Dorsal-to-Ventral Cortical Expansion Is Physically Primed by Ventral Streaming of Early Embryonic Preplate Neurons

Abstract

Summary: Despite recent studies elucidating the molecular mechanisms underlying cortical patterning and map formation, very little is known about how the embryonic pallium expands ventrally to form the future cortex and the nature of the underlying force-generating events. We find that neurons born at embryonic day 10 (E10) in the mouse dorsal pallium ventrally stream until E13, thereby superficially spreading the preplate, and then constitute the subplate from E14. From E11 to E12, the preplate neurons migrate, exerting pulling and pushing forces at the process and the soma, respectively. At E13, they are morphologically heterogeneous, with ∼40% possessing corticofugal axons, which are found to be in tension. Ablation of these E10-born neurons attenuates both deflection of radial glial fibers (by E13) and extension of the cortical plate (by E14), which should occur ventrally, and subsequently shrinks the postnatal neocortical map dorsally. Thus, the preplate stream physically primes neocortical expansion and arealization. : Embryonic radial glial fibers that guide most neocortical neurons are ventrally deflected near their terminal, contributing to further expansion of the neocortical area. Saito et al. demonstrate that this fiber deflection is induced physically by a previously unrecognized ventral stream of the earliest generated preplate neurons. Keywords: neocortex, arealization, protomap, radial glia, preplate, subplate, tangential migration, mechanical factor, axonal traction