centriole

One of two cylindrical cellular structures that are composed of nine triplet microtubules and form the asters during mitosis.

[New Latin centriolum, diminutive of Latin centrum, center. See center.]

A morphologically complex cellular organelle at the focus of centrosomes in animal cells and some lower plant cells. Prokaryotes, some lower animal cells, higher plant cells, and a few exceptional higher animal cells do not have centrioles in their centrosomes. Centrioles typically are not found singly; the centrosome of higher animal cells contains a pair of centrioles (together called the diplosome), arranged at right angles to each other and separated by a distance ranging from 250 nanometers to several micrometers. See also Centrosome.

Centrioles are typically 300–700 nm in length and 250 nm in diameter. Although they can be detected by the light microscope, an electron microscope is required to resolve their substructure. At the electron microscopic level, a centriole consists of a hollow cylinder of nine triplet microtubules in a pinwheel arrangement (see illus.). Within each triplet, one microtubule (the A tubule) is a complete microtubule, while the others (the B and C tubules) share a portion of their wall with the adjacent tubule. In some cells these nine triplet microtubules are embedded in a densely staining cylindrical matrix that is spatially distinct from the pericentriolar material of the centrosome. Structures found in the lumen or core of the centriole include linkers between the triplets, granules, fibers, a cartwheel structure at one end of the centriole, and sometimes a small vesicle.

Diagrams of centriole showing (a) arrangement of microtubules and (b) cross section of proximal end, with nine triplet microtubules (A, B, and C) and central cartwheel structure.

Centrioles have a close structural similarity to basal bodies, which organize the axoneme of cilia and flagella. In many types of mammalian somatic cells, the older of the two centrioles in the centrosome can act as a basal body during the interphase portion of the cell cycle. In such cases, tapered projections, called basal feet, are often observed on the external surface of the centriole that is acting as the basal body. Microtubules are attached to the globular tips of the basal feet and may serve to anchor this centriole in the cell.

During interphase the centrosome nucleates the array of cytoplasmic microtubules; later in the cell cycle the centrosome duplicates, and the daughter centrosomes form the poles of the mitotic (or meiotic) spindle. The terms “centriole” and “centrosome” are sometimes erroneously used interchangeably; centrioles are not the centrosome itself, but a part of it. The centrosome of higher animal cells has at its center a pair of centrioles, arranged at right angles to each other and separated by 250 nm or less.

The only clearly demonstrated role for the centriole is to organize the axoneme (central microtubular complex) of the primary cilium in cells having this structure, and the flagellar axoneme in sperm cells. Other possible functions for centrioles are a matter of debate. Some authorities assert that when present in the centrosome, centrioles contain activities that serve to organize the centrosome, determine the number of centrosomes in a cell, and control the doubling of the centrosome as a whole before mitosis. Others believe that centrioles have no role in the formation and doubling of the centrosomes but are associated with the centrosomes only to ensure the equal distribution of basal bodies during cell division. See also Cell (biology).

Either of the two cylindrical organelles located in the centrosome and containing nine triplets of microtubules arrayed around their edges; centrioles migrate to opposite poles of the cell during cell division and serve to organize the spindles. They are capable of independent replication and of migrating to form basal bodies.

Schematic of centriole showing microtubule triplets

3-dimensional view of a centriole

A centriole is a barrel-shaped organelle^[1] found in most animal eukaryotic cells, though absent in higher plants and most fungi.^[2] The walls of each centriole are usually composed of nine triplets of microtubules (protein of the cytoskeleton). Deviations from this structure include Drosophila melanogaster embryos, with nine doublets, and Caenorhabditis elegans sperm cells and early embryos, with nine singlets.^[3][4] An associated pair of centrioles, arranged perpendicularly and surrounded by an amorphous mass of dense material (the pericentriolar material) constitutes the compound structure known as the centrosome.^[1]

Contents 1 Cell division 2 Cellular organization 3 Ciliogenesis 4 Animal development 5 Centriole duplication 6 References

Cell division

Centrioles are involved in the many organizations of the mitotic spindle and in the completion of cytokinesis.^[5] 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 .^[6] Additionally, mutant flies lacking centrioles can develop almost normally, although the adult flies lack flagella and cilia, a lack that underscores the requirement of centrioles for the formation of these organelles (see below).^[7] Cells whose centrioles have been removed via laser ablation or genetic manipulation lack aster microtubules. These cells often fail to undergo proper asymmetric cell division, as the aster microtubules help to position the spindle within the cell.

Cellular organization

Centrioles are a very important part of centrosomes, which are involved in organizing microtubules in the cytoplasm.^[8][9] The position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of cell organelles.

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

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.^[10]

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.^[5]

References

1. ^ ^{a b} Eddé B, Rossier J, Le Caer JP, Desbruyères E, Gros F, Denoulet P (January 1990). "Posttranslational glutamylation of alpha-tubulin". Science 247 (4938): 83–5. doi:10.1126/science.1967194. PMID 1967194. http://www.sciencemag.org/cgi/content/abstract/247/4938/83. Retrieved on 2008-07-09. 
2. ^ Quarmby LM, Parker JD (June 2005). "Cilia and the cell cycle?". The Journal of Cell Biology 169 (5): 707–710. doi:10.1083/jcb.200503053. PMID 15928206. PMC: 2171619. http://www.jcb.org/cgi/pmidlookup?view=long&pmid=15928206. Retrieved on 2008-07-08. 
3. ^ Delattre M, Gönczy P (April 2004). "The arithmetic of centrosome biogenesis". Journal of Cell Science 117 (Pt 9): 1619–1630. doi:10.1242/jcs.01128. PMID 15075224. http://jcs.biologists.org/cgi/content/full/117/9/1619. Retrieved on 2008-07-08. 
4. ^ Leidel S, Delattre M, Cerutti L, Baumer K, Gönczy P (February 2005). "SAS-6 defines a protein family required for centrosome duplication in C. elegans and in human cells". Nature Cell Biology 7 (2): 115–25. doi:10.1038/ncb1220. PMID 15665853. http://www.nature.com/ncb/journal/v7/n2/abs/ncb1220.html. Retrieved on 2008-07-08. 
5. ^ ^{a b} Salisbury JL, Suino KM, Busby R, Springett M (August 2002). "Centrin-2 is required for centriole duplication in mammalian cells". Current Biology 12 (15): 1287–1292. doi:10.1016/S0960-9822(02)01019-9. PMID 12176356. http://www.current-biology.com/content/article/fulltext?uid=PIIS0960982202010199. Retrieved on 2008-07-08. 
6. ^ La Terra S, English CN, Hergert P, McEwen BF, Sluder G, Khodjakov A (February 2005). "The de novo centriole assembly pathway in HeLa cells: cell cycle progression and centriole assembly/maturation". The Journal of Cell Biology 168 (5): 713–722. doi:10.1083/jcb.200411126. PMID 15738265. PMC: 2171814. http://www.jcb.org/cgi/pmidlookup?view=long&pmid=15738265. Retrieved on 2008-07-08. 
7. ^ Basto R, Lau J, Vinogradova T, et al. (June 2006). "Flies without centrioles". Cell 125 (7): 1375–1386. doi:10.1016/j.cell.2006.05.025. PMID 16814722. http://linkinghub.elsevier.com/retrieve/pii/S0092-8674(06)00654-4. Retrieved on 2008-07-08. 
8. ^ Feldman JL, Geimer S, Marshall WF (June 2007). "The mother centriole plays an instructive role in defining cell geometry". PLoS Biology 5 (6): e149. doi:10.1371/journal.pbio.0050149. PMID 17518519. PMC: 1872036. http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0050149. Retrieved on 2008-07-08. 
9. ^ Beisson J, Wright M (February 2003). "Basal body/centriole assembly and continuity". Current Opinion in Cell Biology 15 (1): 96–104. doi:10.1016/S0955-0674(02)00017-0. PMID 12517710. http://linkinghub.elsevier.com/retrieve/pii/S0955067402000170. Retrieved on 2008-07-08. 
10. ^ Tsou MF, Stearns T (August 2006). "Mechanism limiting centrosome duplication to once per cell cycle". Nature 442 (7105): 947–51. doi:10.1038/nature04985. PMID 16862117. http://www.nature.com/nature/journal/v442/n7105/abs/nature04985.html. Retrieved on 2008-07-08. 

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