The anaphase B spindle in the fungus Nectria haematococca elongates considerably at a nearly constant rate, regulating the rate at which the asters are allowed to separate the spindle pole bodies (SPBs). To elucidate the ultrastructural basis of the elongation process, we determined the three-dimensional architecture of spindles by reconstructing them from serial thin-sections of freeze-substituted mitotic cells. We also quantified the microtubules (MTs) of the spindles using the same digitized data sets and illustrated the ultrastructure of the kinetochores (KCs). KCs were found to have a fibrillar component that bridges the gap between the end of the kinetochore microtubule (kMT) and the chromatin. The kMTs were inserted in the KCs at metaphase but were pulled out during anaphase A. The transition from metaphase to anaphase was further characterized by narrowing of the spindle, bundling of MTs, and a large reduction in the number and total length of MTs as well as in the number of polar MTs (pMTs). Thereafter, as the spindle continued to elongate and narrowed progressively through late anaphase B, the shortest MTs (< 0.5-mu-m long) were selectively depolymerized, whereas longer ones in all length classes were maintained in similar numbers and proportions. There was no increase in either the number of longer MTs or the maximum length of MTs during anaphase, suggesting that MT growth did not contribute to spindle elongation. Furthermore, active sliding between antiparallel pMTs could not have contributed to spindle elongation, because there was no such MT overlap during anaphase. A careful examination of extant models of spindle elongation in light of these and related findings has led us to favor, instead, a telescoping model for N. haematococca in which the bundled MTs at the beginning of anaphase B are pulled out and slid passively past each other by astral forces acting on the SPBs, to form a very narrow, highly elongated central spindle by the end of anaphase B.
