From a flat sheet to a hollow tube
Before a single thought is possible, the brain has to be built — and it starts as nothing but a flat patch of cells on the back of an embryo. Imagine a strip of paper lying flat. Now imagine its two long edges curling up, rising toward each other, and zipping shut down the middle to form a tube. That rolled-up tube is the neural tube, and everything in your brain and spinal cord grows from its walls.
The tube is not solid — it is hollow, with a fluid-filled space running down its center. The cells lining that inner surface are special: they are neural stem cells, the factory workers that will produce every neuron to come. The walls are thin at first, but the factory is about to run an astonishing shift.
Neurogenesis: minting the cells
The making of new neurons has a name: neurogenesis — literally "neuron birth." The stem cells lining the tube divide again and again. At first each division just makes more stem cells, swelling the workforce. Then the schedule flips, and divisions start spinning off young neurons that will never divide again. At peak production the developing human brain can add on the order of hundreds of thousands of neurons every minute.
Here is a fact that surprises most people: the brain deliberately makes far more neurons than it will keep. Overproduction is not a mistake — it is the strategy. The brain builds a generous surplus, then later trims away the cells that fail to find useful work, in a planned die-off called developmental apoptosis. We will return to that pruning of cells in a later lesson; for now, just hold the idea that *making too many on purpose* is normal.
Migration: every neuron has an address
A freshly minted neuron is born deep in the tube wall, right next to the hollow center — but that is almost never where it belongs. The brain's outer rind, the cerebral cortex, is built in layers, and each young neuron must travel outward to reach its assigned layer. This journey is neuronal migration. Picture a crowd leaving a stadium where everyone has a numbered seat — except the newest arrivals must climb *past* those who came before to settle in the outermost rows.
Neurons cannot see, so how do they find the way? Many of them shimmy along living guide ropes. A long, slender cell called radial glia stretches from the inner wall all the way to the outer surface, like a rope strung across the whole thickness of the brain. The young neuron grips this rope and inches along it, hand over hand, until it reaches the surface and climbs off. The cortex is laid down inside-out: the earliest neurons settle in the deepest layers, and each later wave passes them to form a layer nearer the surface.
OUTER SURFACE (last to arrive)
───────────────────────────────
layer 6 ← newest neurons
layer 5
layer 4 each new wave
layer 3 climbs PAST the
layer 2 older ones
layer 1 ← oldest neurons
───────────────────────────────
INNER WALL (where neurons are born)
| | | ← radial glia = guide ropes
| | |Wiring up: the axon goes exploring
Reaching the right address is only half the job. A neuron is useless until it connects — and to connect it must send out its long output cable, the axon, across sometimes vast distances to find the right partner. Steering that cable to its target is axon guidance. At the very tip of the growing axon sits a tiny, hand-shaped explorer called the growth cone, constantly feeling the terrain ahead.
The growth cone reads chemical signposts soaked into the surrounding tissue. Some molecules whisper "come closer" and pull it forward; others shout "keep away" and push it off course. By following these scent trails of attraction and repulsion, the axon navigates a route it has never seen, like a hiker following blazes painted on trees. Once it arrives, it can finally build a junction with its partner — a synapse — and the circuit begins to take shape.
- Make: stem cells divide to mint a surplus of neurons (neurogenesis).
- Move: each neuron climbs a radial-glia rope to its layer (migration).
- Wire: the growth cone steers the axon to its partner (axon guidance).