The ultrastructure of epithelial and fiber cells in the crystalline lens

Crystalline lenses are often simply described as inside-out stratified epithelial-like organs composed of uniform (hexagonal cross-section profiles) crescent-like cells, arranged end-to-end in concentric shells around a polar axis. In this manner, as light is transmitted through lenses, their highly ordered architecture contributes to transparency by effectively transforming the multicellular organ into a series of coaxial refractive surfaces. This review will attempt to demonstrate that such a description seriously understates the structural complexity that produces lenses of variable optical quality in different species as a function of development, growth, and age. Embryological development of the lens occurs in a similar manner in all species. However, the growth patterns and effects of aging on lens fibers varies significantly among species. The terminally differentiatedfiber cells of all lenses are generally hexagonal in cross section and crescent shaped along their length. But, while the fibers of all lenses are arranged in both highly ordered radial cell columns and concentric growth shells, only avian lens fibers defined by different shapes are continuously formed throughout life. The majority of fibers are s-shaped, with ends that do not extend to the poles. Rather, the ends of these fibers are arranged as latitudinal are lengths within and between growth shells. The overlap of the ends of specifically defined groups of such fibers constitutes th elens suture branches. The location, number, and extent of suture branches within and between growth shells are important considerations in lens function because the shapes of fiber ends, unlike that along fiber length, are very irregular. Consequently, as light is transmitted through sutures, spherical aberration (i.e., focal length variation) is increased. The degree of focal length variability depends on the arrangement of suture branches within and between growth shells, and this architecture varies significantly between species. The lifelong production of additional fibers at the circumference of the lens, culminating in new growth shells, neither proceeds equally aroung the lens equator, nor features identical fibers formed aroung the equator. Suture formation commences in the inferonasal quadrant, and continues sequentially in the superotemporal, inferotemporal, and finally the superonasal quadrants. During this proce ss, lens growth produces fibers of specifically defined length and shape as a function of their equatorial location. Utilizing a variation of this growth scheme, primates produce fibers that are arranged in progressively more complex suture patterns as a function of development, growth, and age that correlates with the temporal development of the zones of discontinuity seen by slit-lamp biomicroscopy. fiber morphology also changes as a consequence of aging. This is an important consideration since the lens, owing to its inverted embryological and growth pattern, retains every cell formed throughtout life. As a result fibers develop unique intercellular contacts for adhesion and communication that are specifically altered throughout life. In conclusion, the lens features many organ-specific structural specializations. While the species variations of these specializations are, in general, consistent with those of other epithelial systems, they represent specific modifications necessary for lens function. © 1995 Academic Press Inc.