Taking the Leap: Runx1 in the Formation of Blood from Endothelium.

Blood cell formation in the embryo occurs from multiple anatomic sites and results in the production of hematopoietic stem and progenitor cells that appear in overlapping waves. The transcription factor Runx1 is involved in a dramatic step of this process, for the transition from an endothelial cell that is integrated in a monolayer to a nonadherent circulating blood cell, a process conceptually similar to the epithelial to mesenchymal cell transition. Here we will review the role of Runx1 in the so-called hemogenic endothelium. We will describe the blood cell progenitors for which Runx1 is required, the proximal upstream transcription factors and signaling events that regulate its expression, and some of its important downstream targets in the hemogenic endothelium.