Branching morphogenesis is reduced in the absence of flt-1 Day-8 WT-eGFP (+/+; Ai-iii) or flt-1^-/--eGFP (-/-; Aiv-vi) ES differentiation cultures were fixed and reacted with an anti-PECAM antibody. (A) Note that while each genotype had a range of branching complexities, on average less complexity was seen in the flt-1^-/--eGFP cultures. Scale bar is 50 μm. (B) The average number of branch points per millimeter of vessel length was determined using 10 fields of each genotype in 2 experiments and expressed as a percentage of wild-type branching. ^***P < .001.