SUBPLATE PIONEERS AND THE FORMATION OF DESCENDING CONNECTIONS FROM CEREBRAL-CORTEX

The adult cerebral cortex extends axons to a variety of subcortical ta rgets, including the thalamus and superior colliculus. These descendin g projections are pioneered during development by the axons of a trans ient population of subplate neurons (McConnell et al., 1989). We show here that the descending axons of cortical plate neurons appear to be delayed significantly in their outgrowth, compared with those of subpl ate neurons. To assess the possible role of subplate neurons in the fo rmation of these pathways, subplate neurons were ablated during the em bryonic period. In all cases, an axon pathway formed from visual corte x through the internal capsule and into the thalamus. In half of all c ases, however, cortical axons failed to invade their normal subcortica l targets. In the other half, targets were innervated normally. Subpla te neurons are therefore likely to provide important cues that aid the process by which cortical axons grow toward, select, and invade their subcortical targets.