An analysis of spindle ultrastructure during prometaphase and metaphase of micronuclear division in Tetrahymena

Mitotic micronuclei were isolated from Tetrahymena thermophila in a medium containing hexylene glycol and their ultrastructure was analyzed using thin section techniques. The two stages selected for analysis were early prometaphase and metaphase. A comparison of data from these two stages revealed several differences in nuclear morphology. Metaphase nuclei were longer, they contained more microtubules, and the distribution of microtubules at metaphase was different from that at early prometaphase. Increases in microtubule number and length were clearly evident in peripheral sheath microtubules, which are a unique class of microtubules that can be distinguished from other classes on the basis of their close association to the nuclear membrane. Growth of peripheral sheath microtubules is thought to be significant because it could be the mechanical basis of nuclear elongation. Crossbridges were observed throughout the spindle between all classes of microtubules, but the exact function of these elements remains to be determined.