The establishment of neuronal circuitry during development relies upon the action of cell-intrinsic mechanisms that specify neuronal form as well as plastic processesthat requirethetransmission of neural activity between afferents andtheirtargets. Here, we examinethe role of interactions between neighboring like-type cells withinthe mouse retina upon neuronal differentiation and circuitformation. Two different genetically modified mouse models were used to modulate the density of homotypic neighbors, the Type 7 cone bipolar cells, without affectingthe density oftheir afferents,the cone photoreceptors.We demonstrate a corresponding plasticityin dendriticfield area whenthe density of Type 7 cone bipolar cells is elevated or reduced. In accord withthis variation in dendriticfield area across an invariant population of afferents, individual Type 7 cone bipolar cells are also shown to modulate the number of cone pedicles contacted without varying the number of contacts at each cone pedicle. Analysis of developing Type 7 cone bipolar cells reveals that the dendritic tiling present in maturity is achieved secondarily, after an initial stage of dendritic overlap, when the dendritic terminals are stratified at the level ofthe cone pedicles but are not localizedtothem. These results demonstrate a conspicuous developmental plasticity in neural circuit formation independent of neural activity, requiring homotypic interactions between neighboring cells that ultimately regulate connectivity within the retina. [ABSTRACT FROM AUTHOR]