Emx1 is a marker for pyramidal neurons of the cerebral cortex

The homeobox-containing gene, Emx1, a mouse homologue of Drosophila empty s piracles, is specifically expressed in the developing telencephalic cortex. It has been reported that Emx1 transcripts and the protein product are loc alized in most cells of the cerebral cortex during the process of prolifera tion, migration, differentiation and maturation. We provide evidence here, based on a multitude of experimental approaches in developing rats, in supp ort of the hypothesis that the expression of this gene is restricted to pyr amidal neurons. Specifically, we found that, similar to pyramidal neurons, cells expressing Emx1 are distributed in all cortical layers, except layer I. Using in situ hybridization and immunocytochemistry at the light and ele ctron microscope levels, we have shown that the density, distribution, soma shape and ultrastructural features of these cells were identical to those of pyramidal neurons. Double-labelling experiments confirmed that the vast majority of Emx1-expressing cells also contained glutamate, a marker of pyr amidal neurons. We also found that this gone is expressed by most glutamate -containing neurons in dissociated cortical cell cultures and the vast majo rity of cells in radially arranged clones of pyramidal cells in the cortice s of chimeric mice. Thus, the homeobox gene Emx1 can be reliably used as a marker of the pyramidal cell lineage.