THE FUNCTIONS OF THE PREPLATE IN DEVELOPMENT AND EVOLUTION OF THE NEOCORTEX AND HIPPOCAMPUS

Recently, it has been shown that the early developmental organization of the archicortical hippocampus resembles that of the neocortex. in b oth cortices at embryonic stages, a preplate is present, which is spli t by the formation of the cortical plate into a marginal zone and a su bplate layer. The pioneer neurons of the preplate are believed to form a phylogenetically ancient cortical structure. Neurons in these prepl ate layers are the first postmitotic neurons and have important roles in the development of the cerebral cortex. Cajal-Retzius cells in the marginal zone regulate the phenotype of radial glial cells and may din er neuronal migration establishing the inside-out gradient pf corticog enesis. Furthermore, pioneer neurons form the initial axonal connectio ns with other (sub)cortical structures, A significant difference betwe en the hippocampus and neocortex, however, is that in the hippocampus, most afferents are guided by the pioneer neurons in the prominent mar ginal zone, while in the neocortex most ingrowing afferent axons enter via the subplate, At later developmental periods, most pioneer neuron s disappear by cell death or transform into other neuronal shapes. Her e, we review the early developmental organization of the mammalian cer ebral cortex (both neocortex and hippocampus) and discuss the function s and fate of pioneer neurons in cortical development, in particular t hat of Cajal-Retzius cells. Evaluating the developmental properties of the hippocampus and neocortex, we present the hypothesis that the dis tribution of the main ingrowing afferent systems in the developing neo cortex, which differs from the one in the hippocampal region, may have enabled the specific evolution of the neocortex. (C) 1998 Elsevier Sc ience B.V. All rights reserved.