Introduction

The cornea and the lens are the principal refractive elements of the eye responsible for, respectively, stationary and variable refraction. However, while both the cornea and the lens must be transparent to function properly, the basis of their transparency is quite different. In general, the cornea relies on the continuous pumping of interstitial fluid across its semipermeable surface membranes and a supramolecular organization of collagen fibrils for clarity. In contrast, lens transparency is presumed to be the result of a highly ordered arrangement of its unique fiberlike cells, or fibers, and a gradient of refractive index produced by a variable crystallin protein concentration within the fibers. However, while lens gross anatomy has a major role in determining lens optical quality (variable focusing power), lens ultrastructure is the principal factor in determining lens transparency. Furthermore, while all vertebrate lenses have a similar form, or structure, their anatomy is not identical, and thus their optical quality varies from species to species and as a function of age. In fact, on the basis of structure, four types of lenses can be distinguished. Key differences in lens morphology, caused during specific periods of development and growth, result in quantifiable variations in optical quality. Furthermore, lens structural anomalies, caused during the same periods of development and growth, result in quantifiable degradation in optical quality. Thus, vertebrate lenses are a prime example of form following function and malformation leading to malfunction.