The role of tangential dispersion in retinal mosaic formation

Individual types of retinal nerve cell are spaced across the retina in an orderly manner, ensuring a uniform sampling of the visual field. This regularity in cellular spacing has been commonly attributed to fate determination mechanisms operating around the time of cell birth, an hypothesis presuming that the position of a nerve cell is fixed within the plane of the retina from the time of its determination. At odds with this view, recent results from X-inactivation mosaic mice indicate that certain classes of retinal nerve cell, those known to form orderly mosaics in the adult retina, disperse tangentially during development. Furthermore, studies defining the spatial characteristics of developing and mature retinal mosaics suggest that cell-cell interactions around the time of morphological differentiation lead to mutual repulsion. Modelling studies in turn show that nothing more than a simple minimal spacing rule between neighboring cells of the same type is sufficient for the creation of the global patterning observed in biological retinal mosaics. For some cell types, the size of this "exclusion zone" surrounding individual cells is shown to be an intrinsic characteristic of each cell type, invariant across the retina, and accounting for the variation in mosaic regularity across changes in cell density. These results show how short-distance movements driven by intercellular interactions at the local level may mediate the emergence of the global patterning characteristic of retinal mosaics during development.