The lack of Emx2 causes impairment of Reelin signaling and defects of neuronal migration in the developing cerebral cortex

Neocorticogenesis in mice homozygous for an Emx2 null allele is the topic of this article. The development of both main components of neocortex, primordial plexiform layer deriva- tives and cortical plate, was analyzed, paying special attention to radial migration of neurons forming the cortical plate. The products of the Reelin gene, normally playing a key role in orchestrating radial migration of these neurons, display normal distribution at the beginning of the cortical neuronogenesis but are absent in the neocortical marginal zone of the mutant mice at the time when the cortical plate is laid down. As a conse- quence, the development of radial glia is impaired, and neurons making up the cortical plate display abnormal migration pat- terns. In addition, restricted defects along the rostrocaudal and the mediolateral axes are present in the subplate, suggesting an Emx2-specific role in priming the proper development of this layer.