Comparative biochemistry and biophysics of elasmobranch lenses

This presentation compares the features of the lenses of sharks and skates to those of land animals, including man. Of all the tissues of vertebrates, the chemistry and physical features of the ocular lens have been preserved intact. Functionally, while the transmission of light and images is common to both classes, the elasmobranch lens shares the added task of refraction with the teleosts (as the cornea does not refract in the sea), and the mammal lens is deformed to support accommodation, while the elasmobranch lens is not. Light filtration by lens pigments does occur mainly in diurnal or shallow-swimming species. The same group of lens crystallins is present in sharks (and skates) as is present in squirrels (and humans). The same structural arrangement of a mono-layer of epithelial cells anteriorly and concentric layers of differentiated and fiber cells from the periphery to the center apply to most vertebrates. Fiber cell ribbon shapes and interdigitating knobs are also very similar, as are the fine structures of the fiber cell membranes and the polymerized arrays of actin. Yet the elasmobranch lens is hard and spherical, while the mammalian lens is soft and disc-shaped. What the chemical basis is for the differing physical states of lenses of shark and man remains to be answered. This presentation will document the chemical and morphological bases for elasmobranch and mammal lens structure and function. It also will show that even though a shark is a primitive vertebrate, its lens is still useful as a model to assist in the understanding of lens aging, of lens swelling (or lack of it) and the classical basis of cataracts (or lack of them).