Retinal mosaics: new insights into an old concept

It has been known since the middle of the 19th century that different neuronal types are distributed across the retinal surface in non-random arrays: indeed, these arrays, called 'mosaics', have long been considered to be a fundamental feature of retinal organization. However, until recently, little was known about how such mosaics are established during development. In the hope of stimulating further research, this article reviews the current status of three very different approaches to this intriguing general problem. The first postulates arrays of molecular markers, which are produced by specific cell types shortly after their final mitotic divisions and could be influential in the differentiation of other cell types. The second invokes a tangential dispersion of differentiating cells to generate spatial order, either while these cells are still migrating or soon after they reach their laminar destinations. The third involves the elimination of wrongly positioned cells through the process of naturally occurring cell death.