DEVELOPMENT AND DISEASE Loss of Emx2 function leads to ectopic expression of Wnt1 in the developing telencephalon and cortical dysplasia

During development of the elegantly structured mammalian cerebral cortex, generation and subsequent migration of specialized neurons and glia within the brain follows a characteristic sequence. The earliest differentiated neurons emigrate from the telencephalic ventricular zone to form a superficial primordial layer called the preplate (Allendoerfer and Shatz, 1994). In mice, the marginal zone (MZ, layer I) and subplate (SP) derive from splitting of the preplate at ~11-12.5 days postcoitum (dpc) as successive waves of neurons from the underlying ventricular zone undergo radial migration into the cortical plate proper (layers II-VI). The MZ and SP comprise a heterogeneous population of cells with neuronal properties, including reelin-expressing Cajal-Retzius (CR) cells, subpial granule cells (glutamate-positive) and subpallial-derived interneurons (GABA-positive) (Fairen et al., 2002; Meyer et al., 1998; Parnavelas, 2000; Zecevic et al., 1999). Important roles for MZ and SP cells during tangential and radial migration of cortical neurons have been previously described (Parnavelas, 2000; Xie et al., 2002). For example, proper formation of the MZ and SP has been shown to be necessary for tangential emigration of GABA-expressing interneurons from ventral forebrain regions (e.g. the ganglionic eminence) into the cortical plate (Lavdas et al., 1999). Second, cells in the MZ provide repulsive or stop cues to radially migrating cortical 2275 Development 130, 2275-2287 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00421