What is a Centriole?
A Centriole can be defined simply as the cylindrical (rod-shaped) organelle that is composed of microtubules and stays next to the nucleus within the centrosome in eukaryotic cells. It is one of the nonmembranous organelles of the animal cell. There are two centrioles within the centrosome. The centriole forms a spindle of microtubules ( a mitotic apparatus for meiosis or mitosis) and is sometimes arranged under the cell membrane to form and bear cilia or flagella in ciliated or flagellated cells. However, the centriole is called the basal body, when it bears a cilium or flagellum.
This organelle lacks a membrane and RNA or DNA. However, centrioles exist in most lower plants (algal cells except red algae, some fern cells, male gametes of charophytes, bryophytes, ginkgo, cycads, seedless vascular plants, moss cells), and animal cells. Thus they are not found in cone-bearing and flowering plants such as conifers and angiosperms. Also, they are absent in prokaryotes, yeast, and some non-ciliated or non-flagellated protozoans ( e.g amoeba). Even though centrioles are found in eukaryotic cells, they are not present in higher plants because the plant cells in higher plants do not use centrioles during cell division.
Origin and History of Centrioles
A ciliated cell with centrioles was the last common ancestors of all eukaryotes. However, some eukaryotes lineage like land plants does not have centrioles except in their motile male gametes. Hence the centriole is completely absent from all cells of conifers and flowering plants that do not have flagellate or ciliate gametes. Also, it is unknown if the last common ancestor had one or two cilia. However, an important gene like centrin needed for centriole growth is not found in bacteria or archaea but only in eukaryotic organisms.
Edouard Van Beneden first observed the centrosome as a composition of two orthogonal centrioles in 1883. Although he and Walther Flemming jointly discovered the centrosome in 1876. However, Theodor Boveri introduced the term “centrosome” and “centriole” in 1888 and 1895 respectively. In 1880, Theodor Wilhelm Engelmann named the basal body. Etienne de Harven and Joseph G. Gall independently first worked out the pattern of centriole duplication in 1950.
- A centriole can be visible under a light microscope although its structure is more revealed and detailed under an electron microscope.
- They are cylindrical in structure and are minute microtubular structures with a configuration of 9 triplet fibrils.
- Without possessing DNA, centrioles can form their own duplicates, astral poles, and basal bodies.
- Some accessory structures of the basal body are the basal feet, ciliary rootlets, and transition fibers.
- Centriole lacks a membranous covering. It is a non membranous organelle.
- The centriole’s diameter size varies at about 0.3-0.7um in length. Although some are short in length at about 0.16um and some are long with a length of 8um.
- A cell each has a pair of centrioles in its centrosome and members of each centriole pair are at a right angle to one another. The centrosome is a region near the nucleus.
- Centrioles have a whorl of 9 peripheral fibrils. However, the fibrils are not present in the center. Thus, the arrangement of the fibrils is called 9 + 0.
- The fibrils in the centriole run parallel to one another at an angle of 40°. However, each fibril has 3 sub-fibers, hence it is called a triplet fibril.
- Actually, the 3 sub-fibers are microtubules (triplet microtubules) that are joined together by their margins, hence sharing the common walls composed of 2-3 proto-filaments.
- An amorphous material called pericentriolar material surrounds the triplet microtubules (triplet fibril). This material however contains many molecules needed for the construction of microtubules.
- Each sub-fiber ( each microtubule in a triplet) is made up of small units of tubulin, which is a small monomer that can come together to form long straw-like hollow tubes.
- The diameter of each sub-fiber is 25nm. The 3 sub-fibers are named C, B, and A, starting from outside to inside. Thus, sub-fiber B and C are incomplete because of sharing some microfilaments whereas Sub-fiber A is complete with 13 proto-filaments.
- However, the adjacent triplet fibrils are connected by C – A proteinaceous linkers. The centriole center has a rod-shaped proteinaceous mass called the hub.
- The hub has a diameter of 2.5nm and then 9 proteinaceous strands develop from it called spokes.
- Thus, each spoke posses a thickening called X before it unites with sub-fiber A. However Y, another thickening is present close by and is attached by connectives to both the X thickening and C – A linkers.
- Finally, the centriole gives a cartwheel appearance in the transverse section due to the presence of radial spokes and peripheral fibrils.
Parts of a Centriole
The centriole is composed of 3 main parts, which are:
- Distal part
- Central core
The distal part of the centriole is defined by the triple or double microtubules. However, this part is divided into the distal and sub distal parts. Therefore, the Eukaryotic cells have a total of 9 distal parts/appendages whereas the sub-distal appendages can vary in number depending on the cell function and type.
The structure of the distal appendage looks like turbine blades that are symmetrically arranged at the centriole’s distal end. Here, each of the distal parts is attached to one of the triplets at an angle of 50° to the centriole surface.
Contrary to the distal appendage’s structure, the sub-distal appendages are attached to 2 or 3 triplets forming an angle of 90° with the centriole surface. However, the sub-distal appendages have been known to change shape or morphology and even disappear in some instances.
Additionally, the distal and sub-distal appendages have different functions also. The sub-distal appendages act as centers of nucleation for microtubules while the distal appendage’s role is to attach the centriole during cilium formation in some cells.
The central core of the centriole is the part on which the microtubules triplets are attached to the centriole. This part serves to stabilize the scaffold as a part of the centriole. For instance, in an organism like C. reinhardtii, this part is about 250nm long, having a Y-shaped linker and a barrel-like structure located in its inner core.
One of the most studied sub-centriolar structures is the cartwheel. However, the cartwheel structure consists of a central hub with 9 spokes radiating from it. Then, through a pinhead, each of these spokes/ filaments is connected to the A-tubule of the microtubules.
Moreso, the number of cartwheels varies between organisms and developmental stages. During development in Trichonympha, for example, the number of this structure (cartwheel) can vary between 7-10 layers and about 2-4 layers once they mature.
The pinhead is an important structural part of the cartwheel. The pinhead has a hooklike protrusion and linkers located between the pin body and microtubules. Since the cartwheel has been seen to appear before the 9 microtubules in some species. The cartwheel has been suspected to aid determine the number of the centriole’s microtubules.
Therefore the function of the cartwheel is to establish the ninefold symmetry in the organelle and also to strengthen the arrangement of triplet microtubules.
After knowing about a centriole, what does a centriole do?
- The centrioles are involved in the formation of the spindle apparatus that plays a major function during cell division.
- Centrioles are located in the cell nucleus as it helps in cell division by facilitating the separation of chromosomes.
- However, when the centriole is absent there is a divisional error and delay in mitotic processes.
- Aside from cell division, they assist in the movement of the cell as they are involved in ciliogenesis which is the formation of cilia and flagella on the surface of the cell. Basal bodies direct cilia and flagella.
- The centrioles also function to form the sperm flagellum and aid sperm movement.
- The Sperm centriole aids in the development of the embryo after fertilization. The sperm supplies the centriole that forms the centrosome and microtubule system of the zygote.
- During mammalian development, the proper orientation of cilia through centriole positioning toward the posterior of embryonic node cells is crucial to establish left-right asymmetry.
- The position of the centriole also determines the position of the nucleus.
- Centriole also plays an important role in the spatial arrangement of the cell.
Centrioles in Plant cells
Higher plants do not have centrioles. The centriole is absent from the cells of higher plants although normal mitosis still occurs in the plant with satisfactory results. However, Spindle fibers that facilitate the separation of chromosomes are hence produced by an organelle called the centrosome.
Plant cells lack centrioles but are still capable of forming a mitotic spindle from the centrosome area of the cell located just exterior to the nuclear envelope. How these plants divide without centriole is that they build special vesicles from their Golgi apparatus which are needed for cell division. Also, plant cells have a rigid cell wall that doesn’t undergo any changes in structure during mitosis. However, the cell walls can organize many of the microtubules that form the spindle during mitosis.
Centrioles are essential for animal cells because they pull the cell into two new cells; but in plant cells, instead of the centriole to pull them apart, their cytoplasm will spread, and then the new cell wall will form in the middle which will lead to the formation of two new cells.
Although centrioles are lacking in higher plants, they are seen in some lower plants. The Centrioles have been seen to form during spermatogenesis (a form of cell division) in lower plants. However, centrioles have been found in lower plants like ferns, mosses, male gametes of charophytes, bryophytes, Ginkgo, seedless vascular plants, and cycads.
Centrioles in Animal cell
Centrioles are found mainly in animal cells. They are the paired organelle located near the nucleus in the centrosome. The centrioles are a granular mass in the animal cell that serves as an organizing center for microtubules. They are those cylindrical (rod-shaped) organelles that are composed of microtubules and stay next to the nucleus within the centrosome in animal cells. Centrioles are one of the nonmembranous organelles of the animal cell.
There are two centrioles within the centrosome of the animal cell. The centriole however forms a spindle of microtubules ( a mitotic apparatus for meiosis or mitosis) and is sometimes arranged under the animal cell membrane to form and bear cilia or flagella in animal cells. However, the centriole is called the basal body, when it bears a cilium or flagellum in the animal cell
However, centrioles play a major role in the cell division of the animal cell. In an animal cell, During interphase, the centrioles and other elements of the centrosome are duplicated. Centrioles are essential in animal cells because they pull the cell into two new cells.
In animal cells, the centrioles organize the pericentriolar material to produce microtubules and mitotic spindle fibers. They definitely affect the outcome of mitosis in animal cells.
The structure of a centriole in an animal cell can be visible under a light microscope although its structure will be more detailed under an electron microscope. They appear cylindrical in structure and are minute microtubular structures with a configuration of 9 triplet fibrils.
Without possessing DNA, centrioles can form their own duplicates, astral poles, and basal bodies in the animal cell. However, in an animal cell, the centriole lacks a membranous covering and each animal cell has a pair of centrioles in its centrosome with members of each centriole pair at a right angle to one another.
The Centrioles in an animal cell however have a whorl of 9 peripheral fibrils. Though the fibrils are not present in the center. The fibrils in the centriole run parallel to one another at an angle of 40°. However, each fibril has 3 sub-fibers called a triplet fibril.
Actually, the 3 sub-fibers are microtubules (triplet microtubules) and an amorphous material called pericentriolar material surrounds the triplet microtubules (triplet fibril). This material however contains many molecules needed for the construction of microtubules in the animal cell. Then each sub-fiber ( each microtubule in a triplet) is made up of small units of tubulin. Lastly, the centriole in an animal cell comprises the Distal part, the central core, and the cartwheel.
Centrioles in Mitosis
As one of their major function, centrioles are very essential for cell division.
However, during the S phase of the cell cycle, a new centriole is formed from protein components and is called a procentriole. At this stage the centriole is immature and hence during late mitosis, the procentriole begins to align at an angle of 90° with the pre-existing centriole.
As the procentriole aligns with the pre-existing (mother) centriole, its proximal end gradually gets juxtaposed to the surface of the mature centriole. This process is known as engagement and this arrangement is maintained until interphase.
The protein matrix (pericentriolar material) and centrioles (2 mature centrioles) combine to form the centrosomes. However, the role of centrioles in cell division is mainly about their own duplication. Once the cell is about to divide, the centrioles go to the opposite ends of the nucleus.
During the prophase stage, the chromosomes that were duplicated during the S phase condense and become more compact. Sister chromids are also joined together at the centromere giving them an X-shaped body.
At the second phase of mitosis, the nuclear membrane breaks down by the phosphorylation of the nuclear lamin by kinases called M-CDK( Cyclin-dependent kinases), allowing the spindle fibers to access the chromosomes.
However, the spindle fibers eventually connect at the centromere to the chromosomes as they grow towards the chromosomes. The microtubules (spindle microtubules) attach to a protein complex (kinetochore) assembled at the centromere. However, it is this protein complex that connects the spindle to the centromere of the chromosome.
As soon as the chromosomes are attached to the spindle, they are separated and pulled apart. However, as the chromosomes are pulled apart there is an enzymatic action on cohesin linking the chromatids that help in separating the chromatids.
In the anaphase, the sister chromatids are pulled to the opposite end of the cell and eventually become an independent chromosome.
During telophase the chromosome gathers together, the spindle eventually breaks down and the nuclear membrane and nuclei form again. The mother cell cytoplasm divides to form 2 daughter cells.
Mitosis is essential as it provides new cells for the growth and replacement of worn-out cells. Furthermore, during cell division, the proper development of centrosomes from centrioles is very crucial for cell division. Even though cell division can occur when the centrosome is not present in animals, the whole process can be messy because the organization of microtubules takes more time. Also, the chromosomes can end up getting in the wrong cell or lost even.
Centrioles in Meiosis
Centriole is also involved in meiosis.
The cell’s DNA is copied and results in 2 identical full sets of chromosomes. However, there are two centrosomes outside of the nucleus that contains a pair of centrioles. During the interphase, microtubules extend from the centrosomes.
The copied chromosomes then condense into X-shaped structures. This structure can be seen easily under a microscope. Each of the chromosomes however has 2 sister chromatids that contain identical genetic information. The chromosomes then pair up in a manner that both copies of each chromosome are joined together. The pairs of chromosomes may then exchange bits of DNA. This process of exchanging DNA is called crossing over or recombination.
At the end of this prophase I, the membrane surrounding the nucleus dissolves. Hence the chromosomes are released. However, the meiotic spindle that consists of microtubules and other proteins then extends across the cell between the centrioles.
The chromosome pairs eventually line up next to one another along the cell’s center(equator). Hence placing the centrioles at the opposite end of the cell with the meiotic spindles extending from them. The meiotic spindle fibers then attach to one chromosome of each pair.
The meiotic spindle then pulls the pair of chromosomes apart, pushing one chromosome to one end of the cell and the other chromosome to the opposite end. However in meiosis I, the sister chromatids stay together which is different from what happens in mitosis and meiosis II.
Telophase I and cytokinesis
The chromosomes complete their move to the opposite end of the cell. Hence at each end of the cell, a full set of chromosomes gather together and a membrane is formed around each set of chromosomes. This eventually forms 2 new nuclei. The single cell then pinches in the middle to form 2 separate daughter cells, each of which contains a full set of chromosomes within a nucleus. This process is called cytokinesis.
Now two daughter cells (each with 23 chromosomes. i.e 23 pairs of chromatids) are present. In each of the two daughter cells, the chromosome condenses again into X-shaped structures. The structure can be seen easily under the microscope. In each of the daughter cells, the membrane around the nucleus dissolves away and releases the chromosomes. However, the centrioles duplicates and the meiotic spindle forms again.
In each of the two daughter cells, the pair of sister chromatids (chromosomes) line up end to end along the center of the cell. Hence the centrioles are then at the opposite end of the daughter cells. The meiotic spindle fibers at each end of the cell then attach to each of the sister chromatids.
Due to the action of the meiotic spindle, the sister chromatids are then pulled to opposite ends and these separated chromatids become individual chromosomes.
Telophase II and Cytokinesis
The chromosomes then complete their move to the opposite end of the cell and at each end, a full set of chromosomes gather together. Then a membrane is formed around each set of chromosomes to form two new nuclei. This actually is the last phase of meiosis.
However, cell division is not complete without another round of cytokinesis. Once cytokinesis is complete, four granddaughter cells occur, each with half a set of chromosomes (haploid).
Furthermore, in males, these four cells are all sperm cells whereas, in females, one of the cells is an egg cell, and the other 3 are polar bodies (i.e small cells that do not develop into eggs).
Centriole vs Centrosome
The centrosome is an important organelle located near the nucleus within the cell. Also, like the Centriole, the centrosome is absent in some cells and multicellular organisms. Centriole and centrosome definitely do not mean the same thing but the centrosome is characterized by the combination of centrioles which are surrounded by a protein matrix (pericentriolar material).
Within the centrosome, there are 2 centrioles with a well-defined structure where the centrioles are arranged at an angle of 90° to one another. However contrary to centrioles, the centrosome has an amorphous structure. Hence during cell division, like the centriole, the centrosome also divides as they move to the opposite end of the cell.
In cells that don’t divide, the centriole is involved in the formation of cilia and flagella. However, the centrosome is involved in cell division where it forms spindle apparatus.