The early development of thalamocortical and corticothalamic projections in the mouse

The initial ingrowth of corticothalamic and thalamocortical projections was examined in mice at embryonic and perinatal stages. Fibers, in fixed brain s, were labeled with the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3' -tetra methylindocarbocianine perchlorate (DiI). By E13, the corticofugal fibers h ad entered the lowest intermediate zone through which they ran, turned over the corpus striatum, and left the cortex. The fibers were arranged in scat tered bundles throughout the corpus striatum. At E14 corticofugal axons rea ched the internal capsule and at E14.5-E15 they established contact within the thalamus. Meanwhile, the thalamocortical afferents reached the neocorte x at E13. At this time fibers ran tangentially within the intermediate zone , immediately underneath the cortical plate. By E14, the fibers had started to invade the subplate and, by E15, thalamocortical fibers had begun their radial growth into the cortex. Such radial growth proceeded steadily, inva ding each cortical layer as it differentiated cytoarchitectonically from th e dense cortical plate. The first retrogradely labeled cells were detected at the cortical plate at E15. By the day of birth (E20), thalamocortical fi bers had formed a dense branching system within layers VI and V. Our observ ations indicate that, in mice, the thalamic axons reach the cortex before c orticothalamic projections enter the thalamic nuclei. Moreover, the results suggest that the pathway followed by each fiber system is different. By Di I injections into the internal capsule we have also determined that subplat e cells are the first to send axons to the thalamus.