In the structure of a eukaryotic cell, a special group of organelles is distinguished that perform motor and support functions. Such components are referred to as a protein cytoskeleton formed on the basis of filaments, fibrils and microtubules. The latter form the main frame organelle - the cell center (centrosome), which is based on 2 cylinders called centrioles.
The term was first proposed in 1895 by Boveri. However, at that time, the understanding of what centrioles were was very different from the modern idea. Boveri called so barely noticeable small bodies that were on the border of visibility of a light microscope. Now, not only the structure, but also the functions of centrioles have been studied in detail.
What are centrioles?
As noted above, these organelles are integral 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.
The structure of the centriole isprotein cylinders, from which a network of threads extends - the centrosphere. Both components together are called the centrosome. Electron microscopy allows a detailed examination of the ultrastructure of centrioles.
Cylinders together with the centrosphere form a single microtubule organization center (MCTC). 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, there are usually 2 centrioles, which are located next to each other, forming a diplosome. During division, the cylinders diverge towards the poles of the cytoplasm and form a spindle.
Both the centrioles and the centrosphere are made up of microtubules built from the polymerized protein tubulin.
Building 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 the eukaryotic flagellum. However, in this case, the protein casts do not have motor functions and therefore consist only of tubulin fibrils.
The walls of centrioles are formed from nine triplets of microtubules held together by connecting strands. The cylinders are hollow inside. 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.
There are 2 centrioles in the diplosome:mother and daughter. In the interphase cell, they adjoin each other at a right angle. 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:
- formation of the axoneme (central cylinder) of locomotor structures (flagella and cilia);
- formation of the fission spindle;
- induction of tubulin polymerization.
In all three cases, the centriole plays the role of the center for the formation of microtubules, from which the cytoskeletal matrix is built, the axial cylinder of the flagella, as well as the spindle, along which the daughter chromosomes diverge during mitosis, and chromatids during meiosis.
