The neocortex – the outer region of the mammalian brain –contains distinct layers of cells that develop in a specific spatiotemporal order. Reelin, which is mainly made by Cajal-Retzius (CR) cells in the developing neocortex, is essential for this process, but it is unclear whether this secreted glycoprotein provides positional information or is permissive for cell migration. On , Yoshida and colleagues provide strong evidence for the latter role. CR cells arise from the cortical hem of the developing mouse brain. Unexpectedly, when the researchers genetically ablated the hem, neocortical layers formed in the normal order in mutant embryos, even though most CR cells were lost; in reeler mice, which lack functional reelin but have CR cells, the layers are inverted. The researchers suggest that reelin diffusing in from elsewhere in the brain can compensate for CR cell loss and that,therefore, layer order is not driven by positional information from a localised source of reelin.
