The most dynamic growth occurs in the cerebral cortex, the largest and outermost layer of the brain.During the first months of fetal development, when 250,000 new nerve cells are being created every minute, neurons begin to take on specialized tasks.
First, they inch their way from where they were formed by cell division to their permanent home in other regions of the brain. Most go toward the cortex, but some move into the cerebellum and other portions of the brain. This process, known as migration, is quite remarkable for the distance the neurons must travel as well as their ability to navigate surely along the tangled pathways of the developing brain. Millions of neurons migrate a distance equivalent to a person hiking from Los Angeles to Boston. Amazingly, they manage to arrive at Paul Revere’s house, the U.S.S. Constitution, or Faneuil Hall without ever consulting a map.
Once the migrating neurons reach their destination, they developed axons and dendrites to reach out and make connections with other neurons. Like roads that connect to create a grid for traffic, neurons set up systems of communication that link all parts of the brain. Some pathways receive huge amounts of sensory traffic and become the equivalent of information highways. Others turn into dead ends or decay into crumbling blacktop from lack of use.
You can’t clone a brain. And even if you could, it wouldn’t turn out like the original. Sensitivity to initial conditions in the womb coupled with differences in environment after birth would significantly alter development despite the identical genetic code.
The brain reacts with extreme sensitivity to anything that influence neuronal migration. Only a few decades ago, neuroscientists believed that each neuron had its own special, predetermined location when it set out on its trek across the brain. Now, researchers have found that neurons take on different characteristics because of their journey and their destination. To take just one example, neurons that process oral communication are not inherently preprogrammed to be speech neurons. Instead, they become speech neurons by migrating to the areas of the brain associated with language.
This discovery prompted new understanding of a wide variety of brain disorders. If something interferes with neurons migrating to their intended destinations and not overshooting or undershooting their targets the results can be powerful. Such disorders as autism, schizophrenia, dyslexia, and epilepsy have been at least partly linked with abnormalities in neuronal migration.
Fetal alcohol syndrome has also been linked to problems in migration. The brain’s hypersensitivity to toxins that impede migration underscores the warnings given to expectant mothers to avoid exposing a developing baby to alcohol, tobacco smoke, drugs, or other chemicals that may interfere with healthy brain development.