A long, remarkable journey:: Tangential migration in the telencephalon

Recent studies on the origin of cell populations in rodent and chicken embryonic brains provide evidence for extensive tangential migration within the developing telencephalon. On the basis of these findings, a new concept of corticogenesis has emerged, which proposes that two distinct neuronal populations cooperate in the formation of the cortex. One population consists of radially migrating neurons that originate in the ventricular zone of the pallium (cortex) and give rise to the glutamatergic pyramidal neurons. The second population consists of tangentially migrating neurons that originate in the ventricular zone of the subpallium (subcortical telencephalon) and give rise to GABA (gamma -aminobutyric acid)-producing local circuit neurons. The subpallium is also the origin of other cell types that follow distinct tangential trajectories to migrate to structures such as the olfactory bulb and the striatum. Here, we review evidence that supports the existence of several tangential migration pathways in the telencephalon, and summarize recent findings that describe their regulation.