The number of cells in our body is tightly regulated. There is a complex system that controls the rate of cell division (replication) and the rate of cell death (apoptosis). If cells are no longer needed, the normal removal mechanism activates an intracellular death program. Apoptosis comes from the Greek word meaning “falling off”, as in an autumn leaf as it completes its life cycle and drops from the tree.
The amount of this normal apoptosis that occurs in developing and adult animal tissues can be astonishing. In the developing vertebrate nervous system, for example, up to half or more of the nerve cells normally die soon after they are formed. In a healthy adult human, billions of cells die in the bone marrow and intestine every hour. It seems remarkably wasteful for so many cells to die, especially as the vast majority are perfectly healthy at the time they kill themselves. However, remember that apoptosis conserves the valuable resources of the cell by a type of recycling process so not all is wasted.