Retinal damage by constant light in chimaeric mice: implications for the protective role of melanin.

Adult chimaeric mice, containing varying proportions of albino and pigmented cells in their ocular tissues, were exposed to constant light for 5 weeks and the distribution of the surviving rod perikarya in the retina and of the pigmented cells in various eye tissues were compared. In chimaeras which were mostly albino, the retinal lesion was similar to that in pure strain albino mice; in chimaeras with relatively more pigmented cells in their ocular tissues, the retina was unaffected as in fully pigmented mice. In chimaeras with amounts of pigmented cells in their ocular tissues varying between these two ends, lesions of intermediate degrees could be observed. Surviving rod cells in such chimaeric retinas were always found in regions adjoining the periphery. The location of the rod perikarya in such regions did not show an exact correlation with that of the overlying pigmented cells but regions of the outer nuclear layer with surviving rod perikarya were generally located in the half or quarter of the retina in which the overlying pigment epithelium also contained more pigmented cells than in the other regions. The proportions of the surviving photoreceptor cells varied between such chimaeras. The lesion appeared to be less extensive in individuals with more pigmented cells in the epithelium but no exact correlation was recorded. The findings suggest that while pigmentation in the iris reduces the amount of light reaching the retina, melanin in the pigment epithelium, in addition to preventing light reflection, may also play an antitoxic role, possibly as an antioxidative agent.