Extensive branching of radially-migrating neurons in the mammalian cerebral cortex.

Excitatory neurons of the cerebral cortex migrate radially from their place of birth to their final position in the cortical plate during development. Radially-migrating neurons display a single leading process that establishes the direction of movement. This leading process has been described as being unbranched, and the occurrence of branches proposed to impair radial migration. Here we have analyzed the detailed morphology of leading process in radially-migrating pyramidal neurons and its impact on radial migration. We have compared ferret and mouse to identify differences between cortices that undergo folding or not. In mouse, we find that half of radially-migrating neurons exhibit a branched leading process, this being even more frequent in ferret. Branched leading processes are less parallel to radial glia fibers than those unbranched, suggesting some independence from radial glia fibers. Two-photon videomicroscopy revealed that a vast majority of neurons branch their leading process at some point during radial migration, but this does not reduce their migration speed. We have tested the functional impact of exuberant leading process branching by expressing a dominant negative Cdk5. We confirm that loss of Cdk5 function significantly impairs radial migration, but this is independent from increased branching of the leading process. We propose that excitatory neurons may branch their leading process as an evolutionary mechanism to allow cells changing their trajectory of migration to disperse laterally, such that increased branching in gyrencephalic species favors the tangential dispersion of radially-migrating neurons, and cortical folding.