Ultrastructure of centrioles and organization of the cell center
The basis of the cell center is the centriole, which is a hollow cylinder. The wall of this cylinder is formed by nine microtubule triplets arranged circumferentially and connected by protein bridges. Since the central part of the cylinder contains no microtubules (it is empty), the structural formula of the centriole is written as $(9 \times 3) + 0$.
A complete cell center includes two basic components:
- Diplosome — a pair of centrioles. A non-dividing cell contains only one diplosome, with its centrioles always positioned at right angles to each other.
- Centrosphere — a zone of light cytoplasm immediately surrounding the diplosome.
Special structures called satellites are localized in the centrosphere region. Some of them tightly adhere to the centrioles, while others lie freely in the cytoplasm. The main function of satellites is that numerous microtubules of the cellular cytoskeleton radiate precisely from them (rather than from the centriole surface itself).
Reproduction (duplication) of centrioles
Doubling of the cell center components occurs strictly during the period of cellular preparation for division. During this process, the pre-existing (mother) centriole acts as a structural template.
The new organelle is formed by the polymerization of the protein tubulin and is initiated strictly perpendicular to the mother centriole. The result of duplication is the appearance of two fully formed diplosomes (two pairs) in the cell. It is important to note that each such diplosome always contains one parental centriole and one newly synthesized (daughter) centriole.
Axoneme: basis of flagella and cilia
Motility organelles, such as the cilia of the respiratory ciliated epithelium and the flagella of spermatozoa, are based on the axoneme ("axial filament"). The axoneme is a derivative of cytoskeletal microtubules, but its ultrastructure differs from the centriole and is described by the formula $(9 \times 2) + 2$.
- Periphery: 9 doublets (pairs) of microtubules.
- Center: 1 pair of microtubules enclosed in a special central sheath.
Axoneme elements are rigidly interconnected. Dynein arms (composed of the protein dynein) extend from each peripheral doublet to the adjacent one. In addition, radial spokes extend from the peripheral doublets to the central sheath. Externally, the axoneme is covered by the plasmalemma, and all the space between the microtubules is filled with hyaloplasm. The structure of a flagellum as a whole is more complex than that of a cilium due to the presence of additional structures around the axial filament.
The movement mechanism of these organelles is based on the cyclic attachment and detachment of dynein arms. This process causes the displacement (sliding) of adjacent microtubule doublets relative to each other, which ultimately leads to the bending of the entire structure and the beating of the cilium or flagellum.
Basal body
For secure attachment of the axoneme to the cell cytoplasm, a basal body is used, localized in the superficial layers of the cytoplasm (at the base of the cilium or flagellum).
In its ultrastructure, the basal body is completely identical to the centriole: it consists of 9 peripheral microtubule triplets (formula $(9 \times 3) + 0$) and also contains satellite particles. The transition of the basal body into the axoneme occurs in a special way: only two microtubules from each triplet of the basal body continue upward, forming the peripheral doublet of the axial filament.
Briefly: answers to topical questions
At what specific stage of the cell cycle does centriole duplication occur?
Centriole duplication occurs during the $S$ period (synthetic phase) of the cell cycle interphase. The process is localized near the nucleus in preparation for cell division, simultaneously with DNA replication.
Features of the duplication mechanism:
- The existing centriole acts as a template.
- The new daughter centriole forms perpendicular to the mother centriole.
- The process proceeds via polymerization of the protein tubulin.
As a result, two diplosomes are formed, in each of which one centriole is parental and the second is daughter.
What additional structures surround the axoneme in the sperm flagellum?
In different regions of the sperm flagellum, the axoneme is surrounded by specific structures.
- Middle piece (Pars intermedia) — surrounded by 9 outer dense fibers and a mitochondrial sheath; the region is covered externally by the plasmalemma.
- Principal piece (Pars principalis) — 9 outer fibers remain around the axoneme, with a fibrous sheath present instead of mitochondria; the region is covered externally by the plasmalemma.
- End piece (Pars terminalis) — additional sheaths are absent; axoneme doublets break down into individual microtubules, leaving only the plasma membrane externally.
In which phase of mitosis do diplosomes migrate to the poles of the dividing cell?
The separation of centrioles to opposite poles of the dividing cell occurs in prophase of mitosis. This is the longest phase of cell division.
Events accompanying the separation:
- A mitotic spindle made of microtubules forms between the separating centrioles.
- Condensation and coiling of chromosomes occur.
- Fragmentation (breakdown) of the nuclear envelope and disappearance of nucleoli are observed.
To avoid confusing the formulas, remember: the centriole and basal body are a massive "foundation" (consisting of triplets, 9×3), but they are empty inside (+0). The axoneme is a mobile "protrusion", it is thinner (consisting of doublets, 9×2), but for bending biomechanics, it needs a central axis of support (+2).
Class and exam questions
Where do the cytoskeletal microtubules begin their growth in the cell center?
They form and radiate from satellites located in the centrosphere, rather than from the centriole microtubules themselves.
How do the structures of the basal body and axoneme relate?
The basal body consists of 9 triplets. During axoneme formation, only two out of the three microtubules of each triplet continue to grow, forming 9 doublets of the axial filament.
What provides the direct movement (bending) of cilia and flagella?
Movement occurs due to the attachment and detachment of dynein arms between adjacent axoneme doublets, which leads to their mutual sliding.
What else to study on this topic
- Molecular mechanism of tubulin polymerization during centriole duplication
- Differences in the organization of additional structures in the sperm flagellum versus a typical cilium
- Role and biochemistry of the protein dynein in dynein arm function
- Participation of the cell center in mitotic spindle formation
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