In the mammalian neocortex, different areas have different functions. One model for the formation of this modular structure proposes that neocortical progenitor cells become patterned by extracellular signals to generate a protomap of progenitor cell areas that are defined by their gene expression patterns, which, in turn, generate area-specific neurons. On , Sansom and co-workers provide important insights into the nature of this proposed protomap by systematically analysing gene expression patterns in the developing mouse neocortex. Their results provide little evidence to support the division of neocortical progenitor cells into discrete domains, as seen in the developing spinal cord. Instead, the early neocortical protomap appears to be composed of gradients of gene expression along the rostrocaudal axis. Through their examination of gene expression changes in mutant Fgfr1and Mest (which encodes a mesoderm-specific gene) mice, the researchers show that the formation of the protomap is Fgf-regulated and influenced by Mest.
