Time course and development of light adaptation processes in the outer zebrafish retina

Retinomotor movements are morphological changes in the outer retina in response to changing light conditions. They can be separated into two components: Migration of pigment granules within the microvilli of the retinal pigment epithelium (RPE) and positional changes in photoreceptor cells. These positional changes optimize exposure of the cone and rod photoreceptors to light. The aim of this study was to analyze both the time course of retinomotor movements in the adult zebrafish and the maturation of these processes in the developing fish. We show that retinomotor movements are used as a dark/light adaptation mechanism in zebrafish. In adult zebrafish, melanin granules of the RPE migrate with constant speed and reach the fully light adapted (LA) state approximately after 1 h. In contrast, about two thirds of double cone outer segment movements are finished in 5 min, and are fully completed in 10 to 20 min. During development there are three crucial stages leading to mature retinomotor movements in response to light: at 5 dpf (days post fertilization) the migration of pigment granules begins, at 20 dpf the pigment granules condense in the apical part of the RPE microvilli, and at 28 dpf, concomitant with the functional maturation of rods, the double cones contract as in adult retinas.