Formation of retinal pigment epithelium in vitro by transdifferentiation of neural retina cells

Chick embryonic neural retina (NR) dedifferentiates in culture and can tran sdifferentiate spontaneously into retinal pigment epithelium (RPE). Both, p rimary RPE and transdifferentiated RPE (RPEt), are characterized by pigment ation, expression of RPE-specific protein, eRPE(AG) and lack of expression of the neural cell adhesion molecule, NCAM. In contrast, NR cells are unpig mented and express NCAM but not eRPE(AG). Functionally, both primary RPE an d the RPEt cells display a pH(i) response to bFGF, which is different from that of the NR. We used these characteristics to distinguish cell types in primary cultures of chick NR and follow the changes in phenotype that occur during transdifferentiation. We show that the RPEf forms as small "islands " in the packed regions of the primary, "mother" NR cell sheets, in a stoch astic process. Because of a small number of cells involved in the initiatio n of the transdifferentiation we refer to it as a "leader effect" to contra st it with the "community effect" which requires many competent cells to be present in a group to be able to respond to an inductive signal. The RPEt then expands centrifugally and underneath the surrounding NR sheet. To dete rmine if the RPEt maintains its identity in isolation while displaying the RPE-typical phenotypic plasticity, we explanted the islands of RPEt and tre ated half of them with bFGF. The untreated RPEt maintained its closely pack ed, polygonal pigmented phenotype but the bFGF-ereated RPEt transdifferenti ated into a non-pigmented, NR-like phenotype, indicating that RPEt encompas ses the full differentiation repertoire of native RPE.