Mitotic assembly of the nuclear envelope on the chromosome mass. Live-cell images were acquired from mitotic HeLa cells expressing GFP-Sec61β and H2B-mCherry. (A) 2D time-lapse series showing formation of the nuclear envelope on the chromosome mass (also see Video 1). Arrows indicate the rim of the chromosome mass, at which ER cisternae make initial contacts (0 s). Arrowheads follow the migrating edge of the nascent nuclear envelope from the rim to the center of the chromosome mass (120–420 s). The monochrome images highlight the nuclear envelope. (B) Assembly of the nuclear envelope followed in 3D. Deconvolved images from a mitotic cell acquired at various time points in 3D. The top row shows 2D images from the middle section. The dotted line labeled spindle axis connects the two spindle poles. The columns below show 3D renderings of the right chromosome mass together with the associated nuclear envelope. For clarity, only the bottom half of the chromosome mass is shown. The three time points (−240 s, 0 s, and 120 s) are from the same anaphase cell. The 0-s time point corresponds approximately to the onset of nuclear envelope assembly. The image set labeled completion is from another cell that has just completed nuclear envelope assembly. Views 1–3 show three regions on the chromosome mass surface. View 1 shows spindle pole proximal side; View 2 shows the rim; View 3 shows the spindle pole distal side. Bars, 10 µm.