Cellular mosaics in the rat marginal zone define an early neocortical territorialization

We describe a novel neuronal group of the neocortical primordium that is fo rmed by the pioneer neurons, unlike the Cajal-Retzius cells and the subplat e component of the preplate. These pioneer neurons of the preplate and the marginal zone send transient axonal projections into the nascent internal c apsule, preceding the formation of the axonal projection from the subplate neurons. In wholemount preparations, the pioneer neurons of the preplate an d the marginal zone cover the prospective neocortical territory from embryo nic day (E) 12 to E17. Two subpopulations of pioneer neurons (defined by di fferential expression of calcium-binding proteins) group into well-defined cell clusters, separated by spaces containing a lower packing density of ce lls immunoreactive to the corresponding calcium-binding protein. In both ca ses, cell clustering was concomitant with fasciculation of their axons. Alt hough both subpopulations cohabit in the same areas of the marginal zone, t heir clustering occurs in specific, well-delineated territories, giving a m osaic appearance to the surface of the neocortical primordium before the ar rival of thalamocortical axons. We hypothesize that the fascicles of descen ding axons arising from defined territories of the marginal zone may interv ene in the initial guidance of the subcortical projection from the subplate .