Cell death, is it "suicide" or "accident"?

Audit expert: Ji Shi

Life Field Observer

Humans are born, grow old, get sick and die, and cells are no exception. As the basic unit of organisms, the "birth, aging, illness and death" of cells are closely related to health. In the long process of evolution, the cells of organisms have also experienced various forms of death.

Cell death pathways include apoptosis (which can be further divided into apoptosis and necroptosis), autophagy, pyroptosis, ferroptosis, and necrosis. Looking at these terms, do you feel very confused? What are they? Why are there so many and so confusing? In fact, they can be divided according to a relatively simple classification standard, which is divided into programmed death and non-programmed death.

The structure of animal cells Source: Wikipedia

What is the so-called "programming"? I believe that everyone has a certain degree of understanding of computer programming. Our computers will run step by step according to the set program. This is called programming.

The same is true for our bodies. Cells have a "program" in themselves. If a cell works according to the "program" until it dies, then we call its death programmed death. But if a small accident occurs in the middle of the cell and it is out of the "programmed" control, then the cell that dies is called unprogrammed death.

Unprogrammed cell death

First, let's take a look at the ways of non-programmed death. There is only one situation - necrosis. There are many reasons for cell necrosis, which will be introduced in detail below. In fact, necrosis is usually considered to be a non-programmed, unregulated cell death process. If other cells are "dying of old age", then necrosis is definitely an "accident" on the way. So what will cause this "accident" to happen? What should the injured cells do?

Epithelial cells under a microscope Source: Wikipedia

Since we call it necrosis, the environment in which it dies must be very harsh, such as trauma, ischemia, continuous high temperature, strong acid and alkali environment, radiation, toxins and immune cell attacks.

At this point, the ATP level in the cell drops to the point where the cell cannot maintain its physiological activities (the cytoplasm is the main place where ATP is consumed). At the same time, vacuoles appear in the cytoplasm, the cell membrane is damaged, and the substances contained in the cell, such as broken organelles and chromatin fragments, "escape" desperately because the cell is "powerless", causing an inflammatory response in the surrounding tissues, aggravating the severity of the situation. Finally, the cell is unable to bear it and can only die.

Red blood cells source | pixabay

So what happens to the cells after necrosis? There are four ways out: dissolution and absorption, separation and elimination, organization, and surrounding and calcification. Dissolution and absorption means that the necrotic cells are dissolved and absorbed by the human body. Separation and elimination means simply and roughly cleaning them out. If separation and elimination is throwing away garbage, then organization is the utilization of waste. The tissues surrounding the necrotic tissue will seize the opportunity, expand their strength, and strive to replace the necrotic tissue. And surrounding and calcification is a further organization, targeting a larger range of necrotic tissues and a more complete method.

Programmed cell death

After talking about non-programmed death, the next is the main stage of programmed death. Programmed cell death includes all the other death methods mentioned above except necrosis. According to its occurrence mechanism, programmed cell death can be divided into apoptosis, autophagic programmed cell death, non-apoptotic programmed cell death, cell mitotic catastrophe, oncosis, etc., and the mechanism of each method is very complicated, so we will not explain them in detail one by one.

The origin of cell necrosis and apoptosis丨High school biology (People's Education Edition)

Programmed cell death is diverse and designed by the body, so what are the benefits? Programmed cell death accompanies the entire development process of an organism. It is a natural and orderly cell death method determined by genes, which can play the role of "turning into spring mud to protect flowers".

Illustration of apoptosis Source: Wikipedia

Programmed cell death is quickly removed by other nearby tissues under the command of the body. The body's set "program" keeps everything in order. Programmed cell death, as a cell's instinct, plays an important role in the evolution of organisms, the stability of individual organisms, and the development of multiple systems.

In most cases, it is because of programmed cell death that the human body can complete metabolism and become healthier; while non-programmed cell death will affect the normal functioning of the body and may even cause other unpredictable consequences. Studying the way in which cells die can help us find the cause of human death and even the secret of longevity, so we should pay tribute to the cells that "sacrifice heroically" for our health!