Lens Structure And Function

The transparency of the lens is closely linked to the unique structure and function of its fibre cells. These highly differentiated cells are derived from equatorial epithelial cells that exit the cell cycle and embark upon a differentiation process that produces extensive cellular elongation, the loss of cellular organelles and nuclei, and the expression of fibre-specific proteins (3, 4). At the lens poles, elongating fibres from opposite hemispheres meet and interdigitate to form the lens sutures (5). Since this process continues throughout life, a gradient of fibre cells at different stages of differentiation is established around an internalized nucleus of mature, anucleate fibre cells (Fig. 1A). While the transparent properties of the lens are a direct result of its highly ordered tissue architecture, the lens however, should not be considered a purely passive optical element. Because of its size, the avascular lens cannot rely on passive diffusion alone to transport nutrients to deeper-lying cells, or to transport waste products back to the surface (6, 7). Thus, maintenance of this architecture requires special mechanisms not only to supply the deeper-lying fibre cells with nutrients, but also to control the volume of these cells.

A common feature of all vertebrate lenses studied to date (8), is the existence of a standing flow of ionic current that is directed inward at the poles and outward at the

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