Centriole Structure and Biological Functions
Structure of the Centriole
In general, all cells have two centrioles located near the nucleus or a notch on the concave side of the Golgi apparatus. The centriole is a cylinder measuring 0.15-0.25 µm wide and 0.3-0.7 µm in length, comprising:
- A peripheral part: 0.04-0.05 µm in thickness, which contains nine groups of three or triplets of microtubules (MT).
- A central part: Usually clear, though it may sometimes contain one or two granules.
- A proximal end: The end closest to the center of the cell.
- A distal end: Directed towards the periphery, which encloses a complex system known as a cartwheel, present only during immature centriole elongation.
Microtubule Triplets and Arrangement
The centriolar wall consists of nine triplets. Each triplet is formed by the close association of three microtubules. The virtual axis passing through the center of each of the tubules is at an angle of 40 degrees with the radius of the circle described by the base of the cylinder.
The letters A, B, or C designate each of the three microtubules from the inside out. Microtubules A and B, as well as B and C, have a common wall. Each has a diameter of approximately 250 Å.
The wall of each triplet microtubule consists of protofilaments. The meeting of 13 protofilaments in a cylinder around a clear central cavity leads to tubule A. Tubules A and B have 3 to 4 protofilaments in common, as do B and C. The cross-section of microtubule A is circular, while microtubules B and C have a semicircular shape.
The triplets are linked through dense connexin junctions between microtubules A and C, which are responsible for the inclination of the tubules in relation to the radius.
The Cartwheel Device and Elongation
A cylinder of opaque material is at the center of the distal immature centriole, and thick lines converge towards the radially arranged cylinder. The lines are inserted on the "feet" as dark areas attached to the inner surface of the A tubules; the spokes and the feet are arranged in a plane perpendicular to the axis of the centriole and draw a cartwheel device. The spokes are distributed in a spiral pattern. When the centriole elongation has reached a length of 300 nm, polymerization is stopped and the cartwheel device is destroyed.
Associated Structures and the Diplosome
A centriole is always associated with another, forming a diplosome. Pericentriolar satellites include:
- Masses (macules): Groups of nine located in the vicinity of each end, which are spheres with a diameter of 400-700 Å near the centriole and attached to the microtubules by a short stem.
- Striated Appendices: Four thick arms that extend radially from the wall of each centriole. The microtubules appear to be associated with striated satellites but never come into contact with the centriole.
Chemical Composition of Centrioles
Centrioles are very difficult to isolate, so researchers have studied the composition of basal bodies (corpuscles). They consist of 50% protein (tubulin), 2% RNA, and 2% glucose. The presence of DNA is debated; if DNA is present, centrioles are duplicated and included within semiautonomous organelles.
Biological Functions of Centrioles
The centrioles are involved in the formation of new centrioles and basal bodies. They are considered interconvertible structures.