Microvasculature of the rat optic nerve head

PURPOSE. To describe the arterial blood supply, capillary bed, and venous d rainage of the rat optic nerve head. METHODS. Ocular microvascular castings from 6 Wistar rats were prepared by injection of epoxy resin through the common carotid arteries. After polymer ization, tissues were digested with 6 M KOH, and the castings washed, dried , and coated for scanning electron microscopy. RESULTS. Immediately posterior to the globe, the ophthalmic artery trifurca tes into the central retinal artery and two posterior ciliary arteries. The central retinal artery directly provides capillaries to the nerve fiber la yer and only contributes to capillary beds in the neck of the nerve head. T he remainder is supplied by branches of the posterior ciliary arteries that are analogous to the primate circle of Zinn-Haller. Arterioles arising fro m these branches supply the capillaries of the transitional, or laminar, re gion of the optic nerve head. These capillaries are continuous with those o f the neck and retrobulbar optic nerve head. All optic nerve head capillari es drain into the central retinal vein and veins of the optic nerve sheath. A flat choroidal sinus communicates with the central retinal vein, the cho riocapillaris, and with large veins of the optic nerve sheath. CONCLUSIONS. The microvasculature of the rat optic nerve head bears several similarities to that of the primate, with a centripetal blood supply from posterior ciliary arteries and drainage into the central retinal and optic nerve sheath veins. Association of nerve sheath veins with the choroid repr esents an important difference from the primate.