centriole
/ SEN-tree-ohl /
Imagine a short, hollow barrel built from nine bundled drinking straws arranged in a circle. That picture is close to a centriole: a tiny cylinder made of nine sets of microtubules arranged in a ring. Animal cells usually carry two of them, sitting at right angles inside the centrosome, the cell's main microtubule hub.
More precisely, each centriole is a barrel about nine microtubule triplets wide and open at both ends. They do not themselves grow the cell's microtubules — the dense protein material around them does that — but the centrioles help organize that material and, crucially, they get copied once per cell cycle so that each daughter cell inherits a centrosome. A pair of perpendicular centrioles plus the protein cloud around them makes up one centrosome.
Centrioles have a second job that is just as important. When a cell needs to grow a cilium or flagellum, a centriole moves to the cell surface and becomes the basal body, the foundation that templates the nine-fold ring of microtubules in those beating structures. So the same nine-fold barrel underlies both the cell's division apparatus and its swimming and sweeping machinery — a neat example of one design reused for two jobs.
In the cells lining your windpipe, centrioles move to the surface and become basal bodies for hundreds of cilia that sweep mucus and dust upward and out of your lungs.
The same barrel-shaped centriole serves both cell division and ciliated surfaces.
Centrioles do not directly nucleate microtubules; the surrounding pericentriolar material does. Plant cells lack centrioles entirely yet still organize microtubules and divide normally.