Syncytial Epithelia: Transport in the Absence of Paracellular Pathways

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Abstract
The syncytial epithelium of parasitic flatworms offers the opportunity to examine epithelial transport physiology in the absence of paracellular pathways. The asymmetric enzymatic and permeability properties of the apical and basal membranes confirm the transepithelial transport function of the syncytial epithelium. Although the absence of a paracellular pathway has led to the suggestion that the syncytium is a ‘tight’ epithelium, which would be consistent with its low osmotic and diffusive water permeability, the ion transport mechanisms in the apical membrane are more consistent with those predominating in ‘leaky’ epithelia. Contrary to that expected of an animal covered with a ‘tight’ epithelium, the parasitic flatworms are not good ion regulators. The apical membrane contains a Cl−: Na+ co-transport system characteristic of ‘leaky’ epithelia. Although external acidification occurs by an active H+ extrusion mechanism, a large part of H+ secretion is coupled to Na+ influx as in ‘leaky’ epithelia. Categorization of the syncytial epithelium as ‘tight’ or ‘leaky’ will have to await electrical potential profile determinations which are made difficult by the electrical coupling of the syncytium to the underlying nerve-muscle syncytium.