Persistent and injury-induced neurogenesis in the vertebrate retina.

The brains of all vertebrates are persistently neurogenic. However, this is not true for the neural retinas. Only three extant classes of vertebrates show significant posthatch/postnatal retinal neurogenesis: amphibians, birds and fish. The retinas of these animals contain an annulus of progenitors at the margin, from which differentiated neurons emerge. In posthatch amphibians and fish the vast majority of the adult retina is added from the margin and neurogenesis is lifelong, whereas in posthatch birds neurogenesis is limited. Unique to fish, rod photoreceptors are added in situ from stem cells within the mature retina. Strikingly, for each class of animal retinal lesions stimulate neuronal regeneration, however the cellular source differs for each: the retinal pigmented epithelium in amphibians and embryonic birds, Müller glia in posthatch birds and intrinsic stem cells in fish. The molecular events surrounding injury-induced neuronal regeneration are beginning to be identified.