The avian telencephalic subpallium originates inhibitory neurons that invade tangentially the pallium (dorsal ventricular ridge and cortical areas)

Recent data on the development of the mammalian neocortex support that the majority of its inhibitory GABAergic interneurons originate within the subp allium (ganglionic eminences). Support for such tangential migration into t he pallium has come from experiments using fluorescent tracers or lineage a nalysis with retrovirus, and the phenotypes of mutant mice with different a bnormalities in the developing subpallium. In the present study, we describ e tangential migration of subpallial-derived neurons in the developing chic k telencephalon. Using quail-chick grafts, we precisely identified the neur oepithelial origin, time-course, and pathways of migration, as well as the identity and relative distribution of the diverse tangentially migrated neu rons. The analysis of selective grafts of the pallidal and striatal primord ia allowed us to determine the relative contribution of each primordium to the population of migrating neurons. Moreover, we found that, like in mamma ls, the vast majority of the GABAergic and calbindin-immunoreactive neurons within the pallium (dorsal ventricular ridge and cortical areas) have an e xtracortical, subpallial origin. Our results suggest that the telencephalon of birds and mammals share developmental mechanisms for the origin and mig ration of their cortical interneurons, which probably first evolved at an e arlier stage in the radiation of vertebrates than was thought before. (C) 2 001 Academic Press.