[Capillary ultrastructure and permeability in the posterior lobe of the rat pituitary].

The ultrastructure of capillaries and their permeability to horseradish peroxidase and ferritin in rat posterior pituitary has been studied. The capillary wall is formed mainly by flat endothelium penetrated by regularly situated fenestras. The majority of organelles is concentrated in the perikaryon. Caveoles and pinocytotic vesicles are not numerous except for some perikaryons where they are concentrated along the basal surface. Intracellular clefts are interrupted in the regions of fusion of the outer osmiophilic layers of membranes of neighbouring endothelial cells. Horseradish peroxidase and ferritin penetrate partly from the capillary lumen into the pericapillary space: the majority of markers penetrate across the fenestras and the minority--within pinocytosis vesicles. The former way is more rapid than the latter. Ferritin penetrated into the cytoplasm is captured by lysosomes.