Birth Far from Home
Neurons are not born where they work. After neurogenesis in proliferative zones, they embark on long, guided journeys to reach the circuits they will serve. This process, neuronal migration, sculpts the architecture of the brain.
Radial Migration: Climbing the Scaffold
In the developing neocortex, neural precursors proliferate in the ventricular zone. A key stem cell here is the radial glial cell, whose long fibers stretch from the ventricular surface to the outer pia. Early-born cells leave this niche by somal translocation: a bipolar neuron anchors its leading process to the pia and then drags its cell body forward via nucleokinesis, where a microtubule “cage” around the nucleus elongates and contracts to pull it along.
Radial glia then guide later waves of neurons outward along their fibers. These waves split an early preplate into the cortical plate, forming cortex layers in an inside-out pattern: older neurons end up deep, younger ones closer to the surface. In humans, about 90% of migrating cortical neurons use this glial-guided route.
Tangential Migration: Sideways Journeys
Not all neurons follow the radial highways. Many interneurons migrate tangentially, moving sideways through the brain. A classic example is the stream of interneurons traveling from the ganglionic eminence into the cerebral cortex.
In some animals, such as certain mammals, ongoing tangential migration persists into maturity, as in the rostral migratory stream that ferries cells from the subventricular zone to the olfactory bulb.
Axophilic and Multipolar Modes
Some neurons travel along existing axons—a strategy called axophilic migration. GnRH-expressing neurons, for instance, originate in the nose and migrate into the hypothalamus via axonal tracts. Extracellular cues trigger intracellular cascades, including calcium signals, that remodel the actin and microtubule cytoskeleton, driving movement.
Others adopt a multipolar migration mode. In the cortical intermediate zone, neurons with multiple thin processes extend and retract branches in many directions, moving without strictly following radial glial fibers.
The Takeaway
The brain’s layered elegance hides a restless past. Neurons climb, slide, hitchhike, and wander before settling, and the precision of these migrations underlies everything from sensation to thought.