A comparative study of neurogenesis in the retinal ciliary marginal zone of homeothermic vertebrates

The retina of many fish and amphibians grows throughout life, roughly matching the overall growth of the animal. The new retinal cells are continually added at the anterior margin of the retina, in a circumferential zone of cells, known as the ciliary marginal zone, or CMZ. Recently, Fischer and Reh [Dev. Biol. 220 (2000) 197] have found that new neurons are added to the retina of the chicken via proliferation and subsequent differentiation of neurons and glia at the retinal margin in a zone highly reminiscent of the CMZ of lower vertebrates. In addition, other groups have reported that putative retinal stem cells could be isolated from the ciliary margin of the adult mouse. In light of these findings, we have re-investigated the eyes of three additional species to determine whether other homeothermic vertebrates also possess CMZ cells and whether we could detect evidence for addition of neurons at the retinal margin in mature animals. We examined one additional avian species, the quail, one marsupial, the opposum, and one mammal, the mouse. We find that the CMZ cells have been gradually diminished during vertebrate evolution. The quail has a reduced CMZ as compared to the chicken, while the opposum has only a few cells likely related to the CMZ and we failed to find evidence of CMZ cells at the margin of the mouse retina.