Filling-in after focal loss of photoreceptors in rat retina.

We investigated the fate of isolated photoreceptor lesions in rat retina over a time span of 6 months. With a carefully selected dose of UV-A (380 nm) a complete loss of photoreceptors was caused in sharply demarcated areas of 200, 400 or 800 microm wide, without visible damage to other retinal layers. One day after irradiation, all rods were pyknotic. Three weeks later practically all damaged photoreceptors were removed. The size of the lesion had decreased as the surrounding photoreceptors had migrated into the lesion. The outer segment tips had moved inwards up to 200 microm, but the innermost nuclei in the outer nuclear layer had moved inwards substantially less. The distance over which the photoreceptors migrated increased with lesion size, but only 200 microm defects were filled-in completely on the level of the outer segments. Between three weeks and six months after irradiation, no further decrease in lesion size occurred. We conclude that after local loss of photoreceptor cells the bordering photoreceptors rapidly shift into the lesion area, but complete filling-in is limited to very small lesions. Copyright 1999 Academic Press.