Architecture of metaphase spindles and outer kinetochores. (A) Cryotomographic slice (18 nm) from a Cdc20-depleted cell. Major features are annotated: cell membrane (yellow), mitochondria (salmon), endoplasmic reticulum (white), nucleus (blue). The black dashes outline the spindle. A few spindle MTs are indicated with magenta arrowheads. (B) 3D model of a half spindle, spanning seven sequential sections. Dark and light blue: inner and outer nuclear membranes. Magenta tubes: spindle MTs. Green rings: Dam1C/DASH. Inset: schematic showing the structures that are modeled (saturated shading) and those that are not (washed-out shading). (C) Left, enlargement of the spindle modeled in B and rotated to a view perpendicular to the spindle’s axis. Right, transverse view of the same spindle; for clarity, polar MTs are omitted. Because the short axis crosses multiple cryosection interfaces, we are uncertain how long this spindle was in the unsectioned cell. This particular spindle also has an oval cross section due to microtomy compression along the x axis of the right panel. Black arrow: one example of a kMT with two partial rings. (D–F) Cryotomographic slices (6 nm) of Dam1C/DASH rings around kMTs. Green arrows point to bridges. The bottom panels show schematics of the Dam1C/DASH (green), kMT (magenta), and kMT-associated protein (gray) densities. D and F show front views of a complete and partial ring, respectively. E shows a side view of a complete ring. (G) Rotationally averaged density maps of two individual complete Dam1C/DASH rings in vivo, masked to exclude the kMT, contoured at one SD above the mean. Top row, front view. The middle and bottom rows are sequentially rotated 45° around the horizontal axis. Green arrowheads: protrusions. The plus and minus signs indicate the polarity of the encircled kMT. If four or more decamers (outlined by blue dashes) were absent, there would be a gap >25 nm.