Ciliary Body and Ciliary Epithelium

The ciliary body is a complex, highly specialized tissue that comprises several cell types. The ciliary muscle is situated at the base of the ciliary body and ligaments originating in the ciliary body attach to the lens. Contraction or relaxation of the muscle alters tension on the lens causing it to alter shape and thus shift focus. The surface of ciliary body is elaborated into a series of ridges named ciliary processes. Each ciliary process contains a complicated network of blood vessels that appear leaky to plasma constituents. The ciliary processes are covered by a specialized epithelium bilayer that comprises two distinct epithelial cell types, pigmented ciliary epithelium (PE) and nonpigmented ciliary epithelium (NPE). The ciliary epithelium bilayer constitutes a diffusion barrier between the blood and the aqueous humor in the interior of the eye. Barrier function depends on tight junctions between adjacent NPE cells. The ciliary body is responsible for the production of aqueous humor, a task that requires the polarized cellular distribution and coordinated function of Na, K-ATPase, Na/K/2Cl cotransporter, Na-H exchanger chloride channels and aquaporins in the NPE and PE. There is evidence suggesting an important role for gap junctions between the NPE and PE layers. The rate of aqueous humor secretion can be modified by ion transport inhibitors, by agents that modify gap junction permeability and by maneuvers that change blood flow in the ciliary processes.