Abstract
There is a high degree of specificity in the efferent connections of the cerebral cortex. In the rodent neocortex, the characteristic band of corticospinal neurons within layer V is present at birth even though changes still occur in the areal distribution of these neurons. Disruption of neocortical development with ionizing radiation before, during, or after the production of neurons destined for layer V results in abnormally located corticospinal neurons. One abnormal location in which corticospinal neurons are found is in ectopic cell clusters beneath the cortical white matter bordering the dorso-medial aspect of the lateral ventricle. Corticospinal neurons only occur in these periventricular ectopias in adult rats irradiated on or before embryonic day 17. A second abnormal location of corticospinal neurons is between layer V and the pial surface. These scattered supragranular corticospinal neurons occur in all adult animals irradiated on embryonic days 16, 17, 18, or 19. The fact that neurons having an unusual position project to a subcortical target appropriate for one neocortical sublayer indicates that neither migratory path nor final position is essential to specifying a subcortical target. In addition, the fact that labeled corticospinal neurons are located in periventricular ectopias only when irradiation occurs on or before embryonic day 17 suggests that the initial projections of corticospinal neurons are determined early in their individual ontogeny prior to migration.