Functional Morphology of Rabbit Iridial Ciliary Processes

Using scanning electron microscopy of vascular resin casts, it was found that the capillary network of the iridial ciliary processes is supplied by arterioles of its own and drained posteriorly by long isolated venules. These venules pass the ciliary body without greater connection to the venules of the major ciliary processes (shunt-system). Transmission electron microscopy showed that in contrast to the pigment epithelium (PE) of the major ciliary processes, the PE of the iridial processes develops elongated finger-like cytoplasmic processes which protrude into the stroma, often in the direction of the vessels, forming a 'basolateral labyrinth', like the kidney tubule epithelium. After paracentesis, an enlargement and leakage of the iridial ciliary process capillaries was found, resulting in resin leakage at various sites; no leakage was found in the vasculature of the major ciliary processes. Electron microscopically, the basolateral labyrinth of the PE appeared enormously enlarged, forming wide protein-containing intercellular spaces. These findings suggest that the function of the iridial ciliary processes in rabbits has a dual nature. Under normal circumstances, the main function of the iridial processes may be fluid reabsorption rather than aqueous humor secretion. In cases of paracentesis, for example, the very sensitive blood aqueous barrier of this area breaks down with resulting protein exudation.