Sucrose-induced spindle elongation in mitotic PtK-1 cells.

Brief treatment of mitotic metaphase and anaphase PtK-1 cells with tissue culture medium containing 0.5 M sucrose resulted in spindle elongation without chromosome motion. Spindle birefringence also changed from a uniform appearance to one of highly birefringent bundles. Electron microscopic analysis indicated these birefringent bundles were composed of tightly packed arrays of spindle microtubules. No kinetochores could be seen following a 10 min sucrose treatment. Upon removal of sucrose, metaphase spindles returned to pretreatment lengths and the normal birefringence pattern returned. Reduction in spindle length could be temporally coupled with the reappearance of kinetochores and the reassociation of microtubules with these structures. In contrast to treated and released metaphase cells, anaphase spindles did not return to pretreatment lengths. Replacement of sucrose with medium showed the resumption of chromosome-to-pole motion within 2 min of sucrose removal. Chromosome motion could be correlated with the reappearance of kinetochores and kinetochore microtubules. These data have led us to postulate the existence of two microtubule continuums in the spindle and to discuss their roles in spindle organization and chromosome motion.