Neuronal stem/progenitor cells in the vertebrate eye

We acquire information from the outside world through our eyes which contain the retina, the photosensitive component of the central nervous system. Once the adult mammalian retina is damaged, the retinal neuronal death causes a severe loss of visual function. It has been believed that the adult mammalian retina had no regenerative capacity. However, the identification of neuronal progenitor cells in the retina sheds some light on cellular therapies for damaged retinal regeneration. In this review, we highlight three potential stem/progenitor cells in the eye, the ciliary body epithelium cells, the iris pigmented epithelium cells, and Müller glia. In order to make them prime candidates for the possible treatment of retinal diseases, it is important to understand their basic characters. In addition, we discuss the key signaling molecules that function extracellularly and determine whether neuronal progenitors remain quiescent, proliferate, or differentiate. Finally, we introduce a secreted protein, Tsukushi, which is a possible candidate as a niche molecule for retinal stem/progenitor cells.