Held under arrest: Many mature albino RPE cells display polyploidal features consistent with abnormal cell cycle retention.

A population of peripheral retinal pigment epithelium (RPE) cells in mature pigmented rats are retained in the cell cycle and divide, as determined by Ki67 and BrdU labeling. Their cell cycle rate is approximately 5 days. Ten times as many Ki67 positive cells are found in albinos compared with pigmented animals, but it is not known if they actually divide or only express cell cycle markers. In spite of the increased number of cells expressing cell cycle markers, we show here using BrdU, that levels of cell division in albino RPE are similar to those in pigmented animals and have a similar cell cycle rate. Hence, cell cycle activity does not progress through to cell division in the majority of albino RPE cells. Peripheral RPE cells in albinos are different from those in pigmented animals. Many have very large distorted or highly fragmented nuclei. These data along with patterns of Ki67 and BrdU labeling are consistent with such cells being retained abnormally in the cell cycle, replicating their DNA, but not able to progress through to full cell division. Hence, there are two populations of RPE cells in albinos, those undergoing normal cell division and those that appear to be arrested in the cell cycle. These abnormalities are present from early postnatal stages.