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Centrosome and Organelles of Movement

Centrosoma

For medical students2 min readUpdated 2026-10-10

The centrosome is a vital non-membranous organelle characteristic of animal and human cells, but absent in higher plants. It serves as the primary microtubule-organizing center (MTOC) of the cytoskeleton, and its structural elements form the basis of specialized organelles of movement—cilia and flagella.

Centriole formula(9 × 3) + 0 (nine peripheral triplets, empty center)
Axoneme formula(9 × 2) + 2 (nine microtubule doublets and one central pair)
DuplicationOccurs during preparation for cell division via tubulin polymerization
Mechanism of beatingSliding of axonemal doublets driven by dynein arms

Ultrastructure of Centrioles and Centrosome Organization

The structural foundation of the centrosome is the centriole, which is a hollow cylinder. The wall of this cylinder is formed by nine triplets of microtubules arranged circumferentially and interconnected by protein links. Because the central part of the cylinder lacks microtubules (it is empty), the structural formula of a centriole is written as $(9 \times 3) + 0$.

A complete centrosome consists of two basic components:

Specialized structures known as satellites are localized within the centrosphere region. Some of these are closely apposed to the centrioles, while others lie freely in the cytoplasm. The primary function of satellites is that numerous microtubules of the cellular cytoskeleton radiate from them (rather than directly from the centriolar surface).

Reproduction (Duplication) of Centrioles

Duplication of centrosome components occurs strictly during the preparation of the cell for division. During this process, the pre-existing (mother) centriole acts as a structural template.

New organelles are formed through the polymerization of the tubulin protein and are initiated at a strict right angle to the mother centriole. The result of duplication is the appearance of two complete diplosomes (two pairs) within the cell. Notably, each diplosome always contains one parental centriole and one newly synthesized (daughter) centriole.

Axoneme: The Foundation of Flagella and Cilia

Organelles of movement, such as the cilia of the respiratory tract epithelium and the flagella of spermatozoa, are built upon an axoneme ("axial filament"). The axoneme is a derivative of cytoskeletal microtubules, but its ultrastructure differs from that of a centriole and is described by the formula $(9 \times 2) + 2$:

Axonemal elements are rigidly interconnected. Dynein arms (composed of the protein dynein) extend from each peripheral doublet toward the neighboring one. Additionally, radial spokes extend from the peripheral doublets to the central sheath. Externally, the axoneme is covered by the plasma membrane (plasmalemma), and the space between microtubules is filled with hyaloplasm. The overall structure of a flagellum is more complex than that of a cilium due to additional supportive structures surrounding the axial filament.

The mechanism of movement of these organelles is based on the cyclic attachment and detachment of dynein arms. This process causes adjacent microtubule doublets to slide past one another, ultimately resulting in the bending of the entire structure and the beating of the cilium or flagellum.

Basal Body

To securely anchor the axoneme to the cell cytoplasm, a basal body (kinetosome) is located in the superficial cytoplasm at the base of each cilium or flagellum.

In its ultrastructure, the basal body is completely identical to a centriole: it consists of 9 peripheral microtubule triplets (formula $(9 \times 3) + 0$) and also contains satellite particles. The transition from the basal body to the axoneme occurs in a specific manner: only two of the three microtubules from each basal body triplet extend upward to form the peripheral doublets of the axial filament.

Mnemonic

To avoid confusing the formulas, remember: centrioles and basal bodies act as a massive "foundation" (composed of triplets, 9×3), but they are empty in the center (+0). The axoneme is a mobile "projection", making it thinner (composed of doublets, 9×2), but it requires a central axis for bending biomechanics (+2).

Frequently asked questions

At what specific stage of the cell cycle does centriole duplication occur?

Centriole duplication occurs during the $S$-phase (synthetic phase) of interphase in the cell cycle. The process is localized near the nucleus in preparation for cell division, occurring simultaneously with DNA replication.

Key features of the duplication mechanism:

  • The existing centriole acts as a template.
  • The new daughter centriole forms perpendicularly to the mother centriole.
  • The process proceeds via the polymerization of tubulin protein.

As a result, two diplosomes are formed, in each of which one centriole is parental and the other is daughter.

What additional structures surround the axoneme in a sperm flagellum?

In different regions of the sperm flagellum, specific structures surround the axoneme.

  • Middle piece (Pars intermedia) — surrounded by 9 outer dense fibers and a mitochondrial sheath; enclosed by the plasma membrane.
  • Principal piece (Pars principalis) — 9 outer dense fibers remain around the axoneme, but a fibrous sheath replaces the mitochondria; enclosed by the plasma membrane.
  • End piece (Pars terminalis) — additional sheaths are absent; axonemal doublets break down into individual microtubules, with only the plasma membrane remaining externally.
In which phase of mitosis do the diplosomes migrate to opposite poles of the dividing cell?

The separation of centrioles to opposite poles of the dividing cell occurs during prophase of mitosis. This is the longest phase of cell division.

Events accompanying this migration:

  • The mitotic spindle composed of microtubules forms between the separating centrosomes.
  • Chromosome condensation and coiling take place.
  • Nuclear envelope breakdown (fragmentation) and nucleolar disappearance are observed.
Where do the microtubules of the cytoskeleton begin their growth in the centrosome?

They form and radiate outward from satellites located within the centrosphere, rather than directly from the microtubules of the centrioles.

How do the structures of the basal body and the axoneme relate to one another?

The basal body consists of 9 triplets. During axoneme formation, only two of the three microtubules from each triplet continue to grow, forming the 9 doublets of the axial filament.

What drives the direct movement (bending) of cilia and flagella?

Movement is driven by the cyclic attachment and detachment of dynein arms between adjacent axonemal doublets, resulting in their mutual sliding.

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