The medial ganglionic eminence gives rise to a population of early neuronsin the developing cerebral cortex

During development of the neocortex, the marginal zone (layer I) and the su bplate (layer VII) are the first layers to form from a primordial plexiform neoropil. The cortical plate (layers II-VI) is subsequently established be tween these superficial and deep components of the primordial plexiform neu ropil. Neurons in the early zones are thought to play important roles in th e formation of the cortex: the Cajal-Retzius cells of the marginal zone are instrumental in neuronal migration and laminar formation, and cells of the subplate are involved in the formation of cortical connections. Using the fluorescent tracer 1,1'-dioctodecyl- 3,3,3',3'-tetramethylindocarbocyanine (DiI), we have shown here that a substantial proportion of neurons of the m arginal zone, including cells with features of Cajal-Retzius cells, and of the subplate and lower intermediate zone are not born in the ventricular ne uroepithelium but instead originate in the medial ganglionic eminence (MGE) , the pallidal primordium. These neurons follow a tangential migratory rout e to their positions in the developing cortex. They express the neurotransm itter GABA but seem to lack the calcium binding protein calretinin; some mi grating cells found in the marginal zone express reelin. In addition, migra ting cells express the LIM-homeobox gene Lhx6, a characteristic marker of t he MGE. It is suggested that this gene uniquely or in combination with othe r transcription factors may be involved in the decision of MGE cells to dif ferentiate in situ or migrate to the neocortex.