Directionally controlled spindle disassembly after mitosis in the diatom Pinnularia.

Two aspects of late mitosis have been investigated further. First, the two interdigitated half spindles that form the central spindle of this organism appear to separate before their disassembly. This separation is not dependent upon the cleavage furrow breaking the spindle as appears to be the case of observing normal mitosis; it occurs even when cleavage is artificially prevented with cytochalasin D. After separation, the birefringence of each half spindle fades steadily from the overlap end (now, the "free" end) back to the pole. Disassembly commences about 1 1/2 min after the half spindles separate, synchronously in each daughter cell. This observation suggests that the microtubules in each half spindle are of uniform polarity and their disassembly, presumably influenced by this polarity, proceeds from only one end (the "free" end). The other end, however, may be "capped" by polar structures. Because there is this pause between separation and the initiation of disassembly, breakdown of microtubules is probably not initiated merely by their ends becoming free in the cytoplasm.