The early development of thalamocortical and corticothalamic projections.

The early development of thalamocortical and corticothalamic projections in hamsters was studied to compare the specificity and maturation of these pathways, and to identify potential sources of information for specification of cortical areas. The cells that constitute these projections are both generated prenatally in hamsters and they make reciprocal connections. Fluorescent dyes (DiI and DiA) were injected into the visual cortex or lateral geniculate nucleus in fixed brains of fetal and postnatal pups. Several issues in axonal development were examined, including timing of axon outgrowth and target invasion, projection specificity, the spatial relationship between the two pathways, and the connections of subplate cells. Thalamic projections arrive in the visual cortex 2 days before birth and begin to invade the developing cortical plate by the next day. Few processes invade inappropriate cortical regions. By postnatal day 7 their laminar position is similar to mature animals. By contrast, visual cortical axons from subplate and layer 6 cells reach posterior thalamus at 1 day after birth in small numbers. By 3 days after birth many layer 5 cell projections reach the posterior thalamus. On postnatal day 7, there is a sudden increase in the number of layer 6 projections to the thalamus. Surprisingly, these layer 6 cells are precisely topographically mapped with colabeled thalamic afferents on their first appearance. Subplate cells constitute a very small component of the corticothalamic projection at all ages. Double injections of DiI and DiA show that the corticofugal and thalamocortical pathways are physically separate during development. Corticofugal axons travel deep in the intermediate zone to the thalamic axons and are separate through much of the internal capsule. Their tangential distribution is also distinct. The early appearance of the thalamocortical pathway is consistent with an organizational role in the specification of some features of cortical cytoarchitecture. The specific initial projection of thalamocortical axons strongly suggests the recognition of particular cortical regions. The physical separation of these two pathways limits the possibility for exchange of information between these systems except at their respective targets.