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Apoptosis is a form of programmed cell death, which is a natural process that occurs in multicellular organisms. It is a highly regulated and controlled process that leads to the elimination of cells without causing harm to the surrounding tissues. Apoptosis plays an important role in maintaining the balance between cell growth and cell death and is involved in various physiological processes such as development, tissue homeostasis, and the immune response.

Cells can undergo apoptosis under various circumstances, including: 1. Developmental signals: Apoptosis can be triggered by certain signals during the development of an organism to remove unwanted or unnecessary cells. For example, it is involved in the formation of fingers and toes by removing the tissue between them in the developing embryo. 2. DNA damage: If a cell's DNA is damaged and cannot be repaired, the cell may undergo apoptosis to prevent the potential formation of cancerous cells. 3. Infection: Apoptosis can be triggered by the immune system to eliminate infected cells, especially during viral infections, to prevent the spread of the virus to other cells. 4. Cellular stress: Stress-induced apoptosis can occur when cells are subjected to adverse conditions such as nutrient deprivation, heat shock, or oxidative stress. 5. External signals: Cells can receive external signals from other cells or molecules that can initiate apoptosis, such as the binding of Fas ligand (FasL) to the Fas receptor on the cell surface.

In summary, apoptosis is a form of programmed cell death that is vital for maintaining the balance between cell growth and cell death in multicellular organisms. Cells can undergo apoptosis under various circumstances, including developmental signals, DNA damage, infection, cellular stress, and external signals.