In biology, proliferation refers to the reproduction and growth of cells. In this process, cells proliferate by cell divisions and grow by growing to their genetically intended size and shape. Proliferation plays a major role in humans, especially during the embryonic and growth phases, and thereafter mainly for the replenishment of rejected cells in certain types of tissues and in repair processes.
What is proliferation?
In biology, proliferation is the multiplication and growth of cells. Proliferation refers to tissue proliferation that consists of mitotic cell divisions and cell growth. Cell growth involves a maximum increase in volume of cells to the size and shape that is pre-programmed in the DNA of genes. The stimulus for division is provided by certain hormones, neurotransmitters (messengers) and growth factors. In the adult stage, some types of tissue or cells in humans are no longer capable of proliferation, i.e. they are no longer capable of dividing and thus no longer capable of reproducing. This applies, for example, to the majority of nervous tissue and to most sensory cells. However, in many types of tissue, renewal processes are constantly taking place, which are usually facilitated by proliferation-capable basic cells or even stem cells. The average age of cells in humans varies from a few hours to a lifetime, depending on the tissue type. For example, the cornea renews itself every 28 days. The intestinal mucosa manages this much faster, within a few days. While erythrocytes, the red blood cells released from bone marrow, renew every 120 days, most white blood cells live only a few days.
Function and purpose
For embryonic and postnatal human development, the proliferation of tissue cells is of great importance. Estimates say that at birth we are composed of about 5 trillion cells. This number increases to about 60 to 90 trillion in adults due to the proliferation process. The number of cells has thus increased twelve to sixteen times. After completion of the growth phase, some types of cells lose their ability to proliferate. In other types of cells, a limited proliferative capacity still remains. For tissue types whose cells can no longer proliferate but still need to renew themselves, the body resorts to a type of stem cells that are often already specialized, i.e., have lost their omnipotence and can only grow into cells of specific tissue types. The limited ability to proliferate is necessary to maintain the cell renewal process that takes different lengths of time for different types of tissue. The necessity of the remaining proliferative capacity is vividly expressed by the fact that about 50 million cells die per second and are either recycled, degraded and excreted by the body metabolism or, as in the case of the skin, simply exfoliated to the outside. The cells that are constantly dying and being broken down by body metabolism must be replaced by proliferation in order not to lose cell substance overall. Proliferation plays a special role in injuries. Controlled by messenger substances, a proliferation process starts during the healing phase of injuries with the cooperation of hormones and enzymes. Non-lamellar connective tissue cells (fibrocytes) located in the immediate vicinity of tendons and ligaments migrate into the damaged area and are able to make mutual contact with their projections and contract via contractile elements in their cytoskeleton, allowing the torn ends of ligaments or tendons to tighten again. The repair mechanism shows that the proliferative capacity of certain cells can be reactivated when needed. Since the mid-1990s, it has been known that neurogenesis, i.e. the formation of new nerve cells in the central nervous system, is also possible in adults with certain neural stem cells, something that had not been thought possible until then. Neural stem cells located in a restricted area of the hippocampus give rise to precursor cells (progenitor cells) that also exhibit proliferative capacity for a period of a few days.
Diseases and ailments
The process of wound healing can be seen as an example of the body having the ability to turn the proliferative capacity of cells on and off as needed. This raises the question of why this possibility does not exist in all types of tissue, so that organs destroyed by disease or limbs lost in an accident could grow back. Apparently, nature recognized by evolutionary means that in an unlimited proliferative capacity of cells, the dangers would be greater than the potential benefits. The main danger associated with an unrestricted proliferative capacity is that the complex process can no longer be controlled. This means that once the cells turn on their proliferative capacity, they no longer respond 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 proliferation ability can no longer be stopped. The main difference between benign (benign) and malignant (malignant) tumors is that malignant tumors, in addition to their own ability to proliferate, 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 unrestrained proliferation, which can lead to cancer formations with very different orientations, there is also the problem of restricted proliferative capacity. Often the dysfunctions are triggered by toxins and by drugs such as alcohol and nicotine. For example, chronic alcohol abuse leads to impaired proliferation and differentiation of T lymphocytes, which are an important part of the immune system.
