Stereotyped axonal arbors of retinal ganglion cell subsets in the mouse superior colliculus

Mouse retinal ganglion cells (RGCs) have been classified into around 20 subtypes based on the shape, size, and laminar position of their dendritic arbors. In most cases tested, RGC subtypes classified in this manner also have distinct functional signatures. Here we asked whether RGC subtypes defined by dendritic morphology have stereotyped axonal arbors in their main central target, the superior colliculus (SC). We used transgenic and viral methods to sparsely label RGCs and characterized both dendritic and axonal arbors of individual RGCs. Axon arbors varied in size, shape, and laminar position. For each of 12 subtypes defined dendritically, however, axonal arbors in the contralateral SC showed considerable stereotypy. We found no systematic relationship between the laminar position of an RGC's dendrites within the inner plexiform layer and that of its axon within the retinorecipient zone of the SC, suggesting that distinct developmental mechanisms specify dendritic and axonal laminar positions. We did, however, note a significant correlation between the dendritic field sizes of RGCs and the laminar position of their axon arbors: RGCs with larger dendritic areas, and hence larger receptive fields, projected to deeper strata within the SC. Finally, combining these new results with previous physiological analyses, we find that RGC subtypes that share similar functional properties, such as directional selectivity, project to similar depths within the SC.