These range in number from one to two and are made up of RNA. Electron microsocopic observation has revealed the centrioles to be Paired cylindrical structures 3000 – 5000A long and 1200 -1500A in diameter.
These are open at one or both ends and placed at right angles to each other. Each cylinder is composed of nine fibres spaced equally and running parallel to the cylinder axis. Each fibre is in turn made up of three microtubules (triplet).
In a cross section, these appear to be arranged like the vanes of a pin wheel. The vanes are tiled at an angle of 40°. The wall of the cylinder is constituted by these nine triplet fibres.
The microtubule triplet in a fibre can be designated as A, B and C of which A is innermost. Each microtubule wall is 50A° in diameter and the lumen in 120 – 130A0 (the total diametre varies from 180 – 200A0).
The length of each microtubule is about 560A0. At the inner side of each triplet fibre a dense line runs and it is called triplet base connecting A and C tubules. In addition to this, a dense RNA region called foot is present.
The dense granlar region of cytoplasm surrounding the centrioles is called the centrosphere.
Structurally the microtubules are lipoproteinaceous (Fulton 1971).
Centrosome appear to play an important part in cell division; people believe that centrioles play an important part in the formation of spindle. They however constitute the mitotic pole in higher animals. In primitive animals which are ciliated, centrioles seem to help in the generation of cilia.