Transdifferentiation of pigmented epithelial cells: a source of retinal stem cells?

In urodeles, larval anurans, embryonic chicks and rodents, the retinal pigmented epithelium (RPE) is capable of transdifferentiation and generating new neurons. Recent evidence suggests that pigmented cells in the ciliary body of the adult rodent eye are capable of producing new neurons in vitro. Here we provide data to suggest that the pigmented epithelium at the retinal margin of postnatal chickens is similar to that found in the embryonic retina. Pigmented cells at the retinal margin expressed mitf and pax6, transcription factors that are transiently expressed by the developing RPE. Furthermore, these pigment cells at the retinal margin express high levels of proliferating cell nuclear antigen and accumulate bromodeoxyuridine, indicating that they continue to proliferate long after embryonic stages of development. Exogenous fibroblast growth factor-2 (FGF2) or insulin alone did not affect the proliferation of these cells, while FGF2 plus insulin induced their proliferation and loss of pigmentation. We propose that the pigmented cells at the retinal margin of the postnatal chicken are similar to those found in the embryonic eye, and these cells could be a source of neural regeneration under appropriate conditions.