Role of microtubules in pigment granule migration in teleost retinal pigment epithelial cells.

In cells of the teleost retinal pigment epithelium (RPE), melanin pigment granules migrate in response to changes in environmental light conditions. Melanin granules disperse into the RPE cell's long apical projections in response to the onset of light, and aggregate towards the base of the RPE cell in response to the onset of darkness. The RPE cells possess numerous microtubules and actin filaments, which in the apical projections are aligned longitudinally. Previous cytochalasin studies have shown that intact actin filaments are required for pigment granule dispersion and maintenance of the dispersed state (Burnside, Adler and O'Connor (1983). Invest. Ophthalmol. Vis. Sci. 24, 1). We report here that pigment granule aggregation is strongly inhibited when the highly stable microtubules of RPE apical projections are disrupted by a combination of cold and nocodazole treatments. Pigment dispersion and maintenance of the dispersed and aggregated states are unaffected by microtubule disruption. These results indicate that microtubules are required for RPE pigment aggregation but not for dispersion.