Diffusion processes into and out of optic nerve capillaries and between vitreous and optic nerves were studied with peroxidase in squirrel monkeys. Following intravenous injection of peroxidase, no passage of tracer out of optic nerve capillaries occurred. Diffusion was obstructed by zonulae occludentes of capillary endothelium. After intravitreal injection, tracer diffused freely through the internal limiting membrane into intercellular spaces of the optic nerve parenchyma. Zonula occludentes prevented flow from the perivascular spaces of the optic nerve into the capillary lumen. A blood-nerve diffusion barrier thus exists in the optic nerve.
Peyman GA, Apple D. Peroxidase Diffusion Processes in the Optic Nerve. Arch Ophthalmol. 1972;88(6):650-654. doi:10.1001/archopht.1972.01000030652017