The retinal pigment epithelial cells of the adult newt and rat under conditions of in vitro organotypic culture

To understand why the retinal pigment epithelium (RPE) has different potentials for neural differentiation in lower and higher vertebrates, the RPEs of adult newts and rats were compared under similar in vitro cultivation conditions. The RPEs of both animal species were organotypically cultivated within the posterior eye wall under constant rotation in the serum medium free of growth factors. Comparison of the cell morphology, proliferation, and expression of pan-neural markers demonstrated that the RPE cells of adult newts and rats under similar in vitro conditions displayed both similarities and differemces. They were able to synthesize DNA but rarely divided mitotically. In addition, part of the RPE cells of both the newt and the rat were dislodged from the layer, migrated, and acquired a macrophage phenotype. However, the majority of the cells retained the initial morphology and remained within the layer. In several cases, these cells displayed the initial characteristics of neural differentiation, namely, expression of pan-neural proteins. The difference between the newt and rat RPE cells was in the ability of the former to generate in vitro an additional row of dedifferentiated NF-200-positive cells, characteristic of in vivo newt retinal regeneration. These data demonstrate that the RPE cells of the adult newt and rat retain the potential of manifesting neural cell traits; however, more advanced changes towards differentiation are characteristic of only the newt RPE.