BRANCHING PATTERN OF CORTICOTHALAMIC PROJECTIONS FROM THE SOMATOSENSORY CORTEX DURING POSTNATAL-DEVELOPMENT IN THE RAT

In adult animals corticothalamic (CT) axons pass through the thalamic reticular nucleus (Rt) where they give off collateral branches innerva ting the Rt neurons. The postnatal development of CT projections from the somatosensory cortex, with particular reference to the branching p attern within Rt, ventrobasal (VB) and posterior (PO) nuclei, was inve stigated in the rat with anterograde tracing. Biotinylated dextran-ami ne (BDA) was iontophoretically injected into the somatosensory cortex of rats ranging from postnatal day (P) 0 to P30. At P1 most of the cor tical axons traversed unbranched Rt and terminates in VB and PO, where as at P3 they formed rudimentary branches in these nuclei. From P6 to P9 a progressive increase in the amount of dense clusters of terminal arborizations was evident in Rt, and by the second postnatal week more complex arborizations with a clear topographic arrangement were obser ved in Pt, VB and PO. Our findings indicate that CT fibers show a quan titative increase both in Rt and in somatosensory thalamic nuclei duri ng the first postnatal week, although their terminal arborizations are however still incomplete. The pattern of collateralization of CT proj ections achieves an adult configuration at the end of the second postn atal week.