Centrioles in Eukaryotic Cells

Centrioles

• Eukaryotic cells contain cylindrical, rod-shaped microtubular structures called centrioles.
• Centrioles lack a limiting membrane, DNA, and RNA.
• They form spindle microtubules.
• Centrioles sometimes arrange beneath the plasma membrane, forming basal bodies for cilia or flagella.

Occurrence

• Present in most algal cells, moss cells, fern cells, and most animal cells.
• Absent in red algae, prokaryotes, yeast, cone-bearing and flowering plants (conifers and angiosperms), and amoeba.

Structure

• Cylindrical structures.
• Diameter is approximately 0.15 to 0.25 μm.
• Length is approximately 0.3 to 0.7 µm; length in some cases varies from 0.16 to 8 µm.
• All centrioles possess a consistent ultrastructure.

Ultrastructure

• Cylinder wall: Composed of nine triplets of microtubules arranged around an imaginary cylinder. The space between and around triplets is filled with an amorphous, electron-dense material. The tubules twist, forming a spiral/helical structure. The triplets themselves are arranged like blades of a pinwheel, angled inward toward a central axis.
• Triplets: Three subunit microtubules (A, B, C). A is complete, while B and C are incomplete. They share walls with each other (200 – 260 Å= 20-26nm). Triplet microtubules are generally similar to other microtubules. They nearly run parallel to each other along the cylinder, but that's not always the case. The microtubules are in close proximity at the proximal end (end closer to the center).
• Spokes, Cartwheel, Linkers, Ciliary rootlets: Other components found within the centriole structure.
• Basal feet and satellites: Dense processes that are perpendicular to the basal body. In some cells, structures arise from the basal end of the centriole. Satellites are dense structures near the centriole.

Linkers

• 'A' tubule is linked to the 'C' tubule of the next triplet by protein linkers, running the entire length.
• Linkers hold the cylindrical array of microtubules, maintaining radial tilt.

Cartwheel

• No central microtubules or special arms are present.
• Protein spokes radiate from a central core to each triplet, forming a cartwheel pattern.
• Cartwheel configuration provides structural and functional polarity to the centriole (proximal end vs. distal end).
• Growth occurs from the distal end (away from the center).

Ciliary rootlets

• From basal ends of basal bodies
• Two types: Tubular root fibrils and striated rootlets

Basal Feet and Satellites

• Basal feet are dense processes perpendicular to the basal body.
• They create structural asymmetry related to ciliary beat direction.
• Satellites are electron-dense structures near the centriole, possibly nucleating sites for microtubules.

Functions of Centrioles

• Formation of basal bodies and cilia.
• Centrosome: Pair of centrioles act as a focal point, organizing cytoplasmic microtubules during interphase and duplicating at mitosis to create the mitotic spindle poles.
• Basal bodies can form centrioles prior to cell division.
• Involved in ciliary or flagellar beat.

Cilia and Flagella

• Flagella: Present in Class Flagellata, sponges, gametes of plants and algae, and gametes of animals.
• Cilia: Present in Class Ciliata, larvae of certain Platyhelminthes, Echinodermata, Mollusca, and Annelida, and respiratory tracts of animals.

Functions of Cilia and Flagella

• Locomotion in cells or organisms.
• Create food currents in lower aquatic animals.
• In respiratory tracts, ciliary movement helps eliminate solid particles.
• Eggs of amphibians and mammals are expelled from the oviduct due to ciliary movement.