THE SPECIALIZED JUNCTIONS OF THE LENS

This chapter presents a comparison between the structure and chemical composition of different types of junctions in the lens. It also distinguishes between the possible functional roles of these junctions in ion transport and the maintenance of lens-fluid balance. Lens is a transparent and moderately deformable organ whose principal function is to focus light on the retina. Electrolytes and water move throughout the lens, either by crossing plasma membranes of lenticular cells (transcellular pathway) or by diffusing through the extracellular clefts between cells (paracellular pathway). Cells in the lens are connected by four types of plasma membrane contacts: (1) tight junctions, (2) gap junctions, (3) 18–20 nm junctions, and (4) 11–13 nm junctions. These junctions mediate and control different pathways for the diffusion of ions and small nonelectrolytes within and out of the lens and play fundamental roles in lens-fluid balance and transparency. The most important plasma membrane specialization regulating the permeability of the paracellular pathway to ions and nonelectrolytes is the tight junction. In the lens, tight junctions are found only in the epithelium. Such an asymmetric localization of tight junctions transforms the extracellular clefts in the posterior pole into the pathway of least resistance to the diffusion of molecules within the lens. © 1992, Academic Press Inc.