Section Summary

Brain segmentation is controlled at the level of gene expression and can be subject to disease through both abnormal gene expression and environmental exposures. The cells of the brain are produced from neural stem cells that occupy the neural tube and differentiate into neurons and glia. Some of these cells, such as glutamatergic cortical neurons, produce a layered structure using an inside-out pattern. These mechanisms produce a fundamental unit that will have tremendous plasticity to sense and respond to the environment during both subsequent prenatal and postnatal development.

A differentiating neuron undergoes significant changes which produce a growth cone that migrates through a milieu of signaling to find a target tissue. Upon selection, additional cues are provided to promote the formation of a mature synapse. Specific signaling events regulate maturation of the synapse as well as neuronal survival prior to birth, but after birth the nervous system continues to undergo refinement of synapses. In the subsequent sections, we will focus more on critical periods and provide examples of disorders that arise as a consequence of changes in our environment.

Critical periods for brain development, with long-term consequences for behavioral function, are found throughout development, from the embryonic stages of neural tube formation up to the final circuit refinement stages of adolescence. This plasticity is at its root adaptive–it allows our brains to develop in response to our specific environment, making the brain highly adaptable. However, it also creates points of vulnerability. The early embryo can be derailed by teratogens and environmental stimuli, which can cause significant structural abnormalities. Later changes in environmental stimuli, such a neglect or stress, can also have long-lasting effects on the developing brain and its function.