Animal cells consist of various structural components. The cytoskeleton of animal cells contains microtubules, typically nucleated by the centrosome. The centrosome is a complex structure that consists of a pair of centrioles surrounded by amorphous, electron-dense pericentriolar material (PCM).
Centrioles are cylindrical barrel-shaped structures. These are 0.2 μm in diameter and 0.4 μm in length. The centriole lacks membrane, DNA, and RNA. These form a spindle of microtubules, a mitotic apparatus during mitosis or meiosis. These arrange just beneath the plasma membrane and possess and produce flagella or cilia in flagellated or ciliated cells. The centriole-bearing flagella or cilia are called basal bodies.
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Occurrence of Centrioles
The centrioles are present in most algal cells (except red algae), moss cells, some fern cells, and almost all animal cells. The prokaryotes, yeast, cone-bearing, flowering plants, and non-flagellated and non-ciliated protozoans (like amoebae) lack centrioles. Some amoebae have flagellated and the amoeboid stage. The centrioles develop during the flagellated stage but disappear during the amoeboid phase.
Structure of Centrioles
Centrioles are cylindrical structures with 0.15-0.25 µm diameter and 0.3-0.7 µm in length, but some can be as short as 0.16 µm or longer than 8 µm. The structure of centrioles consists of five components; cylindrical walls, triplets, linkers, cartwheels, and satellites or pericentriolar bodies.
Cylinder wall
The array of nine triplet microtubules is spaced equally around an imaginary cylinder’s perimeter. The space between and around the triplet fills with amorphous, electron-dense material. The transverse section of the centriole shows the arrangement of the triplets are like blades or vanes of a pinwheel or turbine. Each triplet tilts inside the central axis at a 45° angle to the circumference. The tubules in each triplet twist from one end to the other in a helical course. Since the centriole has no outer membrane, the tubules act as the wall of the cylinder. It arbitrarily defines the inside and outside of the centriole.
Triplets
The three subunit microtubules are A, B, and C, where A is the innermost. The diameter of individual tubules is 200-260 A°. A tubule is round, whereas others are C-shaped or incomplete circles and share their wall with the preceding tubule. The structure of tubules A, B, and C are similar to other forms of microtubules. The A tubule has thirteen 40-45 A° globular subunits around its perimeter. The A tubules share their 3-4 subunits with B tubules that, in turn, share several of their subunits with the C tubules. The triplets often run parallel to one another and the long axis of the cylinder but turn in a long-pitched helix concerning the cylinder axis.
Linkers
The tubule A of each triplet links with the tubule C of the neighboring triplet by protein linkers at intervals along their entire length. The linkers keep the cylindrical array of the microtubules in place and maintain the typical radial tilt of the triplets.
Cartwheel and Satellites or pericentriolar bodies
The centrioles have no unique arms as they lack central microtubules. Sometimes protein spikes radiate out from the central core of each triplet, forming a pattern like a cartwheel. The cartwheel configuration determines the centriole’s proximal end, providing a structural and functional polarity. The growth of centrioles occurs from the distal end, where the formation of procentrioles occurs at an angle of 90°.
These are electron-dense structures that lie near the centriole. These are probable nucleating sites for the microtubules.
Composition of Centrioles
The microtubules of centrioles are composed of protein, tubulin, and lipid molecules. The centrioles have a high concentration of ATPase enzyme, but the presence of nucleic acids (DNA and RNA) is controversial.
Functions of Centrioles
The centrioles have the following functions:
- The centrioles are responsible for forming basal bodies, which ultimately include cilia.
- The centrioles in most animal cells act as focal points for the centrosome. The centrosome helps in organizing the array of cytoplasmic microtubules during interphase and duplicates at mitosis for nucleating the two poles of the mitotic spindle.
- Sometimes centrioles can perform two functions consecutively before each division in Chlamydomonas, the two flagella absorb, and the basal bodies leave their position to act as mitotic poles.
- One centriole give rise to the tail fibre or flagellum in spermatozoon.
- Centrioles are found to be involved in ciliary and flagellar beats.
- Centrioles have a role in the reception of optical, acoustic, and olfactory signals.
- It can also serve as a device for locating the directions of signal sources.
References
- Centriole. Genome.gov. (n.d.). Retrieved November 3, 2022, from https://www.genome.gov/genetics-glossary/Centriole
- Verma, P. S., & Agarwal, V. K. (2019). Centrioles and Basal Bodies. In Cell Biology, genetics, Molecular Biology, evolution and ecology (13th ed., pp. 304–307). essay, S. Chand and Company Limited.
