Cellular pattern formation in the retina: retinal regeneration as a model system.

Like many structures in the central nervous system, the neural retina is highly organized at the cellular level. Examples of this cellular organization include the laminar profile of the vertebrate retina, the hexagonal array of ommatidia in the retinas of insects, and non-random two-dimensional patterns of specific vertebrate retinal neurons. These organized cellular ensembles are taxonomically robust, and their importance in visual processing is, although typically not well understood, virtually axiomatic. The presence of non-random cellular patterns in the retina also begs questions concerning the spatial nature of the patterns, and the underlying mechanisms that coordinate their assembly during retinal development and growth. What are the spatial characteristics of the non-random cellular patterns? What molecular signaling schemes might account for their assembly? What are good model systems for investigating these issues? In this review we attempt to provide some preliminary answers to these questions. We present recent advances in our understanding of cellular patterns in the vertebrate retina and the mechanisms that underlie their assembly, the ability of adult anamniote retinas to regenerate following injury, and how these seemingly disparate topics can be successfully merged into an effort to better understand both processes. We combine insights from retinal assembly mechanisms in Drosophila with empirical, quantitative, and theoretical investigations in vertebrates, to propose an inclusive model for retinal cell patterning.