Functional morphology of the episcleral vasculature in rabbits and dogs: presence of arteriovenous anastomoses.

Scanning electron microscopic analysis of corrosion casts of the intra- and episcleral vasculature in the limbic region of two species (rabbit and dog) revealed typical arteriovenous anastomoses representing shunts between the long radial arterioles, branches of the anterior ciliary arteries, and the episcleral venous plexus. The arteriovenous anastomoses consist of a small arteriolar segment and a wide, funnel-like venous segment. Using immunohistochemical techniques it was shown that the wall of the arteriolar segment consists of smooth muscle cells containing alpha-smooth muscle actin. In the dog, the vessel wall of the arteriovenous anastomoses also contain typical epithelioid cells, the structure of which was analyzed by transmission electron microscopy. In the episcleral vasculature, a capillary network is lacking, an elaborate wide venous plexus is developed instead. It is assumed that this plexus with the help of the newly described arteriovenous anastomoses may influence aqueous humor drainage and thereby also intraocular pressure.