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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.
@simrin-- Actually radiotherapy is not really any better than chemotherapy when it comes to apoptosis. The truth is that all cancer treatments can and do cause damage to cells' DNA. And if the cells can't repair themselves, apoptosis becomes inevitable. That's a risk that comes with all cancer therapies.
And to some extent, we wish that all cells were that vulnerable to apoptosis. Some people actually have an apoptosis gene which prevents some cells from dying. This is a huge problem if that same cell is diseased and mutated. It's one of the problems that doctors are dealing with right now. This gene and these undying cells cause anti-cancer treatments to fail.
So apoptosis is a good thing. It's vital for us.
@simrin-- In regards to your HIV example, I think the difference is that the apoptosis there is by immune dysfunction whereas necrosis is triggered by disease.
With necrosis, the cell dies because it becomes affected by the disease itself, it's not the programmed activity of the cell. In apoptosis, the cell dies because the body tells it to. Under normal circumstances, this is normal and good. But when there is a disease with causes the immune system to dysfunction, such as HIV, then apoptosis is triggered when it's not supposed to be.
This is why HIV positive individuals are treated with apoptosis inhibitors. The HIV causes healthy T-cells to die off which further weakens the immune system. Apoptosis inhibitors limit and prevent this from happening.
I hope this made sense.
How come the extrinsic apoptosis mechanism is not considered necrosis? HIV or cancer is a disease too. How is it that extrinsic apoptosis in reaction to HIV for example is normal and desirable whereas necrosis in response to HIV is not?
I'm glad the article mentions chemotherapy. I do not approve of chemotherapy for exactly this reason, that it causes apoptosis- the death of healthy cells in the body. Medical professionals find this to be a small price to pay for the destruction of cancerous cells.
But just as the article mentioned, there are downsides to apoptosis too, especially if the leftover fragments of the cell are not consumed. Since chemotherapy is basically an uncontrolled method of killing cancer cells unlike treatments like radiotherapy, I think the risks associated with excessive apoptosis is fairly large.
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