Endothelial cell divisions are increased in the absence of flt-1 Day-6 WT-eGFP (+/+;A) or flt-1^-/--eGFP (-/-; B) ES differentiation cultures were imaged by time lapse at one-minute intervals for the indicated times. Day-8 cultures of both genotypes were also imaged for quantitation (C). (A-B) Endothelial cell predivision rounding up is denoted by white arrows, and cytokinesis is denoted by gray arrows. Time in minutes is at the bottom left. Note that over a similar time span there are more divisions scored in the flt-1^-/- mutant vessels (Supplemental Movies S6-S7). Original magnification × 20. (C) Time-lapse movies from day-6 WT-eGFP (n = 6), day-6 flt-1^-/--eGFP (n = 8), day-8 WT-eGFP (n = 8), and day-8 flt-1-/--eGFP (n = 8) were analyzed for a mitotic score (no. of endothelial cell divisions/eGFP-positive area/h). ^***P < .001.