Cleavage furrow establishment by the moving mitotic apparatus

The midpoint of the mitotic apparatus is fixed in the future division plane long before the division mechanism develops, and this static relationship has been considered essential in speculations concerning division mechanism establishment. The purpose of the present investigation was to determine whether prevention of the static relationship affects the establishment process. Sand dollar eggs were reshaped into cylinders by confinement in an elastic capillary tube and, beginning about 20 min before cleavage, the mitotic apparatus was kept in reciprocal motion by alternately compressing the poles. When the movement was continuous and the excursions were 25, 50 or 75 microm, furrow activity developed near the midpoint of the region underlain by the mitotic apparatus. The acuteness of the furrow decreased as the distance the mitotic apparatus was moved increased. When the movement was made discontinuous by allowing the mitotic apparatus to pause at the end of each excursion, the results depended upon the duration of the pause. Pauses 30 s long resulted in a single furrow formed in the midpoint of the entire region underlain by the mitotic apparatus. When the pauses were 45 s long, furrowing activity developed in both regions where the mitotic apparatus was allowed to pause. The results indicated that the normal static relation between the mitotic apparatus midpoint and the division plane is unnecessary for division mechanism establishment. They also demonstrate that a restricted region of contractile activity can be established in the cortex despite experimentally induced spreading and dilution of mitotic apparatus effect.