Spindle dynamics during mitosis in Dictyostelium discoideum.

We studied mitosis in vegetative amebae of Dictyostelium discoideum by phase-contrast microscopy of live and flat-embedded cells, by time-lapse microcinematography, and by electron microscopy of ultrathin sections. The mean time from early prophase to the end of cytokinesis was 10 min 32 s (range: 6 min 23 s to 15 min 40 s). The start of anaphase divided the mean total time for mitosis into two almost equal intervals. Spindle elongation was analyzed in detail in 10 amebae. It was composed of an active first and a passive second phase. The mean duration of the active phase was 94.7 s, the mean increase in spindle length for that interval was 6.1 micrometers, and the mean velocity of elongation was 4.0 micrometers/min. The spindle possibly elongates via an interaction between interdigitating microtubules of the central spindle (cf. [14]), but the mean elongation factor of 2.9 indicates that these microtubules themselves must elongate concomitantly.