If an animal cell is looked at under a light microscope, a small ring can be seen near its nucleus. The extra resolution of an electron microscope over a light microscope makes it clear that this is in fact two centrioles located close together and perpendicular to each other. Together these organelles make up a centrosome, also known as the microtubule organizing centre, an area in the cell where microtubules are produced.
A centriole is a hollow cylinder about 0.4 um long, formed made up of a ring of nine groups of microtubules. Each group consists of three fused microtubules.
The microtubules which constitute the centrioles are used to grow the spindle fibres which separate chromosomes during nuclear division. During cell division, the centrosome divides and the centrioles replicate, forming two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, the microtubules grow into a spindle which is responsible for separating replicated chromosomes into the two daughter cells.