An electron micrograph showing that a centriole is constructed from a ring of nine triplet microtubules. Most animal cells contain two to four centrioles. In longitudinal view, a centriole looks like a rectangle about 3-times as long as it is wide. Centrioles are commonly arranged in pairs with one perpendicular to its neighbor. This pair of centrioles is usually located at the middle of an animal cell's 'centrosome,' the structure that serves as the origin of many of its cytoplasmic microtubules. Microtubules grow from an apparently amorphous matrix called the 'pericentriolar material' (PCM) that surrounds one of the centrioles. PCM is formed from long proteins that form a meshwork; it connects the surface of a centriole to a protein complex called the 'gamma-tubulin ring complex', which initiates the growth of microtubules. In this way, centrioles participate in organizing the cell cytoskeleton. During the cell growth and division cycle, the centrioles duplicate at a time around the beginning of the S-phase, when DNA replicates. Each cell enters mitosis with two centrosomes, each built around a pair of centrioles; these structures serve as the poles of the mitotic spindle, helping to organize the microtubules that segregate the duplicated chromosomes. A centriole can also serve as the initiator for a more complex array of microtubules, called an 'axoneme,' which is the array of nine doublet microtubules that comprises the motile core of every cilium and flagellum. Many interphase cells in a multicellular animal contain a small, non-motile cilium, or 'primary cilium' that grows from one centriole. Other cells, like those that line the breathing tube, or 'trachea,' grow multiple cilia. Each cilium grows from a single centriole-like structure called a 'basal body,' which develops from a centriole. Thus, centrioles participate in at least two processes of microtubule organization; the formation of a cilium or flagellum and the assembly of a centrosome, which will in turn organize many of a cell's cytoplasmic or mitotic microtubules. With only two to four centrioles per cell, clear images of centrioles in multicellular organisms are difficult to find. Images like the one shown here requires a fortunate section that happened to be cut in a plane that exactly perpendicular to the axis of the centriole.