Pericapillary permeability of the ciliary processes: role of molecular charge

The pericapillary permeability of the ciliary processes to intravenously injected native (anionic) ferritin, neutral ferritin, and two cationic ferritin derivatives was studied in normal rats by electron microscopy. Anionic and neutral ferritin were largely confined to the circulatory compartment. Those particles that entered the pericapillary region of the ciliary processes were randomly scattered within the basal lamina. In contrast, cationic ferritin left the circulatory compartment and accumulated in the pericapillary region in association with the endothelium, the endothelial fenestrations, and the subendothelial basal lamina. The most cationic of the tracers used exhibited the greatest penetration and accumulation. The results indicate that the localization of tracer within the pericapillary region of the ciliary processes is directly related to the tracer's isoelectric point. The findings suggest that this region of the ciliary processes contains fixed anionic groups that influence its permeability.