In some eukaryotic cells, near the nucleus, there is the presence of two cylindrical structures. They are rod-shaped and microtubular. They are known as the centrioles.
A limiting membrane is absent in it.
DNA or RNA is also absent in it.
It also forms the spindle of microtubules.
It is also found to be arranged beneath the plasma membrane for the formation of cilia or flagella.
The basal body is the centriole having the flagella or the cilia.
The basal body is also called:
The centriole is present in:
Algal cells (except red algae)
The centriole is absent in:
In it, different structures help to form spindle microtubules.
In the early days, cell biologists use the light microscope for the study of the centrioles and centrosomes.
Van Beneden observed the centrioles as the two dots inside the centrosome.
Later electron microscopy was used and studied by different scientists like Harven and Bernherd, Fawcett and Porter.
Structure of Centrioles and Basal Bodies
The shape of the centriole is cylindrical which is also the same in the basal bodies.
Its diameter is 0.15–0.25μm.
Its length is 0.3–0.7μm.
It is short as 0.16μm.
It is long as 8μm.
It is visible under the light microscope.
For the detailed study, an electron microscope should be used.
When each centriole duplicates, the replication of the centrosome occurs.
It occurs during the process of mitosis.
Members of centrioles, i.e first centriole is perpendicular to the next centriole.
A cell consists of the centrosome where a pair of centrioles is present.
Two centrosomes are formed from the four centrioles.
The number of microtubules may vary from one hundred to thousands.
When the cell is functioning normally, attachment of the motor proteins occurs in microtubules.
Similarly, it also carries the thing which needs to be exported.
The scaffold of the microtubule is present in the core of centrioles.
It is formed from the nine triplet microtubules which are arranged in a radial array in a cylinder.
The centriole is based on protein.
The basal body performs the different functions of the cell. It organizes the cilium or flagellum.
In the different types of the cell, the centriole is converted into the basal body.
The centriole is the polar structure.
From the proximal site of the centriole, there is the formation of the new centriole.
It uses the cartwheel. At the central part, there is the presence of the hub and spokes resembling the structure of the cartwheel.
Centriole seems like the turbine because of the presence of the triplets.
When observation is done from the proximal end, an anticlockwise twist is seen in the triplet microtubule blade.
Doublet microtubule is present in the Drosophila.
Singlet microtubule is present in the Caenorhabditis elegans.
Ultrastructural components of Centrioles and Basal Bodies
A. Cylinder Wall
It consists of nine sets of microtubules. In each group, triplets are present. i.e 3 microtubules.
They are equally spaced
There is the presence of amorphous and electron-dense material in the space
Absence of the outer membrane in the centriole.
Triplets are responsible for the formation of the cylinder’s wall.
Presence of A, B, and C subunit microtubules.
The shape of the A tubule is round while B and C are concave.
A triplet microtubule consists of the A-tubule which is a complete microtubule.
Other tubules are incomplete.
Protofilaments in the A-tubule consist of 13 α- and β-tubulin.
The partial microtubules, B-tubules, and C-tubules are assembled in it. i.e 10 protofilaments
By the sharing of 3, protofilament B and C tubules make 13 protofilament microtubules.
Structurally A, B, C tubules are similar to other microtubules.
Triplets are assumed to be aligned parallel to one another and in the long axis of the cylinder.
But it does not happen in all of the cases.
There is a reduction in the diameter of the cylinder, in the basal bodies of some organisms.
Because at the proximal ends, the triplets get closer in such cases.
In some of the centrioles, though the triplets are aligned parallel to one another, to the cylinder axis, they may be turned in a long pitched helix.
Due to the presence of the three concentric rings of the microtubules, triplets are strong.
In the triplet, a microtubule is present which is made up of tubulin.
Tubulins are small monomers that can join together forming the tubes. It resembles the straws.
γ-tubulin is required for the formation of the microtubules which may be singlet or triplet.
In a study, it was found that in the organism where triplet microtubule is absent, the gene encoding δ and ε-tubulin are also absent. For example in C. elegans and Drosophila.
δ-tubulin is present abundantly in the testes of mammals.
In the somatic cell, it is present in the cytoplasm and also in the nucleus. During the process of mitosis, it is present in the spindle poles.
The assembly of the basal body and its maintenance is aided by the ε-tubulin.
With the help of the protein linkers, tubule A gets linked with tubule C.
It occurs at intervals.
The typical radial tilt of the triplets is maintained by these linkers.
In the centrioles, there is an absence of the central microtubules.
Absence of the special arms.
Formation of the cartwheel-like pattern.
The proximal end is determined by the cartwheel configuration.
From the distal end, growth of the centrioles takes place.
Cilium is formed in the basal bodies from this end.
At the proximal end, the procentrioles are located. They form the right angles to the centriole.
E. Ciliary Rootlets
Ciliary rootlets originate from the basal end in the basal bodies.
Ciliary rootlets are of two types:
Tubular root fibrils
Its diameter is 200A°
Presence of regular crossbanding.
The repeating period is usually 55 to 70 nm.
Striations can be found in most of the ciliary rootlets.
The composition of the striated fibers of the rootlets is parallel microfilaments.
Its diameter: 3 to 7 nm and is formed of the globular subunits.
F. Basal Feet and Satellites
Basal feet are the dense processes.
Basal feet are arranged perpendicularly to the basal body.
The structural asymmetry on the basal body is related to the direction of the ciliary beat.
Microfilaments make the basal foot. It terminates in a dense bar.
The microtubules get converged in it.
Satellites or pericentriolar bodies lie near the centriole. They are the electron-dense structures that might be the nucleating sites for microtubules.
Functions of Centrioles and Basal Bodies
The main function of the centrioles is the formation of basal bodies and the cilia in the cell.
It acts as the focal point for the centrosome in most animal cells.
The centrosome is also called the cell center. During the interphase, it organizes the cytoplasmic microtubules. Then the duplication occurs at the mitosis. It nucleates the two poles of the mitotic spindle.
In the case of a spermatozoon, the flagellum or the tail fiber is formed by one centriole.
Centrioles and the basal bodies are responsible for the ciliary and the flagellar beat.
They receive different signals. i.e optical, acoustic, olfactory.
Centrioles could locate the directions of the signal source.
Microtubules in a cell help in the transportation of the substances to various locations of the cell.
The special glycoproteins like the sugar and protein tag the products. Then to the specific motor proteins, it acts like the signals. (Scheer, 2014)
Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S.Chand and Company Ltd.
Scheer, U. (2014). Historical roots of centrosome research: Discovery of Boveri’s microscope slides in Würzburg. Philosophical Transactions of the Royal Society B: Biological Sciences, 369(1650). https://doi.org/10.1098/rstb.2013.0469.