In recent years, tremendous progress has been made in understanding the mechanisms underlying the specification of projection neurons within the mammalian neocortex. New experimental approaches have made it possible to identify progenitors and study the lineage relationships of different neocortical projection neurons. An expanding set of genes with layer and neuronal subtype specificity have been identified within the neocortex, and their function during projection neuron development is starting to be elucidated. Here, we assess recent data regarding the nature of neocortical progenitors, review the roles of individual genes in projection neuron specification and discuss the implications for progenitor plasticity.

We have used a monoclonal antibody against the neuron-specific class III beta-tubulin (TuJ1; Lee et al., 1990b) to study the distribution and morphology of immature neurons in the proliferative ventricular and subventricular zones of the developing telencephalon. Mouse brains from embryonic day 12 (E12) to postnatal day 5 (P5) were fixed either with a non-cross-linking agent, HistoChoice, or with 4% paraformaldehyde, and processed for TuJ1 immunohistochemistry. TuJ1 immunoreactivity first appeared in the proliferative zones of the developing cerebral cortex at E13-E14 as the cortical plate was emerging. After E14, tangentially oriented TuJ1-positive cells were abundant at the interface between the ventricular and subventricular zones. This tangential pattern was less conspicuous in the developing striatum. Within the cortical and striatal ventricular zone TuJ1-positive cells were less numerous and displayed a variety of orientations and morphologies. Postnatally, after the period of neurogenesis has ended, TuJ1 immunoreactivity continued to increase in the subventricular zone and remained high until the last developmental stage examined (P5). Anti-MAP2, another neuron-specific marker, never labeled the cells of the ventricular and subventricular zones, pre- or postnatally. To determine the birthdates of TuJ1-positive cells in the cortical-ventricular and subventricular zones, brains were double labeled with TuJ1 and bromodeoxyuridine according to different pulse-chase schedules. TuJ1-positive cells were postmitotic and generated throughout the period of cortical neurogenesis. Collectively, the results suggest that TuJ1 immunoreactivity distinguishes two neuronal populations: those that remain for an indefinite period of time in the proliferative zones, and those that leave the proliferative zones soon after being generated. Although the fate of the TuJ1-positive cells that reside in the proliferative zones remains unclear, their tangentially aligned orientation and their distribution suggest that they migrate independent of radial glial fibers.