Microanatomy and modulation of the ciliary vasculature.

Scanning electron microscopy of primate anterior uveal methylmethacrylate vascular casting has revealed a complex, multifaceted, highly anastomotic vasculature. The anterior and long posterior ciliary arteries interconnect to form three collateral vascular circles: the episcleral circle; the intramuscular circle; the 'major' arterial circle. Located at the iris root, the 'major' arterial circle is the most discontinuous with its circumferentially oriented vessels frequently ending in iris and ciliary process arterioles. The ciliary process microvasculature is also highly complex, with two types of arterioles: anterior and posterior. Anterior arterioles possess focal constrictions near their origins and supply capillaries to the anterior regions and margins of the major ciliary processes. Posterior arterioles are less constricted and supply the minor ciliary processes and the central portions of the major processes. These findings, and the demonstration of multiple other channels, suggest several mechanisms by which blood can be shunted between and around the major and minor ciliary processes. In addition, we illustrate the use of scanning electron microscopy of methacrylate luminal castings to demonstrate alterations of the ciliary microvasculature in response to both pharmacologic and surgical stimuli. Quantitative studies of methacrylate castings may allow future localisation of the sites of vasomotor substance activity and improve our understanding of the role of blood flow in aqueous humour production.