The Most Common Way We Lose Brain Cells

Surprisingly, neither disease nor trauma wipes out the greatest number of brain cells. The grimmest reaper in our brains is normal development. In some parts of the nervous system, it wipes out half the total number of nerve cells generated.

During development, the number of nerve cells rises to a maximum as cells proliferate and migrate to their final destinations. Nerve cells send out “feelers” called growth cones to find one another and form connections. Cells that fail to make appropriate connections are tidily eliminated.

Although this process may seem wasteful, it permits much more flexibility than would be possible if every nerve cell and connection were specified by our genes. Instead, it provides a mechanism through which brain anatomy and function in humans and other complex animals can respond to environmental influences.

The form of cell death that neatly eliminates unwanted cells is apoptosis. The term is derived from a Greek root that means “dropping of leaves off a tree.” During apoptosis cells shrink and display signals on their surfaces that tell other cells to eat them. Neighboring cells or white blood cells known as macrophages engulf the dying cells. Apoptosis is carefully controlled and does not injure the surrounding tissue.

In contrast, following infection, stroke, or trauma (such as when a big rock conks Wile E. Coyote on the head), brain cells mainly die via necrosis. Necrosis is much messier than apoptosis. It involves the leakage of cell contents and inflammation, which can damage nearby cells.

This is not to say that apoptosis is always a good thing. Sometimes apoptosis gets activated abnormally. For example, exposing the developing brain to alcohol can activate apoptosis and delete millions of nerve cells. The resulting damage is responsible for the most disabling features of fetal alcohol syndrome.

Evidence is mounting that apoptosis also plays an important role in many disorders characterized by slow degeneration of the central nervous system, such as Lou Gehrig’s disease (ALS), Parkinson’s disease, Huntington’s disease, and Alzheimer’s disease. Therefore, a great deal of research is directed at understanding what cellular signals activate apoptosis.

In modern society, the most common cause of intellectual deterioration is Alzheimer’s. In the United States, about 2 percent of the population is affected, but more than half of individuals over age 85 may have the disease, according to an article in the American Journal of Medicine. Cell numbers may decline by 20 to 80 percent in certain regions of the brain over the course of one or two decades as the disease progresses.