Activation of the old SPB is accompanied by compromised nuclear integrity and defective SPB insertion in cut12.1 cells at 36°C. (A–C) Transmission electron micrographs of different nuclei where a disruption of the nuclear envelope (NE) is found next to the SPBs. (A) Two SPBs linked by the bridge structure. (B) A single noninserted SPB adjacent to a large hole in the envelope. (C) The cell is clearly in mitosis, as microtubules (MT) emerge from the SPB adjacent to the hole in a characteristic V-shape monopolar fashion. (D) Nucleoplasm leaking into the cytoplasm. (E) An example of an active SPB that has fallen into the nucleus. Note the herniation of the membrane arising from the polymerization of microtubules from this SPB to a point on the membrane that is devoid of an SPB. Similar images arise in cut11 mutant mitoses (West et al., 1998). HE, hole edge. Bars, 500 nm.