Ding R, McDonald K L, McIntosh J R
Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder 80309-0347.
J Cell Biol. 1993 Jan;120(1):141-51. doi: 10.1083/jcb.120.1.141.
Mitotic spindles of Schizosaccharomyces pombe have been studied by EM, using serial cross sections to reconstruct 12 spindles from cells that were ultrarapidly frozen and fixed by freeze substitution. The resulting distributions of microtubules (MTs) have been analyzed by computer. Short spindles contain two kinds of MTs: continuous ones that run from pole to pole and MTs that originate at one pole and end in the body of the spindle. Among the latter there are three pairs of MT bundles that end on fibrous, darkly staining structures that we interpret as kinetochores. The number of MTs ending at each putative kinetochore ranges from two to four; all kinetochore-associated MTs disappear as the spindle elongates from 3-6 microns. At this and greater spindle lengths, there are no continuous MTs, only polar MTs that interdigitate at the spindle midzone, but the spindle continues to elongate. An analysis of the density of neighboring MTs at the midzone of long spindles shows that their most common spacing is approximately 40 nm, center to center, and that there is a preferred angular separation of 90 degrees. Only hints of such square-packing are found at the midzone of short spindles, and near the poles there is no apparent order at any mitotic stage. Our data suggest that the kinetochore MTs (KMTs) do not interact directly with nonkinetochore MTs, but that interdigitating MTs from the two spindle poles do interact to form a mechanically stable bundle that connects the poles. As the spindle elongates, the number of MTs decreases while the mean length of the MTs that remain increases. We conclude that the chromosomes of S. pombe become attached to the spindle by kinetochore MTs, that these MTs disappear as the chromosomes segregate, that increased separation of daughter nuclei is accompanied by a sliding apart of anti-parallel MTs, and that the mitotic processes of S. pombe are much like those in other eukaryotic cells.
通过电子显微镜（EM）对粟酒裂殖酵母的有丝分裂纺锤体进行了研究，使用连续横截面从经冷冻置换超快速冷冻和固定的细胞中重建了12个纺锤体。所得微管（MTs）的分布已通过计算机进行分析。短纺锤体包含两种微管：从一极延伸到另一极的连续微管，以及起源于一极并终止于纺锤体中部的微管。在后者中，有三对微管束终止于纤维状、深色染色的结构，我们将其解释为动粒。终止于每个假定动粒的微管数量在2到4根之间；随着纺锤体从3 - 6微米伸长，所有与动粒相关的微管都会消失。在这个及更长的纺锤体长度下，没有连续的微管，只有在纺锤体中区相互交错的极微管，但纺锤体仍继续伸长。对长纺锤体中区相邻微管密度的分析表明，它们最常见的中心间距约为40纳米，并且存在90度的优选角间距。在短纺锤体的中区仅发现这种方形排列的些许迹象，并且在任何有丝分裂阶段，纺锤体两极附近都没有明显的秩序。我们的数据表明，动粒微管（KMTs）不直接与非动粒微管相互作用，但来自两个纺锤体极的交错微管确实相互作用形成一个机械稳定的束，连接两极。随着纺锤体伸长，微管数量减少，而剩余微管的平均长度增加。我们得出结论，粟酒裂殖酵母的染色体通过动粒微管附着于纺锤体，这些微管在染色体分离时消失，子核分离增加伴随着反平行微管的相互滑动，并且粟酒裂殖酵母的有丝分裂过程与其他真核细胞非常相似。
