<P> Programmed cell death (or PCD) is cell death mediated by an intracellular program . PCD is carried out in a regulated process, which usually confers advantage during an organism's life - cycle . For example, the differentiation of fingers and toes in a developing human embryo occurs because cells between the fingers apoptose; the result is that the digits are separate . PCD serves fundamental functions during both plant and metazoa (multicellular animals) tissue development . </P> <P> Apoptosis or Type I cell - death, and autophagy or Type II cell - death are both forms of programmed cell death, while necrosis is a non-physiological process that occurs as a result of infection or injury . Necrosis is cell death caused by external factors such as trauma or infection, and occurs in several different forms . Recently a form of programmed necrosis, called necroptosis, has been recognized as an alternate form of programmed cell death . It is hypothesized that necroptosis can serve as a cell - death backup to apoptosis when the apoptosis signaling is blocked by endogenous or exogenous factors such as viruses or mutations . </P> <P> Mitotic catastrophe is a mode of cell death that is due to premature or inappropriate entry of cells into mitosis . It is the most common mode of cell death in cancer cells exposed to ionizing radiation and many other anti-cancer treatments . </P> <P> Autophagy is cytoplasmic, characterized by the formation of large vacuoles that eat away organelles in a specific sequence prior to the destruction of the nucleus . </P>

After age ____ or so the rate of cell breakdown beings to exceed the rate of cell renewal