Figure 3.

A spindle pole body and its associated microtubules from the budding yeast, Saccharomyces cerevisiae (O'Toole et al. 1999). A shows a 3-nm slice from a tomographic reconstruction of one end of a forming mitotic spindle. Six layers of the spindle pole body are indicated by numbers (1, outer plaque [OP]; 2, inner layer 1 [IL1]; 3, inner layer 2 [IL2]; 4, central plaque [CP]; 5, inner plaque [IP]; 6, the minus ends of the spindle microtubules [MT]). The six small panels to the right of A are slices cut from the tomogram perpendicular to the slice shown on the left, and the placement of the slices corresponds to the positions of the labeled lines in A on the left. Order is quite evident in the arrangement of subunits in IL2. However, the shape of the plaque is distorted by the collapse of the section under the influence of the electron beam (see O'Toole et al. 1999 for details). B and C show minus ends of microtubules at IP and OP respectively (arrows). Both spindle and cytoplasmic microtubules are capped, though the shapes of the caps are not the same. D shows the plus ends of two spindle microtubules, revealing their flared shape (arrows). Reprinted with modifications and with permission from the authors and Molecular Biology of the Cell. Bars: (A) 50 nm; (D) 25 nm.