Patterning of the Brain, Neural Proliferation, and Migration

Abstract

By gastrulation the head–tail axis and a ventral and a dorsal side are formed from 3 weeks after conception. Noggin and chordin may trigger the formation of neuronal plate, which will develop to the neural groove and then the neural tube. This will be closed in both ends about 25 days after conception. Failure of this closure leads to anencephaly or spina bifida. This can be partially prevented by folic acid. The neural tube is then expanded in the cranial end forming the brain hemispheres, i.e., ballooning of the brain.

There are about 100 billion neurons in the adult brain. Most of them are generated during fetal life, mainly between the 10th and 20th gestational week. New neurons may be formed in the hippocampus but probably not in the cortex, where we believe the consciousness is located. The neurons, which are born in the ventricular zone near central cavity of the cerebrum, migrate radially along glial threads to the developing cerebral cortex at the outer surface. Nerve cells can also migrate in a horizontal direction. Synaptogenesis boosts during the last trimester of gestation and peaks at around 1 year, when up to one million per second can be formed. In the body the nerves find the way to the various organs by axonal guidance mechanisms. The last step is myelination of the nerves to increase the velocity of the action potentials.