Centrioles: Structure and Function

Centrioles are tiny, cylindrical organelles found in most eukaryotic cells. They are a core component of the centrosome, a major microtubule-organising centre (MTOC) in animal cells. These organelles play crucial roles in cell division and the formation of cilia and flagella. 

1.0Centriole Ultrastructure 

• Each centriole is a hollow cylinder composed mainly of a protein called tubulin. 
• Its diameter is about 0.25 µm, and its length is between 0.2 and 0.5 µm. 
• The most distinctive feature of a centriole's ultrastructure is its unique arrangement of microtubules.

A centriole consists of nine triplet microtubules arranged in a cartwheel pattern.

• Triplet Microtubules: Each of the nine groups consists of three fused microtubules, labelled A, B, and C. Microtubule A is a complete microtubule with 13 protofilaments. In contrast, microtubules B and C are incomplete, each sharing three protofilaments with the adjacent microtubule.
• Cartwheel Structure: The triplets are connected to a central hub via protein spokes, forming a distinct cartwheel-like structure. This intricate arrangement is essential for maintaining the centriole's cylindrical shape.
• A pair of centrioles, typically arranged perpendicularly to each other, forms a centrosome. 
• This centrosome is located near the nucleus in the cytoplasm. 
• An amorphous protein mass surrounding the centrioles, called the pericentriolar material (PCM),is crucial for microtubule nucleation.

2.0Centriole Functions 

Centrioles are multifunctional organelles primarily known for their roles in cell organisation and movement. Their main functions include:

• Cell Division (Mitosis & Meiosis): During cell division, the centrosome, which contains the centrioles, duplicates. The two new centrosomes move to opposite poles of the cell and form the spindle fibres (microtubules). These spindle fibres attach to the chromosomes and pull them apart, ensuring that each daughter cell receives a complete set of chromosomes.
• Ciliogenesis & Flagellogenesis: Centrioles are the foundational structures, or basal bodies, for the formation of cilia and flagella.

A basal body is a modified centriole that organises the nine microtubule doublets of a cilium or flagellum, which are essential for cellular movement and fluid transport.

• Microtubule Organisation: As a component of the centrosome, centrioles act as microtubule-organising centres (MTOCs). They regulate the number, polarity, and orientation of microtubules within the cell, which is crucial for maintaining cell shape, organelle positioning, and intracellular transport.

While centrioles are critical for these functions in animal cells, it is noteworthy that most plant cells and fungi lack centrioles and organise their microtubules differently.

3.0Centriole vs. Centrosome

While the terms are often used together, they refer to distinct structures:

• Centriole: The core, cylindrical organelle made of triplet microtubules.
• Centrosome: The entire complex, consisting of two centrioles and the surrounding pericentriolar material. The centrosome is the functional MTOC.