The Entangled Bank. Definition: Spindle fibers are aggregates of microtubules that move chromosomes during cell division. During prophase of mitosis, spindle fibers form at opposite poles of the cell. During metaphase, spindle fibers extend from the cell poles toward the midpoint of the cell known as the metaphase plate.Chromosomes are held at the metaphase plate by the equal forces of the spindle fibers pushing on the centromeres of the chromosomes.In anaphase, spindle fibers pull sister chromatids toward the spindle poles. Spindle fibers not connected to chromatids lengthen and elongate the cell.uring metaphase, spindle fibers extend from the cell poles toward the midpoint of the cell known as the metaphase plate. Chromosomes are held at the metaphase plate by the equal forces of the spindle fibers pushing on the centromeres of the chromosomes.