The crystalline lens. A system networked by gap junctional intercellular communication.

The vertebrate eye lens is a solid cyst of cells which grows throughout life by addition of new cells at the surface. The older cells, buried by the newer generations, differentiate into long, prismatic fibers, losing their cellular organelles and filling their cytoplasms with high concentrations of soluble proteins, the crystallins. The long-lived lens fibers are interconnected by gap junctions, both with themselves and with an anterior layer of simple cuboidal epithelial cells at the lens surface. This network of gap junctions joins the lens cells into a syncytium with respect to small molecules, permitting metabolic co-operation: intercellular diffusion of ions, metabolites, and water. In contact with nutrients at the lens surface, the epithelial cells retain their cellular organelles, and are able to provide the metabolic energy to maintain correct ion and metabolite concentrations within the lens fiber cytoplasms, such that the crystallins remain in solution and do not aggregate (cataract). Gap junctions are formed by a family of integral membrane channel-forming proteins called connexins. Gap junctions between lens epithelial cells are composed of a connexin which is common between many different cell types, notably myocardial cells and connective tissue fibroblasts. The gap junctions between epithelial cells and lens fibers have not yet been biochemically characterized. The gap junctions formed between lens fibers are composed of at least two different connexins, one of which has not been detected between other cell types. The unusual physiology and longevity of the lens fibers may require the special set of connexins which are found joining these cells.