centrosome
/ SEN-troh-some /
Imagine the spot in a city from which all the main roads radiate outward. Traffic across the whole city is organized around that one hub. In most animal cells there is a similar hub for microtubules: the centrosome, usually sitting near the nucleus. From it, microtubules grow outward in all directions like spokes, so the centrosome sets up the cell's internal road map.
Structurally, the centrosome is a small region of dense protein material (called the pericentriolar material) that contains a special starter form of tubulin able to nucleate new microtubules. Embedded in most animal centrosomes is a pair of barrel-shaped structures called centrioles, sitting at right angles to each other. The protein material around them is what actually seeds the microtubules; the centrioles help organize and duplicate the centrosome. Because of this role, scientists call the centrosome a microtubule organizing center.
The centrosome's biggest moment comes during cell division. Before a cell divides, the centrosome copies itself, and the two centrosomes move to opposite ends of the cell. From there they build the mitotic spindle, the microtubule machine that pulls the duplicated chromosomes apart so each new cell gets a complete set. Notably, plant cells lack centrosomes yet still build perfectly good spindles, a reminder that the centrosome organizes microtubules but is not strictly required to make a spindle.
If you stain a resting animal cell for microtubules, you typically see them fanning out from a single bright dot near the nucleus — that dot is the centrosome.
The centrosome is the single hub from which most cellular microtubules originate.
It is easy to assume cells need a centrosome to divide, but plant cells and many other organisms build their spindles without one — the centrosome is a microtubule organizer, not an essential spindle requirement.