Precise patterns of cell division and migration are crucial to transform the neuroepithelium of the embryonic forebrain into the adult cerebral cortex. Using time-lapse imaging of clonal cells in rat cortex over several generations, we show here that neurons are generated in two proliferative zones by distinct patterns of division. Neurons arise directly from radial glial cells in the ventricular zone (VZ) and indirectly from intermediate progenitor cells in the subventricular zone (SVZ). Furthermore, newborn neurons do not migrate directly to the cortex; instead, most exhibit four distinct phases of migration, including a phase of retrograde movement toward the ventricle before migration to the cortical plate. These findings provide a comprehensive and new view of the dynamics of cortical neurogenesis and migration.

The neocortex of the adult brain consists of neurons and glia that are generated by precursor cells of the embryonic ventricular zone. In general, glia are generated after neurons during development, but radial glia are an exception to this rule. Radial glia are generated before neurogenesis and guide neuronal migration. Radial glia are mitotically active throughout neurogenesis, and disappear or become astrocytes when neuronal migration is complete. Although the lineage relationships of cortical neurons and glia have been explored, the clonal relationship of radial glia to other cortical cells remains unknown. It has been suggested that radial glia may be neuronal precursors, but this has not been demonstrated in vivo. We have used a retroviral vector encoding enhanced green fluorescent protein to label precursor cells in vivo and have examined clones 1-3 days later using morphological, immunohistochemical and electrophysiological techniques. Here we show that clones consist of mitotic radial glia and postmitotic neurons, and that neurons migrate along clonally related radial glia. Time-lapse images show that proliferative radial glia generate neurons. Our results support the concept that a lineage relationship between neurons and proliferative radial glia may underlie the radial organization of neocortex.

Neurogenesis must be properly regulated to ensure that cell production does not exceed the requirements of the growing cerebral cortex, yet our understanding of mechanisms that restrain neuron production remains incomplete. We investigated the function of microglial cells in the developing cerebral cortex of prenatal and postnatal macaques and rats and show that microglia limit the production of cortical neurons by phagocytosing neural precursor cells. We show that microglia selectively colonize the primate SVZ and phagocytose neural precursor cells as neurogenesis nears completion. We found that de-activating microglia in utero with tetracyclines or eliminating microglia from the fetal cerebral cortex with liposomal clodronate significantly increased the number of neural precursor cells, while activating microglia in utero through maternal immune activation significantly decreased the number of neural precursor cells. These data demonstrate that microglia play a fundamental role in regulating the size of the precursor cell pool in the developing cerebral cortex, expanding our understanding of the mechanisms that regulate cortical development. Furthermore, our data suggest that any factor that alters the number or activation state of microglia in utero can profoundly affect neural development and affect behavioral outcomes.

Interneurons of the cerebral cortex represent a heterogeneous population of cells with important roles in network function. At present, little is known about how these neurons are specified in the developing telencephalon. To explore whether this diversity is established in the early progenitor populations, we conducted in utero fate-mapping of the mouse medial and caudal ganglionic eminences (MGE and CGE, respectively), from which most cortical interneurons arise. Mature interneuron subtypes were assessed by electrophysiological and immunological analysis, as well as by morphological reconstruction. At E13.5, the MGE gives rise to fast-spiking (FS) interneurons, whereas the CGE generates predominantly regular-spiking interneurons (RSNP). Later at E15.5, the CGE produces RSNP classes distinct from those generated from the E13.5 CGE. Thus, we provide evidence that the spatial and temporal origin of interneuron precursors in the developing telencephalic eminences predicts the intrinsic physiological properties of mature interneurons.

The dramatic evolutionary expansion of the cerebral cortex of Homo sapiens underlies our unique higher cortical functions, and therefore bears on the ultimate issue of what makes us human. Recent insights into developmental events during early proliferative stages of cortical development indicate how neural stem and progenitor cells might interact to produce cortical expansion during development, and could shed light on evolutionary changes in cortical structure.