Under proliferation in biology one understands the reproduction and growth of cells. The cells multiply through cell division and grow through growth to their genetically intended size and shape. In humans, proliferation plays a major role, especially during the embryonic and growth phase, afterwards mainly for the replenishment of rejected cells in certain types of tissue and in repair processes.
What is Proliferation?
A tissue proliferation is called proliferation, which consists of mitotic cell divisions and cell growth. Cell growth involves a maximum increase in cell volume to the size and shape that is preprogrammed in the DNA of the genes. Certain hormones, neurotransmitters (messenger substances) and growth factors provide the incentive to divide.
In the adult stage, some types of tissue or cells in humans are no longer capable of proliferation, i.e. no longer able to divide and thus no longer reproduce. This applies, for example, to most of the nerve tissue and to most of the sensory cells.
However, renewal processes are constantly taking place in many types of tissue, which are usually made possible by proliferative base cells or even stem cells. The average age of cells in humans varies from a few hours to lifelong, depending on the type of tissue. For example, the cornea renews itself every 28 days. The intestinal mucosa manages this much faster, namely within a few days. While the erythrocytes, the red blood cells released from the bone marrow, renew themselves every 120 days, most white blood cells only get a few days old.
Function & task
The proliferation of tissue cells is of great importance for the embryonic and postnatal development of humans. It is estimated that we are made up of around 5 trillion cells at birth. The process of proliferation increases this number to around 60 to 90 billion in adults. The number of cells has thus increased twelve to sixteen times. After the growth phase is complete, some types of cells lose their ability to proliferate. Other cell types still have a limited ability to proliferate.
In the case of tissue types whose cells can no longer multiply, but which nevertheless have to renew themselves, the body uses a type of stem cells that are often already specialized, i.e. have lost their omnipotence and can only grow into cells of certain tissue types. The limited possibility of reproduction is necessary in order to maintain the cell renewal process that takes different lengths of time in different types of tissue.
How necessary the remaining ability to proliferate is clearly demonstrated by the fact that around 50 million cells per second die and are either recycled, broken down and excreted by the body's metabolism or, as in the case of the skin, simply exfoliated to the outside. The cells, which are constantly dying and broken down by the body's metabolism, have to be replaced by proliferation in order not to lose any cell substance.
Proliferation plays a special role in injuries. Controlled by messenger substances, a proliferation process begins during the healing phase of injuries with the help of hormones and enzymes. Non-damaged connective tissue cells (fibrocytes), which are in the immediate vicinity of tendons and ligaments, migrate into the damaged area and are able to make contact with their processes and to contract via contractile elements in their cytoskeleton, so that Tighten the torn ends of ligaments or tendons. The repair mechanism shows that the ability of certain cells to proliferate can be reactivated if necessary.
It has been known since the mid-1990s that neurogenesis, i.e. the formation of new nerve cells in the central nervous system, is possible in certain neuronal stem cells in adults, which was previously not thought possible. From neural stem cells, which are located in a limited area of the hippocampus, precursor cells (progenitor cells) develop, which also have the ability to proliferate for a period of a few days.
Illnesses & ailments
The process of wound healing can be seen as an example of the fact that the body has the ability to switch the proliferation capacity of cells on and off again as required. The question arises as to why this possibility does not exist with all types of tissue, so that organs destroyed by illness or limbs lost in an accident can grow back.
Apparently nature has recognized through evolution that in an unlimited ability of cells to proliferate, the dangers would be greater than the potential benefits. The main danger associated with an unrestricted ability to proliferate is that the complex process can no longer be controlled. This means that once the cells have switched on their ability to proliferate, they no longer react to messenger substances, enzymes and hormones. Uninhibited cell growth would be the result.
This is exactly the case with tumors whose tissue is subject to constant growth, i.e. the ability to proliferate can no longer be prevented. The main difference between benign (benign) and malignant (malignant) tumors is that the malignant tumors, in addition to their own ability to multiply, can also feed themselves, since they have their own network of vessels through the process of vascularization and are capable of metastasis.
In addition to the possibility of unchecked proliferation, which can lead to cancers with very different orientations, there is also the problem of limited ability to proliferate. The dysfunction is often triggered by toxins and drugs such as alcohol and nicotine. For example, chronic alcohol abuse leads to a disruption of the proliferation and differentiation of the T lymphocytes, which are an important part of the immune system.
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