Fundamentals of Neuroscience/Development
Stages of Embryonic Development
While the cells in a fertilized embryo start out as undifferentiated cells capable of becoming any tissue type in the body, it is not until about three weeks into development that the first neural precursor cells emerge. Although these cells have not yet differentiated completely into specialized cell types like glia or neurons, they still have differentiated enough so that they will only form cells of the nervous system.
After this, the first major milestone in the development of the human nervous system involves a structure known as the neural tube. This structure is made from tissue that has folded in on itself, creating a cavity known as the neural tube, and pinching off a small section of neural-precursor cells in the process which is called the neural crest. The neural tube become the spinal chord and brain, while the neural crest becomes the peripheral nervous system. As development continues, the brain region starts enlarging and already the cells of the brain are beginning to form the distinct subregions that will come to define the adult brain. At first buds for the eyes and ears are visible, then a cerebellum-like structure is seen sprouting from the top of the spinal chord, and finally by around the 20th week the cerebrum gains its hallmark wrinkles.
Childhood and Adolescence
Although the gross anatomy of the brain undergoes few radical changes post-birth, infancy and early adolescence remain times of crucial change in the brain. Neurons undergo significant changes such as myelination and pruning- the process by which unnecessary connections are cut, leaving only desirable neural pathways. Areas like the hippocampus require years to achieve full functionality, accounting for why most people's earliest childhood memories only date back to around three to four years of age. Furthermore, profound changes in brain regions associated with language use occurs as children hone skills such as learning to talk and then read. In fact, early childhood is still so important for brain development that there exists certain critical periods where skills have to be acquired or else it may be difficult or impossible to ever acquire them later in life. Critical periods include aspects of visual abilities, motor development, and intellectual skills like language and socialization.
By adolescence, brain development slows, however there is still at least once crucial area which has not yet reached fully-functional maturity. The prefrontal cortex continues to undergo development in realms such as getting its axons myelinated, forming proper connections, etc. up until around age 21, and because this brain region is so intimately linked to so many higher level and vital functions, ranging from reasoned judgement to impulse control, the potential consequences of this ongoing development should not be ignored.
The fact that neurons do not regularly divide and regenerate to the same capacity as much other tissue's cells means that the nervous system finds itself at risk of degeneration and decreased efficiency. While the effects of aging can be combated-or accelerated- to a large extent through lifestyle choices, as a general trend brain volume starts to decline noticeably after about age 50, up to about 0.1-0.2% per year. In addition, myelin degrades at a somewhat faster pace, and incidence rates of neurological disorders increase substantially at this point in life. Despite this, these declines in brain matter are not necessarily correlated to a decline in function, as the plasticity and resiliency of the brain is remarkably effective at preserving functionality.