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 organelles of movement (flagella and cilia) have a non-membrane structure. What are they educated with? In this paper, 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 interested scientists is the optional presence of this organoid. So, in lower fungi - chitridiomycetes - 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 mitosis and meiosis. Somatic cells divide in the first way, and sex cells in another. An obligatory participant in both processes is the centrosome. The divergence of its centrioles to the poles of the dividing cell and the tension between them of the spindle of the division spindle provides further separation of the chromosomes attached to these threads and to the poles of the mother cell.
Microscopic studies revealed structural features of the cell center. It includes from one to several dense little bodies - centrioles, from which microtubules fan-like diverge. We will study in more detail the appearance, as well as the structure of the cell center.
Centrosome in an interphase cell
In the cell’s life cycle , the cell center can be seen in a period called interphase. Two microcylinders are usually located near the core membrane. Each of them consists of protein tubes collected in three pieces (triplets). Nine of these structures form a centriole surface. If there are two of them (which happens most often), then they are located at right angles to each other. During the life period between two divisions, the structure of the cell center in the cell is almost the same in all eukaryotes.
Centrosome Ultrastructure
A detailed study of the structure of the cell center became possible 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, their diameter is 0.2 microns. The number of centrioles before the start of division necessarily doubles. This is necessary so that the maternal and daughter cells themselves as a result of division receive a cell center consisting of two centrioles. The structural features of the cell center are that the centrioles that make it up are not equivalent: one of them - mature (maternal) - contains additional elements: a pericentric satellite and its appendages. The immature centriole has a specific area called the cart wheel.
The 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 an elementary unit of living nature, which is a cell. The structure of the cell, the localization and function of the cell, as well as its organoids, are examined by cytology. Despite the fact that scientists conducted a lot of research, the cell center is still not well understood, 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, centrioles diverge to the poles of the mother cell, and then the fission spindle filament is formed. They are attached to the centromeres of the primary constriction of chromosomes. Why is this necessary?
Anaphase cell spindle
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 bipolar with respect to the poles of the cell, and the threads of the spindle 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's life cycle.
Chemical analysis of organelles
For a better understanding of the functions and role of the centrosome, we will study what organic compounds are included in its composition. As expected, the leading ones are proteins. It is enough to recall that the structure and functions of the cell membrane also depend on the presence of peptide molecules in it. Note that in the centrosome, proteins have contractility. They are part of microtubules and are called tubulins. Studying the external and internal structure of the cell center, we mentioned auxiliary elements: pericentric satellites and appendages of centrioles. They include cenexin and myricitin.
There are also proteins that regulate the metabolism of the organoid. This is kinase and phosphatase - special peptides responsible for the nucleation of microtubules, that is, for the formation of an active seed molecule, with which begins the growth and synthesis of radial microfilaments.
Cell center as an organizer of fibrillar proteins
In cytology, the concept of the centrosome as the main organelle responsible for the formation of microtubules has finally become entrenched. Thanks to generalizing studies, K. Fulton is able to assert that the cell center provides this process in four ways. For example: polymerization of spindle filaments, the formation of percentrioles, the creation of a radial system of microtubules of an interphase cell and, finally, the synthesis of elements in the primary cilia. This is a special formation characteristic of maternal centrioli. Studying the structure and functions of the cell membrane, scientists find 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 phase of interphase, as well as in the early stages of prophase, the cilium disappears. By its chemical composition, it consists of tubulin molecules and is a label by which it is possible to determine the mature maternal centriole. So how does the centrosome mature? 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 identical in structure. So, the mature structure is bordered by a layer of pericentric substance - the mitotic halo. Full ripening of the daughter centriole takes longer than one cell life cycle lasts. At the end of stage G1 of the second cell cycle, the new centriole already acts as the organizer of microtubules and is capable of forming spindle threads, as well as the formation of special movement organelles. They can be cilia and flagella, found in unicellular protozoa (for example, green euglena, ciliates), as well as in many algae, such as chlamydomonas. The flagella formed by the microtubules of the cell center are equipped with many spores in algae, as well as the reproductive cells of animals and humans.
The role of centrosome in cell activity
So, we were convinced that one of the smallest cellular organelles (occupies less than 1% of the cell volume) plays a leading role in the regulation of metabolism of both plant and animal cells. Violation of the formation of the spindle division leads to 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 is 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 of the key role of the centrosome in controlling cell division. Recent experimental data indicate 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 - apuptosis. That is why modern cytologists consider the centrosome to be an important uniting organoid of the cell, responsible both for its division and for the entire metabolism as a whole.