Molecular guidance of retinotopic map development in the midbrain

- Each axis of the retina is mapped independently, using distinct mechanisms in the SC.
- Counter-balanced forces drive mapping along the A-P axis.
- Pre-target sorting and directed branching drive mapping along the L-M axis.

Topographic maps are utilized in many sensory and motor systems to efficiently transfer information between brain regions. The retina's projection to the superior colliculus has served as a model for the identification of molecular cues and mechanistic strategies by which topographic maps are formed. Evidence from both in vitro and in vivo studies points to graded cell surface cues playing a central role, but support for axon-axon competition and selective degeneration have also been advanced recently. In combination with mathematical models, these studies suggest that topographic maps are established using a complex combination of strategies to ensure precise connectivity.