How the Brain Builds Itself

From a flat sheet of cells to 86 billion neurons: the neural tube, the burst of neuron production before birth, migration, wiring, pruning and myelination — the timeline of brain development.

Every human brain begins as a thin sheet of cells — the neural plate — on the back of an embryo only about three weeks old. What follows is one of the most remarkable construction projects in biology.

The neural tube

During the third and fourth weeks of pregnancy, the edges of the neural plate rise, fold and fuse to form the neural tube, which becomes the brain at its front end and the spinal cord along the rest. Closure is normally complete by about day 28 — often before a person knows they are pregnant. Failure to close causes neural tube defects such as spina bifida (at the lower end) or anencephaly (at the top). Taking folic acid before and in early pregnancy greatly reduces the risk, which is why many countries fortify flour with it.

Making neurons

The front of the tube swells into three, then five, brain vesicles that become the forebrain, midbrain and hindbrain. Stem-like cells lining the tube divide rapidly. Neuron production peaks in mid-pregnancy; averaged over gestation the brain makes on the order of hundreds of thousands of neurons per minute. Most cortical neurons are made before birth.

Moving and connecting

New neurons migrate to their final positions — in the cortex, climbing along the fibres of radial glia — then extend axons that navigate, sometimes over long distances, to their targets. Synapses form in enormous numbers.

After birth

A newborn's brain is about a quarter of its adult size; it roughly doubles in the first year, reaching about 80–90% of adult volume by age five or six. Much of this growth is not new neurons but dendrites, synapses, glia and myelin. Synapse numbers in many regions peak in early childhood and are then pruned, guided by experience. Myelination proceeds from the back of the brain to the front, continuing in the prefrontal cortex into the mid-twenties.

Genes plus experience

Genes set up the basic plan, but experience shapes the details — especially during critical periods when certain circuits are most sensitive. Nutrition, stress, toxins (such as alcohol in pregnancy) and the quality of early care all affect how the brain develops.