Most brain neurons migrate in a radial pattern during development, moving almost directly outward from their origin. But a few classes of neurons migrate in more complex patterns. Interneurons of the cortex, for example, are generated by ventral progenitors that then migrate dorsally, thus introducing a different neuronal subtype in the region.
A novel set of such “tangentially” migrating cells, which the authors call corridor cells, are now shown to express a membrane-bound form of neuregulin-1. In combination with soluble neuregulin-1 isoforms produced in the cortex, these molecules allowed the growth of thalamic axons toward the cortex. In the absence of corridor cells, their section of the brain was nonpermissive for thalamic axons, which thus failed to extend toward the cortex.
The creation of these mammal-specific axonal connections was probably an evolutionary breakthrough. They allow sensory and motor information to be relayed to our center of conscious information processing. This big breakthrough apparently required small changes. “The guidance cues can be generated,” says Garel, “by putting just a few new cells there that will be permissive in a restrictive environment.”