GABA-immunoreactive cells of the cortical primordium contribute to distinctly fated neuronal populations.

The roles of GABA during development, as either a putative neurotransmitter or a nonsynaptic trophic factor, are being discussed intensely in recent literature. We offer an anatomical framework to better understand these possible roles in the developing cerebral cortex. During the early development of the cerebral cortex, GABA-containing cells constitute an outstanding cell population in the primordial plexiform layer, but they later distribute into at least four compartments. These include (1) Cajal-Retzius cells in layer I and (2) the subplate cells. Certain of these GABA-containing cell groups may disappear either by ceasing their expression of GABA, dilution in a growing brain volume, or cell death, possibilities that are reviewed here. The chemical tags that characterize Cajal-Retzius cells, both in the forming isocortex and Ammon's horn, are discussed. Another cell population that also belongs to the primordial plexiform layer is formed by (3) the tangentially migrating cells of the deep intermediate layer. These migrate away from the isocortical primordium to invade, and contribute cells to, the forming stratum oriens of the Ammon's horn. Since these cells cross cortical area boundaries, their tangential migration is relevant to the issue of cortical area specification during development. Finally, GABA-immunoreactive cells in the developing cortical plate are considered to be (4) the future GABAergic interneurons. A hypothetical mechanism is presented here to explain their acquisition of laminar positions, which is known to take place simultaneously, and with an identical "inside-out gradient," to the pyramidal cells generated contemporarily.