Stem cells in the vertebrate retina

The capacity for retinal regeneration in cold-blooded vertebrates has long been recognized. Regeneration occurs, in part, through a population of retinal stem cells residing at the peripheral margin of the retina. It has generally been thought that homeothermic vertebrates, such as birds and mammals, lack this so-called ciliary marginal zone. Recent studies have, however, provided evidence that birds too possess a zone of cells at the retinal margin analogous to the ciliary marginal zone of fish and amphibians. In addition, there is an indication that, under certain conditions, Müller glia of the chicken retina can transdifferentiate into retinal progenitor/stem cells. These progenitor/stem cells then generate certain types of retinal neurons. Taken together, these studies have revealed an unexpected capacity for retinal regeneration in birds.