The organelle centriole is cylindrical in shape and is made of the protein tubulin.
- Every animal cell contains two centrioles. They aid the cell division process.
- They perform their duties during mitosis and meiosis.
- They could be found in some lower plants, such as Chlamydia, but not in the majority of fungi, angiosperms (flowering plants), or pinophyta (conifers).
- When the cell is not dividing, they are typically present near the nucleus but are invisible.
- Near the nucleus, the cytoplasm of certain eukaryotic cells includes two cylindrical, rod-shaped microtubular structures termed cen trioles.
- Centrioles lack limiting membrane and DNA or RNA and form a spindle of microtubules, the mitotic apparatus during mitosis or meiosis. In flagellated or ciliated cells, centrioles are occasionally positioned slightly under the plasma membrane to create flagella or cilia.
- When a centriole contains a flagellum or cilium, it is referred to as the basal body. Basal body is synonymous with kinetosome, blepharoplast, basal granule, basal corpuscle, and proximal centriole.
- The classic studies of Henneguy and Lenhossek (1897) suggest that the basal bodies of cilia and flagella are identical to the centrioles present in the mitotic spindle.
Occurrence of Centrioles
- Centrioles are found in the majority of algal cells (red algae being an exception), moss cells, some fern cells, and mammal cells.
- They are lacking in prokaryotes, red algae, yeast, coniferous and flowering plants (conifers and angiosperms), and certain non-flagellated or non-ciliated protozoa (such as amoebae).
- Some species of amoebae have a flagellated stage and an amoeboid stage; during the flagellated stage, a centriole forms, but it vanishes during the amoeboid stage.
Structure of Centrioles
Centrioles and basal bodies are cylindrical structures with a diameter of 0.15–0.25µm and a length of typically 0.3–0.7µm, however some are as small as 0.16µm and as long as 8m. Both consist of the following ultrastructural elements:
1. Cylinder Wall
- The array of nine triplet microtubules evenly placed around the circumference of an imaginary cylinder is the most prominent and typical ultrastructural feature of centrioles and basal bodies.
- An amorphous, electron-dense substance fills the region between and immediately surrounding the triplet. In transverse section, the triplets are organised like pinwheel or turbine vanes or blades.
- Each triplet or blade is inclined inward to the central axis at an angle of approximately 450 degrees to the circumference; within each blade, the tubules twist or follow a helical path from one end to the other.
- Since centrioles lack an outside membrane, the triplets are thought to constitute the cylinder’s wall and arbitrarily delineate the centriole’s interior and exterior.
- The nine triplets that make up the wall have centrioles and basal bodies that are essentially identical. A is the innermost of the three subunit microtubules called A, B, and C, respectively.
- Individual tubules have a diameter of 200–260Ao. The A tubule is the only complete, round tubule; the rest are partial, C-shaped, and share their wall with the preceding tubule. Typically, the C tubule terminates before the A and B tubules at both ends.
- A, B, and C tubules have a substructure identical to that of other microtubules.
- The A tubule has thirteen 40–45Ao globular subunits encircling its periphery. Three or four of these subunits are shared with the B tubules, which share a number of their subunits with the C tubules.
- It is commonly believed that the triplets run parallel to one another and the cylinder’s long axis, but this is not always the case.
- In the basal bodies of certain species, the triplets grow closer to the proximal end, causing the cylinder’s diameter to decrease.
- In certain centrioles, the triplets are parallel to one another but rotate in a long-pitch helix relative to the cylinder axis.
- The A tubule of each triplet is intermittently connected to the C tubule of the adjacent triplet by protein linkers along their entire length.
- These linkers support the cylindrical array of microtubules and preserve the triplets’ usual radial tilt.
- There are no central microtubules or unique arms in the centrioles. However, weak protein spokes extend out from a central core to each triplet, generating a cartwheel-like pattern.
- This configuration determines the proximal end of a centriole and confers structural and functional polarity.
- From the distal end of the centriole, growth occurs, and in the case of basal bodies, cilium is created. Furthermore, the procentrioles that originate at right angles to the centriole are positioned near the proximal end.
5. Ciliary Rootlets
In some cells, the ciliary rootlets emerge from the basal ends of the basal bodies and are of two types:
- Tubular root fibrils: The diameter of tubular root fibrils is 200Ao.
- Striated rootlets: The majority of ciliary rootlets are striated, with a crossbanding pattern that repeats every 55 to 80 nm. The striated fibres of rootlets are made up of parallel microfilaments with a diameter of 3 to 7 nm, which are composed of globular subunits. These fibres and filaments may provide structural functions, such as anchoring the base of the organism. Microfilaments give ciliary rootlets a contractile function. The rootlet may be single or double (e.g., in mollusks) (e.g., the frog Rana).
6. Basal Feet and Satellites
- The basal feet are thick, perpendicularly-arranged structures. These activities impose on the basal body a structural asymmetry proportional to the direction of the ciliary beat.
- The microfilaments of a basal foot terminate in a thick bar. It may serve as a focal point for microtubule convergence.
- Satellites or pericentriolar bodies are electron-dense structures that surround the centriole and are likely microtubule nucleation sites.
- In addition to lipid molecules, microtubules of centrioles and basal bodies contain the structural protein tubulin.
- There is a large concentration of ATPase enzyme in the centrioles and basal bodies. There is debate regarding whether centrioles and basal bodies contain DNA and RNA.
- Fulton (1971) has questioned whether these organelles contain nucleic acids.
Origin of Centrioles And Basal Bodies
The notion that new centrioles develop from the division of old centrioles is no longer widely held. Instead, it appears that new centrioles are either generated from scratch or synthesised using an existing centriole as a template (semi-autonomous replication).
1. Origin of centrioles by duplication of pre-existing centrioles
- In cultured fibroblasts, the beginning of DNA synthesis coincides with the beginning of centriole duplication (interphase).
- First, the two members of a pair of centrioles separate; next, a procentriole is created perpendicular to each original centriole, with the two organelles separated by 50 to 100 nm.
- An immature centriole has a ninefold symmetric array of single microtubules; each microtubule apparently serves as a template for the construction of adult centrioles’ triplet microtubules.
- In late prophase, each daughter centriole reaches its full size while retaining its tight proximity and orientation to the mother centriole. Consequently, when the interphase nuclei rejoin at the conclusion of nuclear division, a centrosome comprising two centrioles is present next to each nucleus.
- The development of the centriole (or basal body) has been examined in Paramecium, Tetrahymena, Xenopus, and chicken tracheal epithelium. The stages of development are basically same across the board.
- In an amorphous mass, the development of a basal body begins with the production of a single microtubule. Microtubules are introduced one by one until a ring of nine is formed with uniform spacing.
- As the microtubules appear, the amorphous mass disappears, as if eaten by the microtubule creation process.
- There is evidence that connectives exist between microtubules, which may function to set their spacing apart.
- Thus, a ring of nine full microtubules (i.e., A tubules) is generated, followed by the development of C-shaped B microtubules and C microtubules.
- The hub and the wheel are added centrally. The A-C linkages are not established until development is complete.
2. Origin of basal bodies
- In a ciliated vertabrate cell, which may include hundreds of cilia, the precursor cell’s centrioles give rise to the numerous basal bodies necessary to nucleate the cilia in the mature cell.
- During the development of ciliated epithelial cells that line the oviduct and trachea, for instance, the centriole pair migrates from its typical site near the nucleus to the apical area of the cell, where the cilia will form.
- In this case, instead of generating a single daughter centriole, each pair of centrioles creates several electrondense fibrogranular satellites.
- From these satellites, many basal bodies subsequently travel to the membrane to commence the development of cilia.
3. The de novo origin of centrioles and basal bodies
- In certain instances, centrioles appear to originate from scratch. Unfertilized eggs of many animals, for instance, lack functional centrioles and rely on the sperm centriole for the initial mitotic division (for cleavage); however, under certain experimental conditions, such as extreme ionic imbalance or electrical stimulation, the unfertilized egg can produce a variable number of centrioles.
- Each of these centrioles nucleates the creation of a small aster, one of which can be exploited by the egg for cleavage division, resulting in the parthenogenesis of a haploid organism.
- In fact, centriole precursors are maintained in the cytoplasm of unfertilized eggs and can be activated under certain conditions to produce a new centriole.
- Similar to the centrioles, the basal bodies are capable of self-assembly and abruptly appear in Naegleria as it transforms from an amoeboid to a normal ciliate.
- Centrioles have been hypothesised to be fully autonomous, self-replicating organelles due to their peculiar mechanism of duplication and their continuity over multiple generations.
- Although it is now known that this is not the case and that centrioles can develop spontaneously in the cytoplasm under specific conditions.
- Consequently, it is possible that some information required for centriole development is typically contained inside the centriole itself (just as the replication of mitochondria and chloroplasts depends on extrachromosomal genes carried in the organelles).
- In Chlamydomonas, for instance, a group of genes encoding proteins important in basal body shape and flagellar assembly are carried on a genetic element that segregates independently from the major chromosomes.
- This genetic element’s nature and placement have yet to be discovered.
Functions of Centrioles
- Formation of basal bodies and ultimately the cilia is the specialized function of the centrioles in the cell.
- The normal function of a pair of centrioles in most animal cells is to act as a focal point for the centrosome. The centrosome (also called the cell centre) organizes the array of cytoplasmic microtubules during interphase and duplicates at mitosis to nucleate the two poles of the mitotic spindle.
- Sometimes centrioles can serve first one function and then another in turn : for example, prior to each division in Chlamydomonas, the two flagella resorb and the basal bodies leave their position to act as mitotic poles.
- In spermatozoon one centriole give rise to the tail fibre or flagellum.
- Centrioles and basal bodies are also found to be involved in ciliary and flagellar beat.
- Centrioles and basal bodies have a role in the reception of optical, acoustic and olfactory signals.
- Recently, it has been suggested that centrioles could serve as devices for locating the directions of signal sources. Such a role for them has been conceived by comparing the geometric design of centrioles (with their disposition in pairs at the right angles and their ninefold symmetry) with manmade devices such as radar scanners, that detect directional signals.