While you read this, millions of your body's cells are committing suicide in a process called apoptosis. Our health depends not only on being able to make new cells but the ability of cells to self-destruct on cue. Many tissues and organs of the body contain apoptic cells that are programmed to commit suicide as part of their normal functioning. For example, skin cells kill themsleves in a 21-day cycle to form the protective outer epidermal protein skin layer. The lens of the eye is formed during the embryonic stage of life from apoptotic cells.
Apoptosis is induced by various triggers.
The withdrawal of growth factors from a cell can cause the cell to die. Cells that become infected by viruses, damaged by free radical molecules and poisoned by many toxins may commit suicide. Internal and external messages that confuse the cell's program for duplication may also trigger the cell to kill itself. The sensitivity of a cell apoptosis depends on the cell's state. Irreplaceable cells like neurons are more resistant to killing themselves.