Two types of neuronal precursor cells in the mammalian retina--a short review.

This short review summarizes current evidence that the mammalian retina is populated by two distinct groups of retinal neurons. One of them (ganglion cells, cone photoreceptor cells, horizontal cells, and a subpopulation of amacrine cells) is generated early in ontogenesis, and may be phylogenetically old. The other group (rod photoreceptor cells, bipolar cells, and another subpopulation of amacrine cells) is born late in ontogenesis, and seems to have been acquired later in phylogeny. It is suggested that the two groups of neurons are generated by two different types of precursor cells that may result from an asymmetrical final division of the undifferentiated retinal stem cells. Qualitative and quantitative features of precursor cell proliferation and differentiation are discussed.