title: The elasmobranch kidney. III. Fine structure of the peritubular sheath.
abstract: 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).
mesh_terms: D000818:Animals; D003238:Connective Tissue / Q000648:ultrastructure; D004284:Dogfish / Q000033:anatomy & histology*; D005260:Female; D005614:Freeze Fracturing; D007684:Kidney Tubules / Q000648:ultrastructure*; D008854:Microscopy, Electron; D012754:Sharks / Q000033:anatomy & histology*