For example, early in the development of a human embryo, the fingers and toes are conspicuously webbed but, at the proper time, the unwanted cells are eliminated by apoptosis. Cell death is also of great importance in the development of the brain. Explosive growth in the embryonic brain initially creates many more nerve cells than the newborn needs. Once the brain has wired itself for action, however, the surplus millions of nerve cells commit harikari. If this process is activated inappropriately later in life, neurodegenerative disease may ensue.
Apoptosis is essential to control cell numbers in the immune system.
During an infection, lymphocyte numbers increase dramatically to fight the invading foe. Once the battle is won, however, lymphocyte numbers must subside to normal levels.