The roles of BMP and IL-3 signaling pathways in the control of hematopoietic stem cells in the mouse embryo.

During mouse ontogeny, the first adult-type hematopoietic stem cells (HSC) are autonomously generated at mid-gestation in the AGM (Aorta-Gonad-Mesonephros) region. Successively present in different anatomical sites where they will expand, HSCs will finally colonize the bone marrow (BM) where they will reside during the entire adult life. In the bone marrow, both HSC self-renewal and differentiation are controlled at cellular and molecular levels by interactions with the stromal microenvironment. So far, very little is known about the extracellular factors involved in the regulation of embryonic HSC emergence, survival and expansion. In the present review, we outline the BMP and IL-3 signaling pathways that are critical for the growth and potential of embryonic HSCs. We will also discuss how these pathways might be integrated with the ones of Notch and Mpl/thrombopoietin, also identified as important key regulators of AGM HSC activity.