noun, plural: spindle fibers
Spindle fibers are made up of microtubules. Microtubules are polymers of alpha- and beta-tubulin dimers. Microtubules that form the spindle fibers come from centrosomes, which are organelles located in opposite poles near the nucleus.
In mitosis, these filaments form at opposite poles of the cell and meet at the equatorial plane. Collectively, they form a spindle-shaped structure, which is widest at the middle then tapers at both ends. The spindle fibers form during prophase. During metaphase of cell division, the spindle fibers radiate from the centrioles at the opposite poles. Some of them attach to the kinetochores of the chromosomes while others bind to the arms of the chromosomes, still others continue to grow. When the spindle fibers start to pull the chromosomes (via their kinetochores) to opposite poles, this marks the anaphase of cell division.
Variant: spindle fibre