Spindle–F-actin interactions in mitotic spindles in an intact vertebrate epithelium

Mitotic spindles are well known to be assembled from and dependent on micro-tubules. In contrast, whether actin filaments (F-actin) are required for or are even present inmitotic spindles has long been controversial. Here we have developed improved methods forsimultaneously preserving F-actin and micro-tubules in fixed samples and exploited them todemonstrate that F-actin is indeed associated with mitotic spindles in intact Xenopus laevisembryonic epithelia. We also find that there is an “F-actin cycle,” in which the distribution andorganization of spindle F-actin changes over the course of the cell cycle. Live imaging using aprobe for F-actin reveals that at least two pools of F-actin are associated with mitotic spindles: a relatively stable internal network of cables that moves in concert with and appears to be linked to spindles, and F-actin “fingers” that rapidly extend from the cell cortex towardthe spindle and make transient contact with the spindle poles. We conclude that there is arobust endoplasmic F-actin network in normal vertebrate epithelial cells and that this networkis also a component of mitotic spindles. More broadly, we conclude that there is far moreinternal F-actin in epithelial cells than is commonly believed.