Tubular invaginations with caveolae and coated pits in the sinus endothelial cells of the rat spleen

The fine structure of plasmalemmal tubular invaginations with caveolae and coated pits in the sinus endothelial cells of the rat spleen has been demon strated by scanning and transmission electron microscopy. In addition, the three-dimensional structure of the tubular invagination has been revealed b y computer-aided reconstruction. The tubular invaginations of the plasma me mbrane plunged into the cytoplasm everywhere from the apical, lateral, and basal surfaces of the plasma membrane. The invaginations were tubular and b ranched away, and their plasma membranes were reinvaginated to form numerou s caveolae and occasional coated pits. Numerous caveolae were found in clus ters that looked similar to a bunch of grapes and the coated pits were pres ent at the base of the clusters. The caveolae and coated pits derived from the tubular invaginations were almost ultrastructurally identical to those derived from the surface plasma membrane. From examination of the fractured surfaces of the endothelial cells treated with the aldehyde prefix osmium- dimethyl sulfoxide-osmium method and of ultrathin sections of those infiltr ated by lanthanum nitrate, the tubular invaginations were found to not pene trate any endothelial cells. A computer-aided reconstruction revealed that the caveolae derived from the tubular invaginations were in close appositio n to the surface-connected canaliculi. The reaction product of Concanavalin A conjugated to horseradish peroxidase was present on the outer leaflet of the membranes of the coated pits and coated vesicles and also in the conte nts of the endosomes, but it was absent from any caveolae. Based on our obs ervations, the functional significance of the tubular invaginations in sinu s endothelial cells is discussed.