MOSAICS OF ISLET-1-EXPRESSING AMACRINE CELLS ASSEMBLED BY SHORT-RANGECELLULAR INTERACTIONS

The nervous system has a modular architecture with neurons of the same type commonly organized in nonrandom arrays or mosaics. Modularity is essential to parallel processing of sensory information and has provi ded a key element for brain evolution, but we still know very little o f the way neuronal mosaics form during development. Here we have ident ified the immature elements of two retinal mosaics, the choline acetyl transferase (ChAT) amacrine cells, by their early expression of the ho meodomain protein Islet-1, and we show that spatial ordering is an int rinsic property of the two Islet-1 mosaics, dynamically maintained whi le new elements are inserted into the mosaics. Migrating Islet-1 cells do not show this spatial ordering, indicating that they must move tan gentially as they enter the mosaic, under the action of local mechanis ms. Clonal territory analysis in X-inactivation transgenic mice confir ms the lateral displacement of ChAT amacrine cells away from their clo nal columns of origin, and mathematical models show how short-range ce llular interactions can guide the assemblage of these mosaics via a si mple biological rule.