Cells undergoing endomitosis have normal mitotic timings but regress their cytokinetic furrow during late M phase. (A) Representative stills from live imaging of Tubulin-mNeon/E-cadherin-tdTomato Hep-Org 2 line showing canonical (top) and endomitosis (bottom) M phases. Stills show formation of central spindle in both canonical mitosis and endomitosis, with subsequent membrane regression in endomitosis (marked with arrow) and midbody severing (marked with asterisk). Time is relative to NEB in h:min. Scale bars represent 50 µm. Panels 3–6 showing β-tubulin are maximum projections of two z-slices. (B) Duration of NEB to NER in minutes for canonical, endomitosis, and polyploid M phases in Hep-Org 1 line expressing GFP-NLS/E-cadherin-tdTomato (N = 9 experiments) and Hep-Org 2 line expressing Tubulin-mNeon (N = 6 experiments). Individual measurements are shown for canonical (Hep-Org 1, n = 204 events; Hep-Org 2, n = 62 events), endomitosis (Hep-Org 1, n = 16 events; Hep-Org 2, n = 18 events), and polyploid (Hep-Org 1, n = 7 events; Hep-Org 2, n = 11 events) M phases. Black bars indicate mean (ns = not significant, **P < 0.01, Student’s t test, two-tailed). (C) Duration from midbody assembly to severing in minutes for canonical (n = 62 events), endomitosis (n = 18 events), and polyploid (n = 11 events) M phases in Hep-Org 2 line expressing Tubulin-mNeon (N = 5 experiments). Individual measurements are shown with mean (black bar) (**P < 0.01, Student’s t test, two-tailed). (D) Duration from NEB to furrowing onset for canonical (n = 32 events) and endomitosis (n = 13 events) M phases in Hep-Org 1 line expressing GFP-NLS/E-cadherin-tdTomato (N = 5 experiments). Individual measurements are shown with mean (black bar) (ns = not significant, Student’s t test, two-tailed). (E) Duration from furrowing onset to cytokinetic regression in endomitosis M phase in Hep-Org 1 line expressing GFP-NLS/E-cadherin-tdTomato (n = 11 events). Individual measurements are shown with mean (black bar).