Permeability properties of the tissues in the optic nerve head region in the rabbit and the monkey. An ultrastructural study.

The distribution of the protein tracer, horseradish peroxidase, has been studied in the optic nerve head region in rabbits and monkeys by light and electron microscopy. Following intravenous injection of the tracer, the eyes were enucleated after varying time intervals. Leaking out of the choroidal capillaries in the peripapillary choroid, peroxidase rapidly spread to the adjacent sclera and to the intraneural connective tissue, that is lamina cribrosa and the connective tissue surrounding the intraneural vessels. The tracer then diffused from both the perineural and intraneural connective tissue into the adjacent optic nerve tissue. This diffusion could take place because the intercellular spaces in the astrocytic cell layer interposed between the connective and neural tissues were freely permeable to the tracer. The present investigation confirms the presence of a defect in the blood optic nerve barrier in the optic nerve head. The clinical importance of this defect in the permeability barrier is not known. It may represent a predilection point for pathological events, e.g. in retrobulbar neuritis.