Mice lacking the vascular endothelial growth factor (VEGF) receptor fit-1 die of vascular overgrowth, and we are interested in how fit-1 normally prevents this outcome. Our results support a model whereby aberrant endothelial cell division is the cellular mechanism resulting in vascular overgrowth, and they suggest that VEGF-dependent endothelial cell division is normally finely modulated by fit-1 to produce blood vessels. Fit-1(-/-) embryonic stem cell cultures had a 2-fold increase in endothelial cells by day 8, and the endothelial cell mitotic index was significantly elevated before day 8. Fit-1 mutant embryos also had an increased endothelial cell mitotic index, indicating that aberrant endothelial cell division occurs in vivo in the absence of fit-1. The fit-1 mutant vasculature of the cultures was: partially rescued by mitomycin C treatment, consistent with a cell division defect in the mutant background. Analysis of cultures: at earlier time points showed no significant differences until day 5, when fit-1 mutant cultures had increased beta-galactosidase(+) cells, indicating that the expansion of fit-1 responsive cells occurs after day 4. Mitomycin C treatment blocked this early expansion, suggesting that aberrant division of angioblasts and/or endothelial cells is a hallmark of the fit-1 mutant phenotype throughout vascular development. Consistent with this model is the finding that expansion of platelet and endothelial cell adhesion molecule(+) and VE-cadherin(+) vascular cells in the fit-1 mutant background first,occurs between day 5 and clay 6. Taken together, these data show that fit-1 normally modulates vascular growth by controlling the rate of endothelial cell division both, in vitro and in vivo. (C) 2002 by the American Society of Hematology.
