Anionic sites on the luminal surface of fenestrated and continuous capillaries of the CNS.

We injected intravenously cationic ferritin, pI 8.4, and studied patterns of labeling of the luminal surface of capillaries in the CNS of rats. Cationic ferritin consistently labeled the luminal aspect of the diaphragms of fenestrated capillaries in the choroid plexus, median eminence and pineal. No other feature of these endothelial cells was consistently labeled. Diaphragms spanning the mouths of vesicles open to the lumen were not labeled. Occasional ferritin molecules were seen in the BL of these fenestrated vessels. The luminal surfaces of endothelial cells of the continuous capillaries of the brain, and of reactive capillaries proliferating in a region of cold injury necrosis, were not labeled. Ferritin was not seen in cytoplasmic structures or the basal lamina of these vessels. The findings in the fenestrated capillaries of CNS are in agreement with those reported for other fenestrated endothelia. The absence of labeling of the continuous capillaries of the blood-brain barrier differs from findings reported for other continuous capillaries, with the exception of the blood-air barrier portions of lung capillaries.