiation of epithelial cultures can be achieved by growing primary cultures or continuous cell lines on permeable surfaces such as porous bottom cultures dishes in which the porous bottom is formed by a filter or membrane of collagen, or on floating collagen gels. In many cultures, differentiation varies with the time after the culture was seeded. Certain chemicals that accelerate differentiation in nonepithelial cells also accelerate the differentiation of epithelial cultures. Ultimately, defined media and specific substrates for cell attachment should lead to further differentiation of epithelia in culture.