As the spindle apparatus forms, a microtubule can take on three possible roles. It can become a kinetochore microtubule, a polar microtubule, or an astral microtuble. Strictly speaking, only the first two (kinetochore and polar) of these three types make up the spindle apparatus, but all three play an essential role in moving the chromosomes.

A kinetochore microtubule, as its name suggests, is the type of microtubule that attaches to the kinetochores of the condensed chromosomes (this is the type that draws the chromosomes toward the centrosomes).

A polar microtubule does not attach to a chromosomes. Instead, microtubules of this type emanate from the two centrosomes and overlap with each other in the center of the cell and, by pushing against each other, force the centrosomes toward the poles of the cell.

An astral microtubule extends outward from the centrosome to the cell membrane. It pulls the centrosome toward one of the poles. The actual force causing chromosome movement is produced by motor proteins.