Local, possibly contact-mediated signalling restricted to homotypic neurons controls the regular spacing of cells within the cholinergic arrays in the developing rodent retina.

In the vertebrate retina neurons of the same type commonly form non-random arrays, assembled by unknown positional mechanisms during development. Computational models in which no two cells are closer than a minimal distance, simulate many retinal arrays. These findings have important biological implications, since they suggest that cells are determined as neurons of specific types before entering their arrays, and that local, possibly contact-mediated interactions acting exclusively among the elements of an array account for its assembly. This is here verified by combining experimental manipulations in normal and transgenic models with computational analysis for the cholinergic mosaics, the only arrays so far for which the development of spatial ordering is known quantitatively. When generalised, these findings suggest a plan for vertebrate retinal patterning, where homotypic interactions organise retinal arrays first, then local interactions between synaptic partners suffice to establish the topographical connections that support retinal processing.