The paleocortical ventricle is the origin of reelin-expressing neurons in the marginal zone of the foetal human neocortex

The subpial granular layer (SGL) is a transient cell layer in the cortical marginal zone during the period of neuronal migration into the cortical pla te. The origin of the SGL has been studied by immunocytochemistry for calre tinin (CR) and reelin in human foetuses from 11 to 40 gestational weeks (GW ). At 11 GW, the paleocortical ventricle, a rostral dilatation of the later al ventricle, gives rise to two fountainheads: a medial fountainhead provid es neurons for the marginal zone (MZ) of the rostral cortex and rostral hip pocampal rudiment, while multiple cell streams migrate from a lateral fount ainhead into the MZ of the paleocortex and insula. The latero-medial gradie nt of neuronal packing density in the neocortical MZ indicates that migrati on extends farther into the neocortex. Neurons express CR already in the re trobulbar ventricular zone; they express reelin only as they approach the M Z of the paleocortex and rostral archicortex. At 16/17 GW, large numbers of CR-immunoreactive granule cells originate from the same fountainheads, and then direct medially, toward the surface of the anterior perforated substa nce, and laterally, into the paleocortical MZ, from where they continue int o the neocortical SGL following a ventrolateral to dorsomedial gradient. Fr om 13 to 18 GW, reelin is expressed by a subpopulation of granule cells and by Cajal-Retzius-like neurons. By 22 GW, the paleocortical ventricle under goes regression and no longer supplies the SGL. Our results show that the p aleocortical ventricle gives rise to a stream of neurons which extends over the cortical MZ as the subpial granular layer. The fact that SGL derivativ es express reelin suggests that this transient cell layer may play a signif icant role in the establishment of the complex cytoarchitecture of the cere bral cortex.