Morphological and Physiological Adaptations in Cichlid Crystalline Lenses

Sammanfattning: Vertebrate eyes share a common ancestry as well as the basic design with a pupil, a retina and a lens. The lens is essential for perceiving the details in the world around us and ensures that a sharp image of the surroundings is focused onto the photoreceptors in the retina. Certain lenses have multifocal properties, in short multifocal lenses acts as filters, matching the wavelengths, or colours, focused onto the retina with the spectral sensitivity of the photoreceptors. This matching is thought to improve the visual quality, however, in certain cases the retina changes its spectral sensitivity, permanently or over periods of time. For instance, many fish species, like cichlids, can alternate what photoreceptors are active between night and day through retinomotor movements. During day the colour discriminating cones receive the focused light from the lens and at night the retina transforms so that the more light sensitive rods instead receive the focused light. With such dramatic changes in the retina one can speculate on what happens in the lens. If the light focused by the lens is matched to the light adapted retina, will the focused light and the sensitivity of the retina be miss-matched during the night or will the lens adjust to match the demands of the retina during night? Paper I describe that lenses undergo dark adaptation in what appears to be a meaningful way and that the signalling substance dopamine might be behind the process directly or indirectly. During night the refractive power of the lens cortex, the outermost 40%, increases significantly. Depleting the eye of dopamine increases the refractive properties in a similar but stronger way compared to that of dark adaptation. Paper II detail a straight forward method to test the effects of signalling substances on excised lenses and in paper III this method is used to test the effect of dopamine on the lens. Dopamine decreases the refractive power of the lens cortex in a dose dependant manner through activation of a D1 receptor like pathway. The changes seen in papers I-III are tied to the optical properties of the lens cortex, the outermost 40 percent of the lens radius. How these changes are induced is uncertain since only the outermost 8 percent of the lens radius contain nuclei and other organelles. For these small optical changes to make a difference the lens needed to be suspended firmly in the eye in relation to the retina. Paper IV is a re-examination of the lens suspension in cichlids and other teleosts and confirms that the actual lens suspension is more complex than previously described.