Comparative microvascular anatomy of mammalian ciliary processes

Methylmethacrylate lumenal castings of the ciliary body microvasculature were prepared from eight mammalian species and studied with the scanning electron microscope. In all of these species, the ciliary body is supplied by the major arterial circle, which originates solely from the long posterior ciliary arteries without contribution from the anterior ciliary circulation. In contrast to primates and rabbits, the ciliary processes of the eight species we studied are supplied by only one type of arteriole, which travels posteriorly from the major arterial circle to the iris root, where it gives rise to ciliary process arterioles. Using precise microdissection techniques, we found marked interspecies variations in ciliary process angioarchitecture among the mammalian eyes examined. Rodents (rats and guinea pig) demonstrated several interesting similarities to primates, with extensive interprocess connections and irregularly dilated, concentrically parallel capillaries traveling posteriorly to empty into the choroidal veins. In addition, rat ciliary process arterioles displayed marked focal concentrations suggestive of precapillary 'sphincter' agonal activity. The carnivore ciliary process (cat and dog) is supplied by a single arteriole traveling posteriorly throughout its length and sending capillary arcades to its margin from where they drain outward into venous sinuses at the base of the process. Ungulate processes (sheep, goat, pig and cow) receive blood from multiple arterioles that occupy the process core along with veins that empty into the choroidal circulation. These vessels serve delicate capillaries lying on the sides, margin and head of each process. The anatomic variations described here should be considered in the design and interpretation of physiologic and immunohistochemical studies of ciliary body vascular perfusion in non-primate animal models.