After birth the brain loses many nerve cells and this continues throughout life - most neurons are formed before birth, after which many excess neurons degenerate. However, there are some cells that are still capable of division in old age - in the brains of mice, at least. According to scientists from the Max Planck Institute of Immunobiology in Freiburg, different types of neuronal stem cells exist that can create new neurons. While they divide continuously and create new neurons in young animals, a large proportion of the cells in older animals persist in a state of dormancy... 
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