Ultrastructural differences between rat inner medullary descending and ascending vasa recta

The ultrastructure of rat inner medullary vasa recta was studied by both conventional transmission and freeze fracture electron microscopy. The identity of descending and ascending vasa recta in the inner medulla was established by tracing outer medullary descending vasa recta and ascending vasa recta into the inner medulla, as well as by the incomplete carbon labeling technique to identify isolated descending vessels or loops. As in the outer medulla, descending vessels possess thick continuous endothelium with pinocytotic vesicles budding off the luminal and basal plasma membranes (more numerous in the latter location), but no fenestrae. Ascending vasa show thin, attenuated endothelium with numerous (500 to 800 Å) fenestrae bridged by 40 Å thick fenestral diaphragms. Intercellular junctions in both vessels are zonulae occludens with usually one, and uncommonly two or three zones of outer leaflet membrane fusion. Intramembranous particles are numerous in endothelial plasma membranes of descending vasa recta; similar particles are much fewer in corresponding ascending vasa recta and ascending vasa recta may reflect markedly different permeability properties, particularly to macromolecules, and may be relevant to recent functional studies on rat inner medullary vasa recta.