Any of a network of filaments that collectively form a mitotic spindle (in mitosis) and meiotic spindle (in meiosis). It is chiefly involved in moving and segregating the chromosomes during nuclear division.
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.
Synonym: spindle fibre