INTERPOLAR SPINDLE MICROTUBULES IN PTK CELLS

Spindle microtubules (MTs) in PtK1 cells, fixed at stages from metapha se to telophase, have been reconstructed using serial sections, electr on microscopy, and computer image processing. We have studied the clas s of MTs that form an interdigitating system connecting the two spindl e poles (interpolar MTs or ipMTs) and their relationship to the spindl e MTs that attach to kinetochores (kMTs). Viewed in cross section, the ipMTs cluster with antiparallel near neighbors throughout mitosis; th is bundling becomes much more pronounced as anaphase proceeds. While t he minus ends of most kMTs are near the poles, those of the ipMTs are spread over half of the spindle length, with at least 50% lying >1.5 m um from the poles. Longitudinal views of the ipMT bundles demonstrate a major rearrangement of their plus ends between mid- and late anaphas e B. However, the minus ends of these MTs do not move appreciably fart her from the spindle midplane, suggesting that sliding of these MTs co ntributes little to anaphase B. The minus ends of ipMTs are markedly c lustered in the bundles of kMTs throughout anaphase A. These ends lie close to kMTs much more frequently than would be expected by chance, s uggesting a specific interaction. As sister kinetochores separate and kMTs shorten, the minus ends of the kMTs remain associated with the sp indle poles, but the minus ends of many ipMTs are released from the kM T bundles, allowing the spindle pole and the kMTs to move away from th e ipMTs as the spindle elongates.