Physiological or programmed cell death. In apoptosis only individual cells are affected generally and these react to internal signals (e.g. adjacent cells) or external stress situations. At the beginning of this process the cell nucleus, cytoplasm and mitochondria are shrinking; the cell membrane remains undamaged, so that no inflammatory reaction occurs. The biochemical indicator of apoptosis is the fragmentation of DNA. Along with proliferation the apoptosis is the common instrument to stabilise the cell count.