Since conception, the body undergoes many changes. Developing from just one cell containing the hereditary material of the parents, it grows due to the multiplication and differentiation of cells. This is a constant process of maintaining the life of a multicellular organism, which is based on many intercellular interactions. At each stage of life, the specialization of cells changes and becomes more narrow.
Cells and tissues
A group of cells with the same morphophysiological characteristics located in one place and solving the same tasks is called tissue. The organs are made up of tissues, and the body is made up of organ systems. But in order to go from a germ cell to an organism, it is necessary to overcome many stages of cell differentiation. This process is the preparation of cells to carry out the functions assigned to them, as a result of which, at high stages of development, they lose the ability to divide.
Regeneration
The necessity of undergoing long-term differentiation explains the impossibility of true regeneration of highly specific tissues and organs whose cells are at a high degree of their development. In these organs, mechanical damage is restored by the fusion of living areas with connective tissue. That is, the full restoration of cells that were in this place before, if they were highly differentiated, will never happen.
As examples, it is appropriate to cite the formation of scars in muscle damage, including in the heart. Also, as a result of damage to the brain or nerves, neurons are not restored. After damage to highly differentiated tissue, the body is forced to put up with the loss of its functions. And only the use of stem cells that have not yet passed the stage of transformation under the influence of local cytokines and conditions of stay leaves hope for true regeneration. But for now, this is the technology of the future.
Body growth
The differentiation of cells in the body occurs in stages, depending on the mediators and the signals that they receive from the regulator. Without an external factor, transformation is impossible in the direction in which it is required for development. And when it is received, the process is directed strictly typified in nature, where at each stage there is a system for monitoring and screening failed cytological populations.
Therefore, the growth process from the embryo to the formed organism is a differentiation of cells programmed in a strict sequence. This order should be strictly observed, and, until one important stage has occurred, another stage of separation and cytological specification should not begin. Otherwise, development and growth will initially occur with an error, which leads to the formation of malformations or anomalies of development.
Multicellular evolution
In an adult, this mechanism underlies the formation of tumor cells. It is difficult to imagine how many stages should change each other in the strictest order for the correct differentiation of cells and tissues. This is an incredible mechanism by which a multicellular organism functions. It is a clear demonstration of the thesis that ontogenesis is a brief repetition of phylogenesis. This means that the differentiation of cells occurs in the sequence in which evolution moved.
Hematopoietic differentiation
Differentiation of blood cells is a clear example of the staging of this process in a highly developed organism. In humans, it starts from a common predecessor called a hematopoietic stem cell. It is pluripotent, that is, any blood cell can form from it under the influence of various types of cytokines. More importantly, it is also a product of long-term development and preparation for becoming the precursor to hematopoiesis. She went through the stage of stem cell differentiation, preparing for only one goal - to become the beginning of hematopoiesis sprouts. Other tissue from it will not work out, which distinguishes it from undifferentiated stem cells.
Initial hematopoiesis
At the first stage, two populations develop from a stem cell under the influence of two fundamentally different factors. Under the influence of thrombopoietin and colony stimulating factor (CSF), a large cell group of myelopoiesis precursors is formed. From this group, all monocytes, granular leukocytes, platelets and red blood cells will develop. Just the formation of a primitive progenitor cell is the starting stage of the separation of hematopoiesis into two streams. The first stream is myelopoiesis, and the second is leukopoiesis.
During it, from the same pluripotent precursor cell, but already under the influence of interleukin, a cell population of leukopoiesis is formed. From it, T and B lymphocytes with natural killer cells will develop. The division into two streams is an example of the initial differentiation of cells. This means that before the formation of functioning blood cells several stages will pass, at each of which the phenotype and receptor set will change. Many will change the location where the separation and cytological specification will occur under the influence of cytokines and antigens with antibodies.
Myelopoiesis
The main cell capable of dividing, which gives rise to all myelocytes, is the myeloid germ. Its development proceeds in two streams: the first is the formation of a common precursor with platelets and red blood cells, and the second is the formation of a proto-leukocyte, from which monocyte and granulocyte will originate. The first stream of cell differentiation is the process of their development under the influence of a colony-stimulating factor, thrombopoietin and type 3 interleukin.
The precursors of leukocytes and monocytes are formed under the influence of hematopoietic colony stimulating factor. From a common precursor of platelets and red blood cells under the action of thrombopoietin and erythropoietin, respectively, intermediate cell forms develop. Of these, through the so-called maturation and additional development, adult red blood cell and platelet cells will be formed.
It is noteworthy that platelets are, rather, fragments of the cell preceding them, since at the stage of differentiation they lost unnecessary organelles and the nucleus. In red blood cells, the nucleus was also eliminated, and the cytoplasm was filled with hemoglobin. White blood cells as cells developing on the second stream of myelopoiesis have a nucleus, although their degree of differentiation is also very high.
Leukopoiesis
Lymphocytic cell differentiation is the process of the formation of lymphocytes and natural killer cells from a common precursor of lymphopoiesis. It is carried out mainly under the influence of interleukins and is initially also divided into two streams - B-lymphopoiesis and T-lymphopoiesis. This stage of controlled development gives two populations of unipotent cells, intended only to become an intermediate form for the formation of a single lymphocyte germ.
The precursor of T-killers and T-lymphocytes is formed from the T-sprout zone, and the influence of interleukin-4 forms a B-lymphocytic sprout zone from the B-cell precursor. T-killers are formed under the influence of interleukin-15, an expression factor of the corresponding receptors - differentiation clusters (CD). On their basis, the entire population of lymphocytes will be divided into groups depending on the type of their CD antigen. Accordingly, immunocytes will perform various functions.