Organization of spindle microtubules in Ochromonas danica.

Abstract

The entire framework of microtubules (MT) in the mitotic apparatus of O. danica is reconstructed (except at the spindle poles) from transverse serial sections. Spindles (11) were sectioned and used for numerical data, but only 4 were reconstructed: a metaphase, an early anaphase, a late anaphase and telophase. Four major classes of MT are observed: free MT (MT not attached to either pole); interdigitated MT (MT attached to 1 pole which laterally associate with MT from the opposite pole); polar MT (MT attached to 1 pole); kinetochore MT (kMT). Pole-to-pole MT are rare and may be caused by tracking errors. During anaphase, the kMT, free MT, and polar MT shorten until most disappear, while interdigitated MT lengthen. In the 4 reconstructed spindles, the number of MT decreases between early anaphase and telophase from 881-285, while their average length increases from 1.66 to 4.98 .mu.m. The total length of all the MT in the spindle (placed end to end) remains at 1.42 .+-. 0.04 mm between these stages. At late anaphase and telophase the spindle is comprised mainly of groups of interdigitated MT. Such MT from opposite poles form a region of overlap in the middle of the spindle. During spindle elongation (separation of the poles), the length of the overlap region does not decrease. These results are compatible with theories that suggest that MT directly provide the force that elongates the spindle, either by MT polymerization alone or by MT sliding with concomitant MT polymerization.