The entire framework of microtubules (MTs) in the mitotic apparatus of Ochromonas danica is reconstructed (except at the spindle poles) from transverse serial sections. Eleven spindles were sectioned and used for numerical data, but only four were reconstructed: a metaphase, an early anaphase, a late anaphase, and telophase. Four major classes of MTs are observed: (a) free MTs (MTs not attached to either pole); (b) interdigitated MTs (MTs attached to one pole which laterally associate with MTs from the opposite pole); (c) polar MTs (MTs attached to one pole); (d) kinetochore MTs (kMTs). Pole-to-pole MTs are rare and may be caused by tracking errors. During anaphase, the kMTs, free MTs, and polar MTs shorten until most disappear, while interdigitated MTs lengthen. In the four reconstructed spindles, the number of MTs decreases between early anaphase and telophase from 881 to 285, while their average length increases from 1.66 to 4.98 micron. The total length of all the MTs 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 MTs. Such MTs 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 MTs directly provide the force that elongates the spindle, either by MT polymerization alone or by MT sliding with concomitant MT polymerization.

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