Diffusion barriers in the vaginal epithelium during the estrous cycle in guinea pigs.

In the present study the permeability barriers of the multilayered vaginal epithelium were examined using tracer perfusion techniques, freeze-fracture and thin sectioning. During diestrus and proestrus the upper layers of mucified epithelial cells exhibit tight-junctional belts, which restrict tracer molecules such as lanthanum and horseradish peroxidase. When the highly mucified cells begin to degenerate toward the end of proestrus the underlying epithelium is already keratinized as typical for estrus. The keratinized epithelial cells have a tight-junctional network that joins the basal plasma membranes with the apical membranes of subjacent cells and blocks paracellular diffusion of the tracer molecules. During conversion of the cornified epithelium to a mucified epithelium in metestrus the intercellular space of the epithelium is stained by tracer molecules even though tight-junctional belts can be observed. These results indicate that during cyclic changes of the vaginal epithelium tight junctions can, in general, be considered for the restriction of paracellular diffusion. In metestrus, however, junctions become functionally leaky although they remain morphologically intact. Intercellular lipids, which are normally common in cornified epithelia, are extremely rare and cannot constitute an effective barrier to diffusion in the vagina of the guinea pig. The significance of a strategy that bases the regulation of the permeability on tight junctions rather than on intercellular lipids is discussed.