Molecular Control of Cell Diversification in the Vertebrate Retina

Abstract

Any discussion of the molecular mechanisms that are responsible for the generation of cell diversity in the retina requires some consideration of what constitutes neuronal diversity. Traditionally, retinal neurobiologists have classified retinal cell types on the basis of their morphology, in terms both of their laminar position and their dendritic structure, as well as by their electrophysiological properties. While it is generally accepted that there are five basic classes of retinal neurons, photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells, there is a considerable amount of debate as to how these basic classes are subdivided. While there are critical differences in gene expression between the classes of cone photoreceptors that determine the wavelength of light to which they respond, there is little difference in their morphology; by contrast, the various classes of retinal ganglion cells have critical differences in their morphology and connectivity, but as yet few molecules have been identified that are differentially expressed in subsets of ganglion cells, and it is not at all clear whether these differences correspond to the morphological subclasses of ganglion cells. Nevertheless, most retinal neurobiologists place the number of retinal cell types somewhere between “70 and the low hundreds (Cook 1996).”