A mapping label required for normal scale of body representation in the cortex

The neocortical primary somatosensory area (S1) consists of a map of the bo dy surface. The cortical area devoted to different regions, such as parts o f the face or hands, reflects their functional importance. Here we investig ated the role of genetically determined positional labels in neocortical ma pping. Ephrin-A5 was expressed in a medial > lateral gradient across S1, wh ereas its receptor EphA4 was in a matching gradient across the thalamic ven trobasal (VB) complex, which provides S1 input. Ephrin-A5 had topographical ly specific effects on VB axon guidance in vitro. Ephrin-A5 gene disruption caused graded, topographically specific distortion in the S1 body map, wit h medial regions contracted and lateral regions expanded, changing relative areas up to 50% in developing and adult mice. These results provide eviden ce for within-area thalamocortical mapping labels and show that a genetic d ifference can cause a lasting change in relative scale of different regions within a topographic map.