Centrioles are small hollow cylinders that occur in pairs in most animal cells and lower plant cells in the cytoplasm near the nucleus. Centriole, also called micro centrum or cell-center was first discovered by Van Benden(1887 A.D.)
Centrosome is formed of two darkly stained granules called centrioles, which are surrounded by a transparent cytoplasmic area called centrosphere or cytol centrum. As the centrioles are in pairs, so-called diplosome.
Centrioles also occur at the bases of cilia and flagella, where they are known as basal bodies. When a centriole bears a flagellum or cilium, it is called the basal body.
Centrioles are non-membranous light microscopic organelle found in all the animal cells except mature mammalian RBCs. They are also found in most of the motile plant cells like antherozoids of ferns, zoospores of algae and motile algae e.g. Chlamydomonas, but is absent in prokaryotes fungi and higher plants, the phanerogams. Plant cells lack centrioles although they do produce spindles during nuclear division. The cells are thought to contain smaller MTOCs which are not easily visible even with the electron microscope.
Each centriole is cylindrical and approximately 500 nm long and 200 nm in diameter. These are large sized in oocytes and spermatocytes, so are called giant centrioles. They are only two in a cell but basal bodies are more in number.
Under an electron microscope, two centrioles are hollow cylinders perpendicular to each other. Each centriole is a microtubular structure formed of microtubules arranged in 9+0 arrangement (all the 9 microtubules are arranged in peripheral portion). Each microtubule is a triplet and is formed of three sub tubules. Inside the microtubules, there is an intra-centriolar or cart-wheel structure which is formed of a central hub and nine radial spokes or radial fibres.
fig:centrioles,arrangement,cart a pair in an animal cell
A centriole is chemically formed of;