Physical theory of the orientation of astral mitotic spindles.

A physical theory was developed for mitotic spindle orientation. This is important because spindle position is known to determine the placement of the cleavage furrow separating the offspring of a mitosis. The theory is based on an equation for the force exerted on spindle poles by the interaction of astral microtubules with the cell surface. Expected spindle placements are positions of stable equilibrium where the net force and torque resulting from the action of the astral microtubules on the spindle poles is zero. The theory provides a novel physical explanation of cleavage patterns in the early embryo.