The elasmobranch kidney. III. Fine structure of the peritubular sheath

In the kidney of two elasmobranch fish, the little skate (Raja erinacea) and the spiny dogfish (Squalus acanthias), each tubular bundle is wrapped by a continuous sheath of extremely flattened cells which are ordered in several closely superimposed layers. Thin sections and freeze-fracture replicas demonstrate that extensive tight junctions exist between the cells of this peritubular sheath. The sheath cells lie on a discontinuous basement membrane which suggests that they do not belong to the connective tissue. Conceivably, each peritubular sheath segregates the milieu inside the sheath (surrounding the bundle of 5 tubules and capillaries which form the countercurrent system) from the milieu outside the sheath (connective tissue matrix in which the bundles are embedded).