Reallocation of Olfactory Cajal-Retzius Cells Shapes Neocortex Architecture

The neocortex undergoes extensive developmental growth, but how its architecture adapts to expansion remains largely unknown. Here, we investigated how early born Cajal-Retzius (CR) neurons, which regulate the assembly of cortical circuits, maintain a dense superficial distribution in the growing neocortex. We found that CR cell density is sustained by an activity-dependent importation of olfactory CR cells, which migrate into the neocortex after they have acted as axonal guidepost cells in the olfactory system. Furthermore, using mouse genetics, we showed that CR cell density severely affects the architecture of layer 1, a key site of input integration for neocortical networks, leading to an excitation/inhibition ratio imbalance. Our study reveals that neurons reenter migration several days after their initial positioning, thereby performing sequential developmental roles in olfactory cortex and neocortex. This atypical process is essential to regulate CR cell density during growth, which in turn ensures the correct wiring of neocortical circuitry. [Display omitted] •CR cell density in the neocortex doubles at mid-neurogenesis•Olfactory CR cells re-migrate into the neocortex and regulate cell density•CR cell redistribution relies on NMDA receptors•Neocortical CR cell density regulates layer 1 wiring and E/I ratio in upper layers Cajal-Retzius (CR) cells are transient regulators of neocortical wiring. de Frutos et al. show that CR cell density is maintained during growth by the reallocation of olfactory CR cells, thereby ensuring the proper development of layer 1 and neocortical circuits.