An electron microscopic study on the blood-optic nerve and fluid-optic nerve barrier.

Utilizing horseradish peroxidase as a tracer, electron microscopic studies were done on the blood-optic nerve and fluid-optic nerve barrier to the peroxidase diffusion. Following intravenous injection the peroxidase was observed to fill the lumen of the capillaries of the laminar, prelaminar and orbital portions of the optic nerve but there was no penetratation of the capillary walls. The obstruction of the tracer diffusion out of capillary walls was attributed to the tight junctions between the endothelial cells. Peroxidase penetration was also absent in the capillaries of the pia and dura mater, however, was observed in pinocytotic vesicles of the endothelial cells. Lateral diffusion from the surrounding choroid into the optic nerve was detected but diffusion from the prelaminar optic nerve into the juxta-optic nerve retina was prevented by the Kuhnt intermediary tissue. Tight junctions which prevented peroxidase diffusion were found between the glial cells of the Kuhnt tissue, and this tissue was the barrier between the prelaminar optic nerve and the juxta-optic nerve retina. Peroxidase which was given into the lateral ventricle of the brain appeared in the subarachnoidal space around the optic nerve and penetrated freely into the optic nerve. The pial surface of the optic nerve possess no barrier activity. Peroxidase could be traced along the intercellular space between glial cells and optic nerve fibers. The basal lamina of the optic nerve capillaries was filled with peroxidase but diffusion into the capillary lumen was obstructured by the tight junctions between the endothelial cells.