Centrioles: Structure, Composition, and Biological Function

Centrioles: Anatomy and Cellular Role

The centriole is a small organelle situated in the middle of an ill-defined area known as the centrosphere, existing in all cells capable of dividing. They are identical to the basal corpuscles of cilia and flagella.

Structure

Most cells contain two centrioles located near the nucleus or within a notch of the Golgi apparatus. This perpendicular pair is called a Diplosoma. Each centriole has a cylindrical shape, distinguished by a proximal end (near the center of the cell) and a distal end, which features a cartwheel complex.

• Wall Composition: Formed by nine triplets of microtubules.
• Microtubule Arrangement: The three tubules (A, B, and C) are closely associated, with the virtual axis passing through the center at a 40-degree angle.
• Dimensions: Each tubule has a diameter of approximately 250 Å.
• Protofilaments: Tubule A contains 13 protofilaments, while B and C share 3 or 4 protofilaments with the adjacent tubule.
• Linkages: Microtubules C and A of neighboring triplets are united by a dense link, responsible for the inclination of the tubules relative to the radius.

The distal end of the centriole is occupied by a cylinder of opaque material, with spokes and feet arranged in a plane perpendicular to the centriole axis.

Associated Structures

Centrioles are typically associated with another centriole to form the Diplosoma.

• Pericentriolar Satellites (Mazas): Nine spheres with a diameter of 400 to 700 Å, adjacent to the centriole and attached to microtubules.
• Splined Appendices: Four thick arms that extend radially from the wall of each centriole.

Chemical Composition

Isolating centrioles for study is difficult, but their composition includes:

• Proteins: 50% tubulin.
• RNA: 0.2%.
• Glucose: 0.2%.
• DNA: Presence is disputed, though it is hypothesized that DNA under the centrioles facilitates duplication, classifying them as semiautonomous organelles.

Function

Centrioles play a vital role in the formation of new centrioles and basal corpuscles. These structures are interconvertible; for example, in the unicellular green alga Chlamydomonas, the basal corpuscles of its two flagella act as organizers of the mitotic spindle during cell division.

Biogenesis

Centrioles differentiate through contact with pre-existing centrioles or via a deuterosoma, formed by the fusion of dense fibrous granules derived from nuclear sources.