Most cells in our bodies’ organs and tissues, such as the liver, guts, or skin are continuously renewed. In contrast, the majority of the approximately 100 billion nerve cells in our brain and spinal cord are born — through a process known as neurogenesis — before birth and will last a lifetime. However, a few brain structures add new nerve cells during infancy and a single region adds new cells throughout the lifespan.
Shortly after birth in humans, a substantial number of new nerve cells are produced and added to brain regions called the cerebellum, olfactory bulb, prefrontal cortex, and hippocampus. But by age 2, neurogenesis in most of these regions disappears except in the hippocampus — a region involved in learning and memory. This may be the only location in the brain where new cells are added throughout one’s lifetime.
Other animals, such as fish, amphibians, reptiles, and birds display a continuous addition and high turnover of nerve cells in many brain structures throughout life. In adult mammals, the birth of new neurons becomes more limited. For example, in rodents, only two regions of the brain continue to acquire new nerve cells throughout life; however, in adult humans the only remnant of neurogenesis persists in one part of the hippocampus.
Often, people are disappointed when I tell them that we do not generate new neurons like most other animals. But new is not always better. Maybe during evolution we traded our capacity to regenerate new cells for the stability of our brain cells. This property allows us to learn and remember information throughout our lifespan better than other animals can. The fact that the majority of our neurons are as old as we are might be what allows us to remember our history and build civilization.
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