Could it be that mouse fetal liver cells and adult bone-marrow blood cells originate from a subset of cells that line the blood vessels in the embryo? Several lines of evidence suggest that this is indeed the case.
During embryonic development, haematopoietic stem cells, which give rise to blood cells, and endothelial cells, which line blood vessels, both form from the mesodermal germ-cell layer; but exactly how is debatable. On the one hand, a controversial, century-old theory proposes that both haematopoietic and endothelial cells arise from a mesoderm-derived common precursor called a haemangioblast. On the other hand, a younger theory proposes that haematopoietic stem cells (HSCs) form from a subset of early endothelial cells known as haemogenic endothelium. The relationship between haemangioblasts and haemogenic endothelium has never been resolved. In this issue, however, three papers1,2,3 clarify the potential relatedness and significance of these cell types.
The concept of the haemangioblast initially arose from observations that, in the chick yolk sac, a mesoderm-derived cell can give rise to both primitive red blood cells and endothelial cells. Moreover, the finding that, in the mouse yolk sac, the formation of blood islands — aggregates of blood cells and endothelium — requires the expression of specific genes such as Flk-1 provided further support for the relatedness of the two cell lineages. But the strongest evidence for the existence of haemangioblasts was obtained after the development of the BL-CFC in vitro assay. This assay allows clonal (single-cell) analysis of blast colony-forming cells (BL-CFCs), which are derived from differentiating mouse embryonic stem (ES) cells4. By definition, BL-CFCs can directly form both haematopoietic and endothelial cells, and are therefore the closest detectable equivalent of the theoretical haemangioblasts.
When ES-cell-derived Flk-1-expressing (Flk-1+) mouse cells are grown in culture, characteristic BL-CFC colonies appear that consist of an aggregate of non-adherent blood cells overlying an adherent layer of endothelium. This observation, together with insights4,5 into the molecular regulation of the development and differentiation of colonies that emerge from a BL-CFC (blast colonies), has been enlightening. Nonetheless, little has become clear about the cellular events that herald the generation of blood cells from BL-CFCs.
Lancrin et al.1 (page 892) used time-lapse photography to analyse the sequence of cellular events required for the formation of mature blast colonies from cultured Flk-1+ cells. They find that these colonies form in two stages. First, after 36–48 hours of 'plating' Flk-1+ cells for growth in culture, the cells form tightly adherent clusters. Subsequently, round, non-adherent cells appear, which then proliferate to complete the formation of mature blast colonies. Among the adherent cell clusters at 48 hours, a transient cell population expressing various endothelial (but not mesodermal or BL-CFC) markers appear, displaying the potential to form haematopoietic cells. From this population, both primitive blood-cell colonies eventually form, characterized by their expression of the embryonic version of the haemoglobin protein, together with definitive blood-cell colonies expressing adult haemoglobin.
Lancrin and colleagues' observations suggest that haematopoietic progenitor cells arise from haemangioblasts through a haemogenic endothelial intermediate — the first linear pathway to resolve, at least in vitro, the relationship between haemangioblasts and haemogenic endothelium (Fig. 1). But do these findings alter the definition of the haemangioblast? To answer this question, we must learn more about the fate of haemogenic endothelial cells following the birth of blood cells. For example, it will be interesting to assess whether haemogenic endothelial cells that give rise to primitive blood cells differ from those that produce definitive blood cells.
In many species, HSCs appear as clusters attached to the endothelium that lines the ventral wall of the abdominal aorta during embryonic development; this observation has long implicated the endothelium as the source of developing blood cells. Indeed, when endothelial cells isolated from mouse embryos are grown in culture, a subset has the potential to develop into mature blood cells such as erythroid, myeloid and/or lymphoid cells6. But this and other evidence is only indirect7, and direct proof of haematopoietic progenitor cells emerging from individual endothelial cells has been lacking.
Eilken et al.2 (page 896) tracked the fates of all cells (more than 6,500) generated from individually plated mouse ES-cell-derived mesoderm cells using time-lapse microscopy. Their detailed analysis of the resulting colonies indicates that 1.2% of the colonies display properties of adherent endothelial cells, and that one or more endothelial cells in a colony directly give rise to non-adherent haematopoietic cells. The authors also directly isolated primary endothelial cells with haemogenic potential from early mouse embryos. They therefore demonstrate that haemogenic endothelial cells not only can be generated in vitro from ES cells but are also naturally present in mouse embryos. But the question that these authors2 and Lancrin et al.1 did not address is whether HSCs emerge directly from haemogenic endothelial cells in vivo during mouse development.
In the developing mouse embryo, the transcription factor Runx1 is required for the formation of HSCs and their progenitors. In fact, Runx1 has been considered necessary for the emergence of HSC clusters from the haemogenic endothelium8. Chen et al.3(page 887) show that, within the endothelium, Runx1 expression is indeed essential for the formation of HSCs and their progenitors over a period of roughly 3 days during mouse embryonic development (embryonic days 8.25–11.5). Furthermore, in agreement with another recent report9, they show that most fetal liver cells and adult bone-marrow cells originate from the haemogenic endothelium.
Together, these studies1,2,3 provide substantial evidence that HSCs and their progenitor cells that populate the fetal liver and adult bone marrow originate from the differentiated endothelium that resides in the functional vasculature of the mouse conceptus. The focus can now turn to determining the intriguing molecular mechanisms involved, which might differ between the various embryonic sites of blood-cell production10. What's more, translation of this knowledge to the human system could be of great assistance in generating human HSCs from human ES cells, either by direct cell reprogramming11 or indirectly through induced pluripotent stem cells.
Lancrin, C. et al. Nature 457, 892–895 (2009).
Eilken, H. M., Nishikawa, S.-I. & Schroeder, T. Nature 457, 896–900 (2009).
Chen, M. J., Yokomizo, T., Zeigler, B. M., Dzierzak, E. & Speck, N. A. Nature 457, 887–891 (2009).
Choi, K., Kennedy, M., Kazarov, A., Papadimitriou, J. C. & Keller, G. Development 125, 725–732 (1998).
Huber, T. L., Kouskoff, V., Fehling, H. J., Palis, J. & Keller, G. Nature 432, 625–630 (2004).
Nishikawa, S. I. et al. Immunity 8, 761–769 (1998).
de Bruijn, M. F. et al. Immunity 16, 673–683 (2002).
Yokomizo, T. et al. Genes Cells 6, 13–23 (2001).
Zovein, A. C. et al. Cell Stem Cell 3, 625–636 (2008).
Dzierzak, E. & Speck, N. A. Nature Immunol. 9, 129–136 (2008).
Gurdon, J. B. & Melton, D. A. Science 322, 1811–1815 (2008).
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The Journal of Experimental Medicine (2015)
Journal of Cellular Biochemistry (2015)
Scientific Reports (2015)