Therefore a major goal remains to understand the complex cel

Therefore, a major goal remains to understand the complex cellular and molecular network that controls HSC generation and expansion. In fact, HSCs that are mainly slow cycling (Wilson et al., 2008) and reside in the bone marrow during the entire life of an individual, are not generated there. They are produced and expanded during embryonic development before colonizing in the bone marrow prior to birth (Christensen et al., 2004). Thus, studying HSC development in the embryo is a powerful tool and a reliable model to decipher the complex characteristics and regulation of HSCs. We will describe in this review the ontogeny of the hematopoietic system in different animal models, the controversies concerning the cellular and anatomical origin of HSCs and the recent insights into the field, notably documented by in vivo confocal imaging approaches.
The journey of a hematopoietic stem cell: From embryo to adult Embryonic hematopoiesis has been studied in various animal models (e.g. amphibian, avian, rodent), but the focus of this chapter will mainly be on the mouse embryonic model. Adult HSCs are generated during embryonic development. The first hematopoietic src inhibitors produced in the embryo are differentiated cells that are most likely needed at the time for proper oxygenation and protection of the developing embryo. The initial site of hematopoietic production is the yolk sac (YS) (Moore and Metcalf, 1970; Palis et al., 1999; Ferkowicz and Yoder, 2005). Starting at embryonic day (E)7.25 of mouse development, this extra-embryonic tissue, composed of both visceral endoderm and mesoderm, transiently produces large nucleated erythrocytes termed erythroblasts (Fig. 1A) (Palis et al., 1999), as well as some macrophages and megakaryocytes (Xu et al., 2001). A layer of endothelial cells will surround these first blood cells to form specialized structures called blood islands (Ferkowicz and Yoder, 2005; Haar and Ackerman, 1971). Some evidence (reviewed later) led to the idea that both cell types (hematopoietic and endothelial) are generated from a common mesodermal precursor known as the hemangioblast (Ferkowicz and Yoder, 2005; Shalaby et al., 1995, 1997; Eichmann et al., 1997; Choi et al., 1998; Huber et al., 2004). Mature cells can easily be identified directly under the microscope by specific morphological criteria, or by analysis of surface marker expression using, for example, immunohistochemistry. On the other hand, hematopoietic progenitors are identified retrospectively in a short-term in vitro assay (clonogenic assay). In such a culture system, different types of lineage-restricted progenitors (erythroid, granulocytic, macrophagic and/or megakaryocytic) can be identified and quantified for their ability to form colonies by looking at specific characteristics (e.g. composition, size, color). By using such assays, it was shown that beginning at E7.5-E8, erythroid-myeloid progenitors start to appear in the YS (Moore and Metcalf, 1970; Palis et al., 1999) and in the chorion and allantois (which later fuse to form the placenta and the umbilical cord) (Palis et al., 1999; McGrath et al., 2011; Alvarez-Silva et al., 2003; Corbel et al., 2007; Zeigler et al., 2006). In addition, it is possible to identify erythroid-myeloid cells in E6.75 embryos once the cells have been co-cultured on OP9 stromal cell lines (Furuta et al., 2006) (Fig. 1B). Slightly later, at E8.5, erythroid–myeloid–lymphoid progenitors are found in the YS and the Para-aortic Splanchnopleura (P-Sp, region formed by the dorsal aorta, omphalomesenteric (or vitelline) artery, gut and splanchnopleura) (Godin et al., 1995). Indeed, B progenitors were identified after co-culture of the different tissues on S-17 stromal cell line, while T progenitors were identified in a fetal thymic organ culture assay (Fig. 1A). Culturing of YS and P-Sp before the establishment of circulation showed that these multipotent progenitors are of P-Sp origin (Fig. 1B) (Cumano et al., 1996). At E10, CFU-S are also detected in the YS and the intra-embryonic Aorta-Gonad-Mesonephros region (AGM, region corresponding to the earlier P-Sp) (Medvinsky et al., 1993) (Fig. 1A).