Sensory Map Transfer to the Neocortex Relies on Pretarget Ordering of Thalamic Axons

• Ebf1 inactivation in the subpallium shifts and blurs pretarget ordering of TCAs
• TCAs scrambled en route to the neocortex reroute to their appropriate target area
• Scrambling of TCAs compromises the transfer of the somatosensory map to the neocortex

SummarySensory maps, such as the representation of mouse facial whiskers, are conveyed throughout the nervous system by topographic axonal projections that preserve neighboring relationships between adjacent neurons [1]. In particular, the map transfer to the neocortex is ensured by thalamocortical axons (TCAs), whose terminals are topographically organized in response to intrinsic cortical signals [2, 3, 4 and 5]. However, TCAs already show a topographic order early in development, as they navigate toward their target [6 and 7]. Here, we show that this preordering of TCAs is required for the transfer of the whisker map to the neocortex. Using Ebf1 conditional inactivation that specifically perturbs the development of an intermediate target, the basal ganglia, we scrambled TCA topography en route to the neocortex without affecting the thalamus or neocortex. Notably, embryonic somatosensory TCAs were shifted toward the visual cortex and showed a substantial intermixing along their trajectory. Somatosensory TCAs rewired postnatally to reach the somatosensory cortex but failed to form a topographic anatomical or functional map. Our study reveals that sensory map transfer relies not only on positional information in the projecting and target structures but also on preordering of axons along their trajectory, thereby opening novel perspectives on brain wiring.