http://www.sens.org/research/introductio...tant-cells
Quote: ApoptoSENS: Removing dysfunctional cells
Our cells have built-in programming that sometimes veers them far away from their normal fate. Some of this programming watches out for conditions emerging within the cell that could put the rest of the body at risk; similar systems exist because the body no longer has need for the function that the cell normally fulfils. Pushing these cells to undergo such transformations is favored by evolution because it meets short-term needs, and having a few of these abnormal cells in the body for is nearly harmless. But the number of these cells in our tissues gradually rises over time, until by our fifth decade or so they begin to reach levels that are harmful to normal tissue function. Examples include:
Classic Senescent Cells
The original and most well-studied sort of cells of this type are what are usually called “senescent” cells. Senescent cells began their existence skin cells, or as related cells that normally play supporting roles in other organs, but were forced into an abnormal state where they lost the ability to divide and reproduce themselves as a protective response to some danger. For instance, the senescence program is activated in cells that undergo risky changes in their DNA expression that put them on a path toward becoming cancerous; it is also activated in some cells involved in the wound response, to keep them from overstepping their bounds and generating an overgrowth of fibrous connective tissue.