Fig. 4. klp67A mutants form diminished and unstable central spindles. Selected frames from time-lapse sequences showing central spindle formation in wild-type (A) and (B) klp67A mutant cells. At anaphase onset in wild-type cells (A; 0 minutes) the spindle is linear and has well defined asters that cap each spindle pole. As anaphase ensues the spindle remodels itself into an equatorially positioned central spindle composed of bundles of cytoplasmic, peripheral astral microtubules and interior microtubules within the spindle envelope. This structure initiates and promotes the advancement of the cleavage furrow (20 minutes) before being compacted into a midbody. (B) Central spindle formation is aberrant in klp67A mutants. Before anaphase onset (0 minutes) the cell contained increased numbers of abnormally long astral microtubules that encircled the robust spindle. As the cell advances into anaphase these peripheral astral microtubules collect in the upper portion of the cell, where they initiate a unilateral cleavage furrow that advances but fails to fully cleave the cell (10-29 minutes; arrowheads). Concomitantly, the interior central spindle fails to form ostensible bundles as seen in the wild-type and rapidly degrades (compare arrows at 6 and 29 minutes). The peripheral microtubules are also unstable and as a result this cell lacks a recognisable central spindle. See main text for details. Time is in minutes relative to anaphase onset. Bar, 10 µm.