The lens consists of two morphologically distinct cell types, an unremarkable cuboidal epithelium that covers the anterior surface and concentric layers of fiber cells that account for the remainder, and vast majority, of the tissue volume (Fig. 9.1). The fiber cells are unique in the body. They have an enormous aspect ratio, being no more than a few micrometers wide but often exceeding a thousand micrometers in length. In cross-sectional profile, they appear as flattened hexagons, and their sharply angled membranes enclose a transparent cytoplasm that lacks the organelles found in typical cells. It is striking that these cells of remarkable shape and composition are derived from the more typical cells of the overlying epithelium.
In this chapter, we examine what is known (and, more often, what is not) about the process of terminal differentiation in the lens. We propose a staging system that allows one to discriminate critical periods in the maturation of a lens fiber cell. Using this system, we follow a hypothetical fiber cell through the differentiation program, from the time when it is an unspecialized epithelial cell near the lens equator to the cessation of protein synthesis that occurs when it is a mature fiber cell buried in the lens core. We include speculations on how the differentiation program might act to influence the shape and thus the optical properties of the lens as a whole. Finally, it seems evident that in some cataracts at least, the differentiation program has been interrupted or corrupted.