INFLUENCES OF THE THALAMUS ON THE SURVIVAL OF SUBPLATE AND CORTICAL PLATE CELLS IN CULTURED EMBRYONIC MOUSE-BRAIN

The afferent and efferent connections of the cerebral neocortex develo p simultaneously toward the end of embryogenesis, At this stage, the n eocortex comprises two main cell-dense layers: the thicker and more su perficial cortical plate (future layers 2-6) and the thinner underlyin g subplate, Many early thalamocortical projections temporarily innerva te the subplate before leaving it to locate their ultimate targets in the overlying cortical plate. The subplate then disappears. In this st udy, we performed in vitro experiments on late embryonic murine brain to test whether the thalamus can influence the survival of cortical pl ate and subplate cells at this stage. In isolated organotypic cortical explants from embryonic day 19 mice, most of the cells that had forme d the subplate died. Coculture with a thalamic explant prevented this loss; coculture with additional cortical or cerebellar explants did no t. By contrast, many cells in or destined for the cortical plate survi ved even in isolated cortical explants; coculture with a thalamic expl ant did not alter the numbers of these cells that survived. Our result s suggest that the thalamus provides trophic support for subplate cell s but not for late embryonic cortical plate cells, In vivo, a loss of thalamic-derived trophic support for the subplate late in embryogenesi s, consequent on the movement of thalamocortical axons into the cortic al plate, may contribute to subplate death.