Neurogenesis in the vertebrate retina.

Many studies concerned with the control of neurogenesis in the retina, as well as other parts of the nervous system, impose the dichotomy of lineage restrictions versus environmental regulation on the design and interpretation of experiments. Recent work on retinal development has focused primarily on "environmental regulation," and this article will review some observations from these studies that provide clues about signals and mechanisms that control proliferation and cell type determination in the retina. Although still at an early stage, these studies already indicate that regulatory signals are not easily categorized as affecting only proliferation or differentiation, but are instead pleiotropic. Interpretation of these findings will be considered in terms of recent work in other systems, which further demonstrates that signals and regulatory mechanisms cannot be classified so simplistically and which suggests that polarizing regulatory mechanisms in terms of lineage restrictions and environmental regulation underestimates the interplay between these types of mechanisms.