MECHANISMS UNDERLYING THE EARLY ESTABLISHMENT OF THALAMOCORTICAL CONNECTIONS IN THE RAT

We labeled axonal projections using carbocyanine dyes in the developin g rat brain to study cellular interactions that might underlie the est ablishment of thalamocortical connectivity. By embryonic day 14 (E14), groups of neurons in the ventral diencephalon and the primitive inter nal capsule have established projections to the dorsal thalamus, and t halamic fibers pass in topographic order among them. Simultaneously, a xons from the early-born cells in both subplate and marginal zone (i.e ., the original cortical preplate) establish an ordered array that fil ls the intermediate zone. Thalamic axons and preplate fibers meet in t he lateral part of the internal capsule (at E15 for occipital cortex a nd dorsolateral thalamus). Subsequently, selective labeling of corresp onding thalamic and early corticofugal projections reveals thalamic fi bers growing in association with early corticofugal axons, right up to the cortical subplate. A small carbocyanine crystal implanted at any point in the cortex shortly after the arrival of thalamic axons (E16 f or the occipital cortex) labels a single, tight bundle containing both descending and ascending fibers, rather than two separate tracts, pro viding further evidence for intimate topographic association of the tw o axon systems. Crystals placed in a row, parasagittally or coronally along the hemisphere, reveal separate, topographically distributed, di screte fiber bundles throughout the pathway, leading to spatially orde red groups of back-labeled thalamic cells. These results indicate that the topography of thalamic axons is maintained throughout the pathway and that they reach the cortex by associating with the projections of a number of preexisting cells, including the preplate scaffold.