Guidance of cortical radial migration by gradient of diffusible factors.

During the development of cerebral cortex, newborn pyramidal neurons originated from the ventricle wall migrate outwardly to the superficial layer of cortex under the guidance of radial glial filaments. Whether this radial migration of young neurons is guided by gradient of diffusible factors or simply driven by a mass action of newly generated neurons at the ventricular zone is entirely unknown, a potential guidance mechanism that has long been overlooked. Our recent study showed that a guidance molecule semaphorin-3A, which is expressed in descending gradient across cortical layers, may serve as a chemoattractive guidance signal for radial migration of newborn cortical neurons toward upper layers. We hypothesize the existence of four groups of extracellular factors that can guide the radial migration of young neurons: (1) attractive factors expressing in superficial layers of cortex, (2) repulsive factors enriched in the ventricular zone, (3) pro-migratory factors uniformly expressed in all cortical layers and (4) stop signals locally expressed in the outmost layer of cortex.