Centriole

Centrosome containing two centrioles.

A centriole is a cylinder shaped cell structure found in most eukaryotic cells, though it is absent in higher plants and most fungi. An associated pair of centrioles, arranged perpendicularly and surrounded by an amorphous mass of dense material, called the pericentriolar material, or PCM, makes up a compound structure called a centrosome.

Most centrioles are made up of nine sets of microtubule triplets, arranged in a cylindrical pattern. Deviations from this structure include crabs and Drosophila melanogaster embryos, with nine doublets, and Caenorhabditis elegans sperm cells and early embryos, with nine singlets.

Edouard van Beneden and Theodor Boveri made the first observation and identification of centrioles in 1883 and 1888 respectively, while the pattern of centriole replication was first worked out independently by Etienne de Harven and Joseph G. Gall circa 1950

Cell division

Centrioles are involved in the organization of the mitotic spindle and in the completion of cytokinesis. Centrioles were previously thought to be required for the formation of a mitotic spindle in animal cells. However, more recent experiments have demonstrated that cells whose centrioles have been removed via laser ablation can still progress through the G₁ stage of interphase before centrioles can be synthesized later in a de novo fashion. Additionally, mutant flies lacking centrioles develop normally, although the adult flies lack flagella and cilia.

Cellular organization

Centrioles are a very important part of centrosomes, which are involved in organizing microtubules in the cytoplasm. The position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of the cell. Buehler has suggested that the centriole may form a primitive directional "eye", sensitive to certain wavelengths in the Infra red spectrum. He has also demonstrated that cells are capable of reacting to each other's presence at a distance, and even when separated by a film of glass.

Ciliogenesis

In organisms with flagella and cilia, the position of these organelles is determined by the mother centriole, which becomes the basal body. An inability of cells to use centrioles to make functional cilia and flagella has been linked to a number of genetic and developmental diseases. In particular, the inability of centrioles to properly migrate prior to ciliary assembly has recently been linked to Meckel-Gruber syndrome.

Animal development

Electron micrograph of a centriole from a mouse embryo.

Proper orientation of cilia via centriole positioning toward the posterior of embryonic node cells is critical for establishing left–right asymmetry during mammalian development.^{[citation needed]}

Centriole duplication

Cells in G₀ and G₁ usually contain two complete centrioles. The older of the two centrioles in a pair is termed the mother centriole, whereas the younger is termed the daughter centriole. During the cell division cycle, a new centriole grows from the side of each of the existing "mother" centrioles. After centriole duplication, the two pairs of centrioles remain attached to each other in an orthogonal configuration until mitosis, when the mother and daughter centrioles separate in a manner dependent upon the enzyme separase.

The two centrioles in the centrosome are connected to each other by unidentified proteins. The mother centriole has radiating appendages at the distal end of its long axis and is attached to the daughter centriole at the other proximal end. Each daughter cell formed after cell division will inherit one of these pairs (one older and one newer centriole). Duplication of centrioles starts at the time of the G₁/S transition and ends before the onset of mitosis.

Origin

The last common ancestor of all eukaryotes was a ciliated cell with centrioles. Some lineages of eukaryotes, such as land plants, do not have centrioles anymore. It is unclear if the last common ancestor had one or two cilia. Important genes required for centriole growth, like centrins, are only found in eukaryotes and neither in bacteria or archaeans.