The opening act: vasculogenesis and the origins of circulation.

Previous studies on cellular and molecular mechanisms that regulate vascular development identified key signaling pathways and transcription factors. These findings supported the notion that the formation of vasculature is predominantly regulated by genetic programs, which is generally accepted. However, recent progress in understanding nongenetic factors that can modify the preprogrammed genetic mechanisms added another layer of complexity to our current understanding of vascular development. Here, we briefly summarize historic viewpoints and evolutionary perspectives on vascular development. We also review the cellular and molecular mechanisms that govern the emergence of the endothelial lineage and the subsequent process of vasculogenesis during development, with an emphasis on vascular endothelial growth factor and angiopoietin signaling cascades. Finally, we discuss epigenetic factors such as hemodynamic forces and hypoxic responses that can modulate and override the predetermined genetic mechanisms of vascular development.