Axonal surface molecules act in combination with Semaphorin 3A during the establishment of corticothalamic projections

Interactions between growing axons are considered to play important roles f or the establishment of precise neuronal connections during the development of the nervous system, Here we used time-lapse imaging techniques to exami ne the behavior of neocortical and thalamic axons when they encounter each other in vitro. Results indicate that axonal growth cones are able to respo nd to specific cues expressed on the surface of fibers. Thalamic growth con es often extended along the surface of other thalamic axons and, likewise, cortical growth cones formed fascicles with cortical axons, In contrast, af ter contacts between cortical and thalamic fibers, in most cases growth con es collapsed and retracted from the axons. Collapse assays using membrane p reparations from cortical or thalamic explants demonstrated the existence o f cell-type specific collapsing factors whose activity was enhanced by a me mber of the semaphorin protein family, Sema3A (expressed in the thalamocort ical pathway), as it increased the rate of homotypic fasciculations and at the same time amplified the segregation between cortical and thalamic axons . The interaction between axonal surface molecules and environmental cues m ight mediate the segregation of afferent and efferent fiber tracts in the n eocortical white matter.