It is therefore not surprising that, during embryonic development, the first hematopoietic cells and ECs arise together in close association within the blood islands of the extraembryonic yolk sac and are thought to share a common mesodermal precursor cell type referred to as the hemangioblast ( Choi et al., 1998 Huber et al., 2004). Endothelial cells (ECs), essential to the establishment of a closed circulatory system, are hypothesized to have evolved from amoebocytes, ancestral phagocytic blood cells ( Monahan-Earley et al., 2013). The blood and the vascular systems are intimately connected in their evolutionary and developmental origins. In this review, we will examine the current understanding of how organ- and stage-specific vascular niche specialization contributes to the development of the hematopoietic system. Vascular beds in distinct anatomical sites of extraembryonic tissues and the embryo proper provide the necessary conditions to support these divergent objectives, suggesting a critical role for specialized vascular niche cells in regulating disparate blood cell fates during development. This process must accomplish two important yet divergent goals: the rapid generation of differentiated blood cells to meet the needs of the developing embryo and the production of a reservoir of hematopoietic stem cells to provide for life-long hematopoiesis in the adult. 2Department of Pediatrics, University of Washington School of Medicine, Seattle, WA, United Statesĭuring embryonic development, sequential waves of hematopoiesis give rise to blood-forming cells with diverse lineage potentials and self-renewal properties.1Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, WA, United States.Heck 1, Takashi Ishida 1 and Brandon Hadland 1,2*
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