Abstract
Rat kangaroo kidney epithelium (PtK2) cells develop prominent asters and spindles during anaphase B of mitosis. It has been shown that severing the spindle at early anaphase B in living PtK1 cells results in a dramatic increase in the rate of pole-pole separation. This result suggested that the asters pull on the spindle poles, putting tension on the spindle, while the spindle acts as a governor, limiting the rate of pole separation. To further test these inferences, we used a UV-laser microbeam to damage one of the two asters in living PtK2 cells at early anaphase B and monitored the effects on individual spindle pole movements, pole-pole separation rates and astral microtubules (MTs). Irradiation at the estimated position of a centrosome greatly reduced its array of astral MTs and nearly stopped the movement of the irradiated pole, whereas the sister pole retained its normal array of astral MTs and actually accelerated. Control irradiations, either close to the estimated position of the centrosome or beside the spindle at the equator, had little or no effect on either spindle pole movements or astral MTs. These results support the inferences that during anaphase B in living PtK cells, the central spindle is under tension generated by pulling forces in the asters (presumably MT-mediated) and that the spindle generates counterforces that limit the rate of pole separation. The results also suggest that the central spindle in living PtK cells may be able to generate a pushing force.
Highlights
Mitotic anaphase commonly occurs in two stages: during anaphase A, the sister chromatids migrate toward the spindle poles, and during anaphase B the central spindle elongates as pole-pole separation continues (Inoué and Ritter, 1975)
Maximum size typically occurs during anaphase, it has often been assumed that the asters do not take an active role in separating the chromosomes (Cande and Hogan, 1989; Fuge, 1977; Mazia, 1961; McIntosh and McDonald, 1989)
There is compelling evidence that mitotic asters pull on the spindle poles and help to elongate the central spindle during anaphase B (Aist et al, 1991; Aist and Berns, 1981)
Summary
Mitotic anaphase commonly occurs in two stages: during anaphase A, the sister chromatids migrate toward the spindle poles, and during anaphase B the central spindle elongates as pole-pole separation continues (Inoué and Ritter, 1975). Maximum size typically occurs during anaphase, it has often been assumed that the asters do not take an active role in separating the chromosomes (Cande and Hogan, 1989; Fuge, 1977; Mazia, 1961; McIntosh and McDonald, 1989). There is compelling evidence that mitotic asters pull on the spindle poles and help to elongate the central spindle during anaphase B (Aist et al, 1991; Aist and Berns, 1981). There is considerable evidence that asters in animal cells have a similar role in anaphase B. This evidence has come from five different species and a variety of approaches, including descriptive cytology of normal and aberrant mitoses (Bajer et al, 1980), destruction of microtubules (MTs) by the application of heat (Bergan, 1960), treatment with MT-altering drugs (Daub and Hauser, 1988; Hiramoto et al, 1986) and micromanipulation (Hiramoto and Nakano, 1988; Kronebusch and Borisy, 1982)
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