It is proved that the cells of eukaryotic organisms are represented by a system of membranes that form organelles of protein-phospholipid composition. However, there is an important exception to this rule. Two organelles (cell center and ribosome), as well as movement organelles (flagella and cilia) have a non-membrane structure. How are they educated? In this work, we will try to find the answer to this question, and also study the structure of the cell center of the cell, often called the centrosome.
Do all cells contain a cell center
The first fact that scientists are interested in is the optional presence of this organoid. So, in lower fungi - chytridiomycetes - and in higher plants it is absent. As it turned out, in algae, in human cells and in most animals, the presence of a cell center is necessary for the implementation of the processes of mitosis and meiosis. Somatic cells are divided in the first way, and sex cells are divided in the other way. An obligatory participant in both processes iscentrosome. The divergence of its centrioles to the poles of the dividing cell and the stretching of the fission spindle filaments between them ensures further divergence of the chromosomes attached to these filaments and to the poles of the mother cell.
Microscopic studies revealed structural features of the cell center. It includes from one to several dense bodies - centrioles, from which microtubules fan out. Let's study in more detail the appearance, as well as the structure of the cell center.
Centrosome in interphase cell
In the life cycle of a cell, the cell center can be seen during a period called interphase. Two microcylinders are usually located near the nuclear membrane. Each of them consists of protein tubes, collected in three pieces (triplets). Nine such structures form the surface of the centriole. If there are two of them (which happens most often), then they are located at right angles to each other. During the period of life between two divisions, the structure of the cell center in the cell is almost the same in all eukaryotes.
Ultrastructure of the centrosome
It became possible to study in detail the structure of the cell center as a result of using an electron microscope. Scientists have found that centrosome cylinders have the following dimensions: their length is 0.3-0.5 microns, and their diameter is 0.2 microns. The number of centrioles doubles before division begins. This is necessary so that the mother and daughter cells themselves, as a result of division, receivecell center, consisting of two centrioles. The structural features of the cell center lie in the fact that the centrioles that make it up are not equivalent: one of them, the mature (maternal) one, contains additional elements: the pericentriolar satellite and its appendages. The immature centriole has a specific site called a cartwheel.
Behavior of the centrosome in mitosis
It is well known that the growth of an organism, as well as its reproduction, occurs at the level of the elementary unit of living nature, which is the cell. The structure of the cell, localization and functions of the cell, as well as its organelles, are considered by cytology. Despite the fact that scientists have done a lot of research, the cell center is still insufficiently studied, although its role in cell division has been fully elucidated. In the prophase of mitosis and in the prophase of the reduction division of meiosis, the centrioles diverge towards the poles of the mother cell, and then the fission spindle thread is formed. They are attached to the centromeres of the primary constriction of chromosomes. What is it for?
Spindle of anaphase cell division
The experiments of G. Boveri, A. Neil and other scientists made it possible to establish that the structure of the cell center and its functions are interconnected. The presence of two centrioles located bipolarly with respect to the poles of the cell, and spindle filaments between them, ensures an even distribution of chromosomes connected to microtubules to each of the poles of the mother cell.
Thus, the number of chromosomes will be the same in daughter cells as a result of mitosis, or half as much (in meiosis) as in the original mother cell. Of particular interest is the fact that the structure of the cell center changes and is correlated with the stages of the cell life cycle.
Chemical analysis of the organelle
For a better understanding of the functions and role of the centrosome, let's study what organic compounds are included in its composition. As one would expect, proteins are leading. Suffice it to recall that the structure and functions of the cell membrane also depend on the presence of peptide molecules in it. Note that proteins in the centrosome have a contractile ability. They are part of microtubules and are called tubulins. Studying the external and internal structure of the cell center, we mentioned auxiliary elements: pericentriolar satellites and centriole appendages. They include cenexin and myricitin.
There are also proteins that regulate the metabolism of the organoid. These are kinase and phosphatase - special peptides responsible for the nucleation of microtubules, that is, for the formation of an active seed molecule, from which the growth and synthesis of radial microfilaments begins.
Cell center as an organizer of fibrillar proteins
In cytology, the idea of the centrosome as the main organelle responsible for the formation of microtubules has finally taken hold. Thanks to the generalizing studies of K. Fulton, it can be argued that the cell centerprovides this process in four ways. For example: polymerization of fission spindle filaments, formation of centrioles, creation of a radial system of microtubules in the interphase cell, and, finally, synthesis of elements in the primary cilium. This is a special formation characteristic of the maternal centriole. By studying the structure and functions of the cell membrane, scientists detect it under an electron microscope in the cell center after mitotic cell division or at the time of the onset of mitosis. In the G2 stage of interphase, as well as in the early stages of prophase, the cilium disappears. According to its chemical composition, it consists of tubulin molecules and is a label by which a mature maternal centriole can be identified. So how does centrosome maturation occur? Consider all the nuances of this process.
Stages of centriole formation
Cytologists have established that the daughter and maternal centrioles that form the diplosome are not the same in structure. So, the mature structure is bordered by a layer of pericentriolar substance - a mitotic halo. Full maturation of the daughter centriole takes longer than one cell life cycle. At the end of the G1 stage of the second cell cycle, the new centriole already acts as an organizer of microtubules and is capable of forming fission spindle filaments, as well as the formation of special movement organelles. They can be cilia and flagella, found in unicellular protozoa (for example, green euglena, ciliates-shoes), as well as in many algae, such as chlamydomonas. Flagella formed due to microtubules of the cell center are supplied with manyspores in algae, as well as germ cells of animals and humans.
The role of the centrosome in cell life
So, we have seen that one of the smallest cell organelles (occupies less than 1% of the cell volume) plays a leading role in regulating the metabolism of both plant and animal cells. Violation of the formation of the spindle of division entails the formation of genetically defective daughter cells. Their sets of chromosomes differ from the normal number, which leads to chromosomal aberrations. As a result, the development of abnormal individuals or their death. In medicine, the fact of the relationship between the number of centrioles and the risk of developing cancer has been established. For example, if normal skin cells contain 2 centrioles, then a tissue biopsy in case of skin cancer reveals an increase in their number to 4-6. These results provide evidence for the key role of the centrosome in the control of cell division. Recent experimental data point to the important role of this organelle in the processes of intracellular transport. The unique structure of the cell center allows it to regulate both the shape of the cell and its change. In a normally developing unit, the centrosome is located next to the Golgi apparatus, near the nucleus, and together with them provides integrative and signaling functions in the implementation of mitosis, meiosis, as well as programmed cell death - apoptosis. That is why modern cytologists consider the centrosome an important unifying organelle of the cell, responsible both for its division and for the entireoverall metabolism.
