Centrosome

The centrosome (Latin centrum 'centre' + Greek sōma 'body') (archaically cytocentre) is a non-membrane bounded organelle in the animal cell that serves as the main microtubule organizing centre (MTOC) and a regulator of cell-cycle progression. The centrosome provides structure for the cell. It is thought to have evolved only in the metazoan lineage of eukaryotic cells. Fungi and plants lack centrosomes and therefore use other structures to organize their microtubules. Although the centrosome has a key role in efficient mitosis in animal cells, it is not essential in certain fly and flatworm species. In non-rodent mammals the sperm contributes the major part of the centrosome, the centrioles. Centrosomes are composed of two centrioles arranged at right angles to each other, and surrounded by a dense, highly structured mass of proteins termed the pericentriolar material (PCM). The PCM contains proteins responsible for microtubule nucleation and anchoring — including γ-tubulin, pericentrin and ninein. In general, each centriole of the centrosome is based on a nine-triplet microtubule assembled in a cartwheel structure, and contains centrin, cenexin and tektin. In many cell types, the centrosome is replaced by a cilium during cellular differentiation. However, once the cell starts to divide, the cilium is replaced again by the centrosome.