Dorsal-to-ventral neocortical expansion is physically primed by ventral streaming of early embryonic preplate neurons

Mammalian neocortex exhibits a disproportionally “luxurious” representation of somatotopies in its lateral region, which depends on dorsal-to-ventral expansion of the pallium during development. Despite recent studies elucidating the molecular mechanisms underlying the cortical arealization/patterning, we know very little about how the cortex expands ventrally and the nature of the underlying force-generating events. We found that neurons born earliest (at embryonic day 10 [E10]) in the mouse pallium migrated ventrally and then extended corticofugal axons, which together formed a morphogenetic flow of the preplate that persists until E13. These neurons exerted pulling and pushing forces at the process and the soma, respectively. Ablation of these E10-born neurons attenuated both deflection of radial glial fibers (by E13) and extension of the cortical plate (by E14), which should occur ventrally, and subsequently shrank the postnatal neocortical map dorsally. This previously unrecognized preplate stream physically primes neocortical expansion and somatotopic map formation.