Figure 10.

Models for spindle positioning in mitotic mammalian cells based on pulling forces exerted on astral microtubules. (A) When the spindle is misaligned relative to the long axis of the cell, the lengths of astral microtubules become asymmetric with respect to the spindle axis. Longer microtubules, which interact with more cortically associated dynein, exert stronger forces on the centrosomes and generate a torque toward the long axis of the cell. The forces become symmetric once the spindle is aligned with the long cellular axis, eliminating the torque. (B) A similar mechanism may also center the spindle at the geometric center of the cell. Shorter astral microtubules on the side of the spindle proximal to the cortex exert weaker forces than do longer ones on the opposite side. This produces a net force toward the distal cortex until the spindle reaches a central location.