K-fibers lacking direct connection to the spindle pole visualized by correlative LM/EM. (A) Sequential focal planes and corresponding 150-nm EM sections of a PtK2 cell with GFP-labeled MTs. All kinetochores present in this part of the cell are shown at twofold higher magnification in insets. K-fibers attached to K1′, K1″, K2″, and K3″ can be traced continuously to the spindle poles (A′). In contrast, K-fiber attached to K3′ appears to terminate ∼2.5 µm from the kinetochore (arrows). (A″) tracing of individual MTs superimposed on GFP fluorescence in the area boxed in A. Notice that minus ends of K-fiber MTs (orange) are in close proximity to a MT bundle that was not traced to a particular kinetochore (i.e., nonkinetochore MT bundle). (B) Consecutive 80-nm EM sections from a full series of a metaphase RPE1 cell. (B′) Traces of MTs in the vicinity of kinetochores superimposed on the corresponding fluorescence planes. Notice short MTs that extend from a kinetochore to wall of MTs in a neighboring K-fiber. Arrows denote potential contacts between MTs.