Why don't brain cells regenerate?
"Why don't brain cells regenerate?"
Cells don't often regenerate. In some animals, limbs regenerate, and some parts of the human body, such as the ends of the fingers, can regenerate. Such regeneration almost always results from the production of replacement cells.
Cells are replaced by production of stem cells and, most commonly, precursor cells that are derived from stem cells. Once a cell becomes specialized for a specific purpose, it generally loses the ability to divide. This loss is induced so that higher level processes can decide the lifetimes of various types of cells.
One exception to this lifetime rule is neurons, which are constructed of multiple compartments, allowing a compartment to die without dooming the whole cell. However, when a compartment dies, it is usually not replaced since there is no master plan which would guide its regeneration. The exception to this is that neurons in a neural sheathe (a nerve fiber), mostly peripheral system neurons, can extend a regenerated compartment within the sheathe and grow to the end of the sheathe, and may be lucky enough to reconnect there. There are no neural sheathes in the brain.
Another exception to the lifetime rule is cancer, where stem cells, precursor cells, and ordinary cells, can be modified by the cancer to allow mitosis. This fuels the characteristic runaway growth seen in cancer. It is also the reason most cell growth is limited to the smaller number of precursor cells rather than delegated to all cells, since the odds of a random mutation are less likely to be propagated.
Brain cells don't regenerate because all their value is in their connections, so once they die (or a compartment dies), connection information is lost. This also contributes to the relative rarity of brain cancer.
It is reported (and likely) that precursor cells create new neurons in the hippocampus, and there is also strong evidence of new neurons in the cerebellum. This is not regeneration, but new growth. The hippocampus and cerebellum contain processes that exploit new neurons, and this creation seems to support both of their jobs of learning.