Some cells die because of cell injury but other cells die because they commit suicide. Yes, they do! And why would cells do that? There are various reasons.
One reason is, to remove severely damaged cells and prevent them from being duplicated. Cells generally repair some damages that may develop in them. If the damage is too extensive however, they receive signals to commit suicide rather than pass on their defect to the next generation of cells.
Another reason is to maintain the number of cells in the body. We cannot have too many or too little number of cells. So “old, worn-out cells” that have reached their “expiry date” are told to commit suicide. They are then replaced by new cells through the process of mitosis. In the cell world, there is no such thing as extension of retirement. Any extension is always detrimental to the whole organism. Some cancerous cells for example ignore this “expiry date” so the organism develops cancer.
Still another reason is “body sculpting” during development. In the formation of our fingers and toes for example, the starting form is a whole solid mass. During development, the “in-between” cells in the mass commit suicide and this leads to the formation of separate fingers and toes. Sometimes some cells may not commit suicide so the separation of fingers can be incomplete. Thus, we sometimes see people with partially fused fingers or toes.
The disappearance of the tadpole’s tail as it is transformed to an adult frog is an example of “body sculpting” produced by cell suicide.
There are other reasons for cell suicide but those three are the most important ones.
By the way, the scientific term for cell suicide is – “apoptosis” or programmed cell death.