K-fibers lacking direct connection to the spindle pole visualized by confocal LM and array tomography. (A) Metaphase spindle in RPE1 cell (maximal-intensity projection). Series of individual focal planes separated by 200 nm are shown for the boxed area. Notice a short bundle of MTs that emanates from one kinetochore. This bundle terminates in the vicinity of a different kinetochore (arrow) and appears to form a side connection to this kinetochore′s K-fiber. (B) As in A, but the cell is in early anaphase. Red, MTs (also shown in grayscale); green, kinetochores (CenpA-GFP) and centrioles (Centrin-GFP). (C) Metaphase RPE1 cell reconstructed via array tomography (see Materials and methods). Left panel presents a maximal-intensity projection of the entire reconstructed volume. Notice the high density of MTs in cells that become apparent with this approach. Consecutive 200-nm sections are shown at higher magnification for the boxed area. Arrows indicate the termination of short MT bundles emanating from kinetochores.