Size and folding of the cerebral cortex increased massively during mammalian evolution leading to the current diversity of brain morphologies. Various subtypes of neural stem and progenitor cells have been proposed to contribute differently in regulating thickness or folding of the cerebral cortex during development, but their specific roles have not been demonstrated. We report that the controlled expansion of unipotent basal progenitors in mouse embryos led to megalencephaly, with increased surface area of the cerebral cortex, but not to cortical folding. In contrast, expansion of multipotent basal progenitors in the naturally gyrencephalic ferret was sufficient to drive the formation of additional folds and fissures. In both models, changes occurred while preserving a structurally normal, six-layered cortex. Our results are the first experimental demonstration of specific and distinct roles for basal progenitor subtypes in regulating cerebral cortex size and folding during development underlying the superior intellectual capability acquired by higher mammals during evolution.

The outer subventricular zone (OSVZ) is a germinal layer playing key roles in the development of the neocortex, with particular relevance in gyrencephalic species such as human and ferret, where it contains abundant basal radial glia cells (bRGCs) that promote cortical expansion. Here we identify a brief period in ferret embryonic development when apical RGCs generate a burst of bRGCs that become founders of the OSVZ. After this period, bRGCs in the OSVZ proliferate and self-renew exclusively locally, thereby forming a self-sustained lineage independent from the other germinal layers. The time window for the brief period of OSVZ bRGC production is delineated by the coincident downregulation of Cdh1 and Trnp1, and their upregulation reduces bRGC production and prevents OSVZ seeding. This mechanism in cortical development may have key relevance in brain evolution and disease.

Development of the cerebral cortex depends critically on the regulation of progenitor cell proliferation and fate. Cortical progenitor cells are remarkably diverse with regard to their morphology as well as laminar and areal position. Extrinsic factors, such as thalamic axons, have been proposed to play key roles in progenitor cell regulation, but the diversity, extent and timing of interactions between extrinsic elements and each class of cortical progenitor cell in higher mammals remain undefined. Here we use the ferret to demonstrate the existence of a complex set of extrinsic elements that may interact, alone or in combination, with subpopulations of progenitor cells, defining a code of extrinsic influences. This code and its complexity vary significantly between developmental stages, layer of residence and morphology of progenitor cells. By analyzing the spatial-temporal overlap of progenitor cell subtypes with neuronal and axonal populations, we show that multiple sets of migrating neurons and axon tracts overlap extensively with subdivisions of the Subventricular Zones, in an exquisite lamina-specific pattern. Our findings provide a framework for understanding the feedback influence of both intra- and extra-cortical elements onto progenitor cells to modulate their dynamics and fate decisions in gyrencephalic brains.