Fish Lenses : Anatomy and Optics

Sammanfattning: I have investigated some of the biological regulatory mechanisms governing the development of crystalline lenses. I used fish as model animals because they possess optically interesting lenses, while the geometrical simplicity of fish lenses allows for studies that are difficult or impossible with the lenses of other animals.First we have investigated lens optical plasticity by measuring longitudinal spherical aberration in light and dark adapted fish of two species, Atlantic and Polar cod. We noticed that Atlantic cod, native to regions with daily light/dark changes responded to light/dark adaptation by changing the optics of its lens, whereas the optics of Polar cod, living in the polar region, was unchanged on a daily basis (Paper I). However, we observed that the optics of the Polar cod lens changed annually between seasons corresponding to polar day and night (unpublished data). Our findings can be explained by the existence of two different mechanisms controlling the optics of fish lenses. A short-term one adapting the lenses to daily light/dark cycles (Atlantic cod) and a long-term one evolved for coping with long polar days and nights (Polar cod).The second project involved investigation of the osmolality of fish larvae body fluids. We tested two levels of osmolality in two different ways. The first one involved measuring the rate of optical deterioration of excised fish lenses placed in different immersion media, the second one the quality of a whole eye fixation. In both cases, lower osmolality gave better results for fish larvae. The optical quality of larval lenses deteriorated slower and fixation preserved the larval eye in a more natural shape (Paper II). We concluded that zebrafish larvae have lower osmolality in their bodies than adult fish.The third project was dedicated to the investigation of the cellular structure of fish lenses. First, we developed a method to visualize an equatorial cross-sections of adult fish lenses. Than we used the method to examine lenses in two size groups of fish of the same species. We measured lens fiber thickness in four relative radial positions in the lens. Our measurements showed that fish lens fiber cells have the same thickness along the radius of the lens and in both size groups. The average thickness was much lower than in other vertebrates (Paper III).We followed up that study by measuring full thickness profile along the lens radius in nine fish species. The thickness of a fiber was independent from its radial position in the lens in all but one species. We observed that the average fiber thickness depends on species. Additionally, we developed a model for calculating historical lens fiber thicknesses necessary for the cells to reach their current refractive indices and thicknesses by cell compaction. The model showed that the cells would have to lose 66% of their volumes to reach their current sizes. This unlikely number and the constancy of cell thickness suggest that a different mechanism is at work. (Paper IV). Based on the findings from both papers, we conclude that, at least in fish, protein is transported inwards between denucleated fibers in the growing lens. The cells in the peripheral lens layers have synthetic capabilities and are most likely the source of those proteins.