Optic nerve ultrastructure was studied in nine humans and 11 monkeys. Unmyelinated nerve fibers, belonging to the peripheral nervous system, lie in the adventitia of the central retinal artery, but all axons of the optic nerve itself are myelinated. The ultrastructure of axons and glial cells is the same as elsewhere in the central nervous system. Astrocyte cell bodies and processes insinuate themselves among the nerve fibers and nerve fiber bundles. An astroglial layer lines all the neuroectodermal surfaces that abut upon connective tissue, such as the pia mater, the adventitia of the central retinal vessels, and the septa. Blood vessels are confined to the connective tissue of the septa and the pia mater. The vessels are mainly capillaries with nonfenestrated endothelium. Adjacent endothelial cells seem to be held together by zonulae occludentes (tight junctions).
Anderson DR, Hoyt WF. Ultrastructure of Intraorbital Portion of Human and Monkey Optic Nerve. Arch Ophthalmol. 1969;82(4):506-530. doi:10.1001/archopht.1969.00990020508017