3HOH-osmotic water fluxes and ultrastructure of an epithelial syncytium

Summary Ultrastructural changes associated with osmotically-induced water transport and water permeability were examined in two flatworm species,Schistosoma mansoni andHymenolepis diminuta. The structure of the surface layer of these parasites is unusual in that it is a syncytial epithelial layer that lacks tight junctions and lateral extracellular spaces. The permeability coefficients observed in this study are therefore necessarily associated only with the transcellular route of transepithelial transport. The ultrastructural changes associated with volume transport across the epithelial syncytium were also unusual in that the basally located channels extending distally from the inward-facing membrane into the syncytial layer remained open regardless of the direction of water flow.Despite the structural differences, most of the features of diffusive (P_d) and osmotic (P_osm) water fluxes across the syncytium resembled those observed in other epithelia: (i) Low water permeability with maximum values of 4.1×10^–5 forP_d and 9.6×10^–5 forP_osm.(ii)P_osm>P_d by 2.0- to 3.2-fold. (iii) Outward water permeability less than inward water permeability. This asymmetry could not be attributed to collapsing channels when net volume transport was directed outward since channels in the syncytium remained open regardless of the direction of water flow. The asymmetry could be explained by tissue contraction or swelling when bathed in anisotonic fluids. (iv)P_osm values were not significantly altered by tissue unstirred layers but bothP_osm andP_d values were underestimated when the bulk fluid was not vigorously stirred.The lower permeability inS. mansoni relative toH. diminuta may be attributed to the membranous surface coat of the former species.