Centrioles are two hollow, micro tubular and cylindrical organelles that are mainly implicated in the organization of a spindle. They are present within the cytoplasm of a cell. They are also known as diplosome as they are found to occur in pairs and have active role in cell division.
Centrioles are part of cytoplasm but recent studies prove that they originate from the Kinoplasm (the thick central cytoplasmic substance of centrosome). During the process of duplication, each new matching set of centrioles is made up of the original centriole, along with a newly-made centriole. These organelles are mainly found in most of the animal cells. Plants do not have centrioles in them, but exceptions are there like, algae.
They are mainly 2 in number i.e. they are found to occur in pairs but just before the process of cell division two new centrioles are found to form. Therefore it means that a centrosome is found to have four centrioles altogether.
Centrioles are found to have a length of 700 nm and a diameter of 250 nm .They appear as empty cylinders, a bit closer to the shape of open barrels of 1500Ậ in diameter. They are found to be positioned at right angles to each other but not touching among themselves. They are surrounded by a compact wall of 3-5µm long. This wall is made up of nine parallel micro tubular triplets that are arranged longitudinally. Centrioles are found to be surrounded by a clear homogenous type of cytoplasm called Centrospheres.
There occur nine related structures of electron dense substances. These structures are round in shape and are joined by centriolar microtubules in two rows or junctions. These are termed as pericentriolar satellites or mussels or even spheroid corpuscles.
Electron microscopic structure
A microtubule is composed of three subfibers or sub tubules each possessing a diameter of 250A. A biologist called Threadgold in the year 1968 termed these sub tubules as A, B, C sub fibers. The 'A' sub fiber is tubular in shape whereas the sub fibers 'B' and 'C' are c-shaped. The 'A' sub tubules are composed of 13 globular proteins or tubulin subunits. Generally by extensive research works it is found that 'A' sub tubules are linked with the 'C' sub tubules by the help of a dense microtubule called "C-A linkers".
At one end of a centriole there is an intra centriolar cart wheel arrangement consisting of a central hub from which emerges 9 radial spokes. Each spoke radiates and ends in a "Dense Matter (DM)" termed as X-Body. Apparently each and every X-body is attached to an 'A' subfiber. It has been found that between X-bodies there is a "DM" named as Y-body which is connected to X-bodies via C-A linkers.
1. Cell Division- The main job of the centrioles is to form mitotic poles in higher animals. They are found be engaged in spindle fiber formation to separate the chromosomes during cell division (mitosis). During anaphase stage of cell division the chromosomes are found to move towards the poles which are all because of these centrioles. Without their help the chromosome movement wouldn't have been possible.
However they are also found to organize the pericentrile material along with L-tubulin in order to assemble the centrosome of the cell, the circular, cytoplasmic non-membranous organelle that grips the kinetochore and nonkinetochore microtubules during the process of mitosis and meiosis. Apparently it has been found that though few animal cells which are devoid of centrioles they are capable of cell division by other means, i.e. the assistance of the centriole is not required by them. Again in many animal cells they replicate to produce their identical pair in the cells which are produced from the mother cells.
2. Celiogenesis- It means the process of formation of cilia and flagella (minute hair like projections and extensions of the surface of the cells. This is one of the primitive roles of centrioles. The centrosome divides and travels to the periphery of the cell and form kinetosome or basal body from which cilia and flagella are found to originate.
3. Sperm tail formation- One centriole of the centrosome helps in the development of the tail fiber of the spermatozoa. Recent studies states that centrioles figure the axial filament or axoneme of the tail of spermatozoa.
4. Role for the centrosomes (debated issue) - Many biologists and cytologists are of the opinion that centrioles help in formation and doubling-up the centrosomes itself. However other researchers strictly deny this. This role of centriole is still a matter of dispute among the scientists.
Wrapping up
Although vast and extensive researches have been made but further researches are still undergoing which may lead us to a number of unknown facts and concealed truths about this organelle.