Mice lacking the vascular endothelial growth factor (VEGF) receptor flt-1 die of vascular overgrowth, and we are interested in how flt-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 flt-1 to produce blood vessels. Flt-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. . Some of the mitogenic signals that promote division of endothelial cells and their precursors are known, but how these signals are modulated to initiate cell divisions only when and where they are needed is not known in detail. After blood vessels initially form, maturation and remodeling steps involve the recruitment of ancillary cells, such as smooth muscle and pericytes. These cells and the extracellular matrix that is also produced can negatively modulate endothelial cell division. [4][5][6][7][8] However, modulators of endothelial cell mitogenesis at the earliest stages of blood vessel formation have not been identified.The vascular endothelial growth factor (VEGF) signaling pathway is clearly critical to both early endothelial cell division and morphogenesis, and its regulation is complex (reviewed in Ferrara & Davis-Smyth 9 and Neufeld et al 10 ). Mouse embryos lacking even one copy of the VEGF gene die in utero with severe vascular defects, and vascular development in differentiating embryonic stem (ES) cells is compromised in VEGF-A ϩ/Ϫ and VEGF-A Ϫ/Ϫ ES cells in a dose-dependent manner. [11][12][13] Moreover, even modestly elevated levels of VEGF lead to vascular abnormalities, 14 and large doses of VEGF invariably severely compromise both vascular development and neovascularization in adult organisms. [15][16][17] These findings suggest that VEGF signaling must be precisely controlled during vascularization to result in proper vessels. The location and duration of VEGF expression provide the first level of control, [18][19][20][21] but other components of the pathway are likely to be involved in fine-tuning the signal.Two high-affinity receptors, flk-1 and flt-1, participate in VEGF signal transduction and are candidates to be involved in fine-tuning mechanisms. Both receptors are membrane-spanning receptor tyrosine kinases that bind VEGF with high affinity, 22-26 but their effects on VEGF signaling are very different. Mice or ES cells lacking flk-1 have little or no blood vessel formation, suggesting that many downstream effects of VEGF on endothelial cells are mediated through flk-1. 27,28 Specifically, numerous studies show that VEGF signaling through flk-1 produces a strong mitogenic signal for endothelial cells. [29][30][31][32] In contrast, VEGF binding to flt-1 does not produce a strong mitogenic signal, and flt-1 Ϫ/Ϫ mice die at mid-gestation with vascular overgrowth and disorganization. 23,29,33 This ph...