Morphological factors influencing transepithelial permeability: A model for the resistance of theZonula Occludens

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Abstract
Epithelial cells are joined at their apical surfaces byzonulae occludentes. Claude and Goodenough (1973) demonstrated a correlation between the structure of thezonula occludens as seen in freeze-fracture preparations and the passive electrical permeability of several simple epithelia. In epithelia with high transepithelial resistance, thezonula occludens consisted of many strands. In epithelia with low transepithelial resistance thezonula occludens was much reduced, sometimes consisting of only one strand. Evidence is reviewed here that indicates that in a number of simple epithelia the structure of thezonula occludens is largely responsible for the magnitude of transepithelial conductance. An equation is derived relating transepithelial junctional resistance to the number of junctional strands:R=R min p −n whereR is the transepithelial resistance of thezonula occludens,R min is the minimum resistance of the junction (as when there areno strands in the zonula occludens),p is the probability a given strand is “open” andn is the number of strands in the junction. Using published experimental values ofR andn for different epithelia, the calculated value ofp was found to be as high as 0.4, which suggests that the strands in thezonula occludens are remarkably labile. Other morphological parameters relevant to transepithelial permeability are also considered, such as the width and depth of the intercellular spaces, and the size of the epithelial cells themselves.