Fibre cell organization in crystalline lenses.

This review qualitatively and quantitatively compares the gross shape and size of lenses from different species as a function of their fibre cell organization. Grossly, all vertebrate lenses are asymmetrical, oblate spheroids with size and spheroidicity that varies considerably between species. Correlative LM and SEM analysis of the basic structural element of lenses, the fibre cell, shows that the average equatorial fibre width and thickness is relatively constant between most species. This indicates that inter-species differences in lens size is primarily a function of fibre number. Comparable analysis demonstrates that lens spheroidicity is due at least in part, to differences in the average anterior and posterior fibre end segment thickness in relation to that at the equator. In addition, the above analysis, supplemented by 3D-CAD reconstructions, illustrates how lifelong lens growth produces crescent fibres, that become arranged in age-related, concentric growth shells overlaid in slightly imprecise register. The reason for the non-exact registering of growth shells is that, while the vast majority of fibres are hexagonal in cross-section, a very small minority are pentagonal in cross-section and of inconstant width and thickness. Hexagonal and pentagonal fibres are required because the increase in the circumference of successive growth shells is frequently less than the widths of hexagonal fibres. Thus, lens growth is likely accomplished by a combination of the addition of successive growth shells containing more fibres, as well as by the addition of growth shells containing equal numbers of fibres that are incrementally wider as a function of radial location. Finally, SEM analysis, supplemented by 3D-CAD reconstructions, highlights the fact that the end-to-end arrangement of fibres within growth shells, the suture patterns, is not identical in all vertebrate lenses. This is important because lens optical quality is directly related to lens suture type and a negative influence of sutures on lens optical quality increases with age and as a result of some ocular surgeries (vitrectomy and trabeculectomy). These facts suggest that future research efforts should be directed at determining the factor or factors that influence or direct the differences in fibre shape, size and organization in branched and unbranched suture lenses.