A structure formed from microtubules in the cytoplasm during cell division that moves chromatids (see mitosis) or chromosomes (see meiosis) diametrically apart and gathers them in two clusters at opposite ends (poles) of the cell. Broad in the middle and narrowing to a point at either pole, its construction is directed by a microtubule-organizing centre, the centrosome. In the preliminary stages of cell division the centrosome divides, and the two daughter centrosomes move to opposite poles. Each organizes three sets of microtubules (or ‘spindle fibres’): the first set consists of a tuft of fibres, called the aster, radiating towards the cell periphery; the second set extends towards the centre of the cell to attach to chromatids or chromosomes; the third set also extends through the centre to overlap with its counterpart from the opposite pole at the spindle equator, the region halfway between the poles. The spindle becomes fully formed by metaphase, when the chromatids are attached to spindle fibres via their centromeres and lie at the spindle equator. During anaphase this set of fibres shortens and hauls the attached chromatids towards the corresponding pole of the cell. Also, the overlapping fibres at the equator actively engage and slide past each other to elongate the entire spindle. See also muscle spindle.