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Centriole |
A centriole is a barrel-shaped organelle1 found in most animal eukaryotic cells, though absent in higher plants and 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.34 An associated pair of centrioles, arranged perpendicularly and surrounded by an amorphous mass of dense material (known as the pericentriolar material) constitutes the compound structure known as the centrosome.1
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Centrioles are involved in the many organizations of the mitotic spindle and in the completion of cytokinesis.5 Centrioles were historically 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 undergo mitosis.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 (either 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.
Centrioles are a very important part of centrosomes, which are involved in organizing microtubules in the cytoplasm.89The position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of cell organelles.
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.
Additionally, proper orientation of cilia via centriole positioning toward the posterior of embryonic node cells is critical for establishing left–right asymmetry during mammalian development.
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
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