Normally occurring loss of single cells and repair of resulting defects in retinal pigment epithelium in situ.

The retinal pigment epithelial cells, which form a monolayer between the choroid and photoreceptor cell layer of the eye, generally do not divide after birth. There is, however, a gradual loss of retinal pigment epithelial cells die and the mechanism by which the integrity of retinal pigment epithelium is maintained after cell death has not been examined. Confocal laser scanning microscopy of whole mounts of retinal pigment epithelium of 12 to 16 day old chick embryos showed that among the great majority of retinal pigment epithelial cells which were regular in size and hexagonal in shape, single, scattered, irregularly shaped, dying cells are present. The distribution of the dying retinal pigment epithelial cells, their morphology and the presence of apoptotic bodies, including pyknotic and fragmented nuclei, above such defects suggests that the death occurs by apoptosis. The defects created by the dying or dead cells were repaired by spreading of the surrounding normal retinal pigment epithelial cells and a series of stages in the repair of the defects could be identified. During the repair of the defects, fine microfilament bundles running parallel to the edge of the defect in each of the surrounding retinal pigment epithelial cells could be detected by confocal laser scanning microscopy giving a 'spider-web' appearance to the region around the defect. Since induction of proliferation in retinal pigment epithelial cells during healing of the defect requires cell migration, we would not expect the spreading of retinal pigment epithelial cells into the single cell defects to trigger cell proliferation. The death of single cells and the spreading of adjacent ones in the absence of cell proliferation would however explain at least in part the increase in the average size of retinal pigment epithelial cells with age.