The brain you have been touring is not the brain you were born with. It took twenty-five years to build, and the order in which it was built matters as much as the final structure. To understand adult disorders — particularly the developmental and degenerative ones — you have to understand the sequence of construction.
Brain development begins around week three of embryogenesis. A flat sheet of cells along the back of the embryo folds inward to form the neural tube. From this single tube, the entire central nervous system grows. The rostral end balloons outward to become the forebrain, midbrain, and hindbrain. The caudal end becomes the spinal cord. Failures at this stage produce neural tube defects — spina bifida, anencephaly — which is why folate supplementation before conception matters so much.
Inside the developing brain, neurogenesis peaks in the first half of pregnancy. The fetal brain produces neurons at a rate of roughly 250,000 per minute. These neurons then migrate outward from their birthplace near the ventricles to their final cortical positions, guided by radial glial scaffolding. Failures of migration produce cortical malformations associated with epilepsy and intellectual disability.
Once neurons reach their destinations, their axons grow toward targets sometimes meters away in the adult body. The growth cones at the tips of axons read molecular gradients — semaphorins, netrins, ephrins, slits — that guide them to their correct partners. Synapses form in waves, with massive overproduction followed by selective pruning. By birth, the human brain has hundreds of trillions of synapses.
Postnatal development continues for twenty-five years. Myelination — the wrapping of axons with insulating fatty sheaths that dramatically speed signal propagation — follows a specific, conserved schedule. Sensory cortex myelinates first, then motor cortex, then language and association areas, and finally — last of all — the prefrontal cortex completes its myelination in the mid-twenties.
This is why adolescents make the decisions adolescents make. It is why brain trauma in childhood produces different outcomes than the same trauma in adulthood. It is why neurodegenerative diseases tend to follow predictable spatial patterns — the regions that mature last (frontotemporal) and the regions richest in plasticity (hippocampus) tend to be the first to fail.
Hold the principle: the brain you treat today carries the imprint of how it was built. Development is not just a pediatric topic. It is the substrate of all adult neuropsychiatry.