Transitin, a nestin-related intermediate filament, is expressed by neural progenitors and can be induced in Müller glia in the chicken retina.

The purpose of this study was to test whether transitin, the avian homologue of nestin, is expressed by retinal progenitors in the developing and postnatal chicken. Because nestin has been widely used as a cell-distinguishing marker of neural progenitors in the mammalian nervous system, we expected to find transitin expressed specifically by the neural progenitors of the retina. In early stages of development, transitin is expressed by neural progenitors in the retina and by cells in the developing ciliary body. During later stages of development, transitin expression persists in differentiating Müller glia but is down-regulated by these cells as maturation proceeds. In the postnatal chick, transitin expression is restricted to neural progenitors at the peripheral edge of the retina. We found that the expression of transitin in mature Müller glia was induced by intraocular injections of insulin and fibroblast growth factor-2 (FGF2) but not by ciliary neurotrophic factor. In response to insulin and FGF2, the expression of transitin was induced in the nonpigmented epithelium (NPE) of the ciliary body. In the postnatal retina, acute retinal damage transiently induces transitin expression in Müller glia. We propose that the expression of transitin by retinal Müller glia and NPE cells in the postnatal animal represents a state of de-differentiation and a step toward becoming neurogenic progenitor cells. Taken together, our findings indicate that transitin is expressed by neural progenitors in the embryonic and postnatal chicken retina. However, transitin is not exclusively expressed by neural progenitors and is also expressed by non-neurogenic cells.