Apoptosis is the term used to describe the generally normal death of the cell in living organisms. Since new cells regenerate, cell death is a normal and constant process in the body. Human embryos, for example, have far more cells than do adult humans. As the embryo develops, certain cells are selected for execution so that normal development takes place. When these cells do not go through apoptosis, they may cause deformity in the growing embryo.
This term should not be confused with necrosis, cell death through disease or infection. Apoptosis is part of the cell’s function in the organism. When the processes are incomplete, this can lead to the development of both benign and malignant tumors, for instance.
Apoptosis has several distinct stages. In the first stage, the cell starts to become round as a result of the protein in the cell being eaten by enzymes that become active. Next, the DNA in the nucleus starts to come apart and shrink down. The membrane surrounding the nucleus begins to degrade and ultimately no longer forms the usual layer.
As the nucleus of the cell is no longer protected, the cell’s DNA breaks in uneven fragments. The nucleus is now broken into many bodies with uneven quantities of DNA. The cell itself goes through a process called blebbing, where parts of the cell begin to break off. Finally, the cell is completely broken into pieces and is consumed by small cells that are called phagocytes.
There is danger if this final step of phagocytic digestion in apoptosis is not completed. Undigested cell fragments can accumulate in the body and have been shown to cause death in mouse embryos and mouse neonates.
Apoptosis can occur because of signals inside the cell (intrinsic), or signals outside the cell (extrinsic). When it's caused by intrinsic signals, it may be the result of lack of sufficient nutrition to the cell or damage to the DNA in the nucleus. Extrinsic apoptosis may occur in response to a virus, or treatments like chemotherapy. Sometimes, a cell initiates the process in an attempt to fight a virus, like HIV.
The study of apoptosis has become quite important, and most of our current understanding of cell death is the result of studies conducted in the 1990s and in present day. Being able to induce cell death is desirable, for example, when attempting to kill tumor tissue. As well, understanding how the process works furthers research on the study of stem cells and their possible applications in medicine.