In the structure of a eukaryotic cell , a special group of organelles is identified that perform motor and supporting functions. Such components belong to the protein cytoskeleton formed on the basis of filaments, fibrils and microtubules. The latter form the main frame organelle - the cell center (centrosome), the basis of which are 2 cylinders, called centrioles.
The term was first proposed back in 1895 by Bowery. However, at that time, understanding what centrioli was was very different from modern notions. Boveri called so subtle little bodies that were on the line of sight of a light microscope. Now, not only the structure, but also the functions of centrioles have been studied in detail.
What is centrioli?
As noted above, these organelles are constituent components of the centrosome. During interphase, it performs a supporting structural function, and during mitosis or meiosis, it participates in the formation of the division spindle.
By structure, centrioles are protein cylinders, from which the network of filaments - the centrosphere - departs. Both components are collectively called the centrosome. Electron microscopy allows a detailed examination of the ultrastructure of centrioles.
The cylinders, together with the centrosphere, form a single center for microtubule organization (CTOM). Therefore, for a better understanding of what centrioles are, it is necessary to consider them not as separate structures, but as a functional part of the centrosome.
In an interphase cell, usually 2 centrioles are present, which are located next to each other, forming a diplosome. During division, the cylinders diverge to the poles of the cytoplasm and form a spindle.
Both centrioles and the centrosphere consist of microtubules built from the polymerized tubulin protein.
Structural features
If we consider what centrioles are from the point of view of ultrastructure, it turns out that the principle of organization of this organelle is very similar to the skeletal framework of a eukaryotic flagellum. However, in this case, the protein cylinders do not have motor functions and therefore consist only of tubulin fibrils.
The walls of the centrioles are formed of nine triplets of microtubules fastened by connecting cords. Inside the cylinders are hollow. This design is indicated by the formula (9 × 3) + 0. The width of each centriole is about 0.2 μm, and the length varies from 0.3 to 0.5 μm.
In the diplosome, 2 centrioles are distinguished: maternal and daughter. In an interphase cell, they adjoin each other at right angles. During mitotic division, protein cylinders diverge towards the poles, where they form their own daughter centrioles. This process is called duplication.
Centrioles are present in all animal cells and in some lower plant cells.
Functions
Centrioles have 3 main functions:
- the formation of the axoneme (central cylinder) of locomotor structures (flagella and cilia);
- spindle formation;
- induction of tubulin polymerization.
In all three cases, the centriole plays the role of the center of microtubule formation, from which the cytoskeletal matrix, the axial cylinder of the flagella, and the spindle along which the daughter chromosomes diverge and during meiosis, chromatids are built.