Origin of Hematopoietic Stem Cells
Authors: Mikkola Hanna 
Published: October 2019

Abstract
Abstract SCI-40
During development, the embryo needs to rapidly produce differentiated blood cells to provide oxygen for its survival and growth, as well as establish a pool of undifferentiated HSCs for lifelong hematopoiesis. These opposing goals are achieved by segregating fetal hematopoiesis into multiple waves that occur in distinct anatomical niches that promote either differentiation or “stemness”. However, it has been unclear which anatomical sites and cellular precursors give rise to the different hematopoietic waves, impeding our ability to study the regulatory mechanisms dictating the fate of these cells. Understanding the development of self-renewing HSCs in the embryo will be crucial for generating these cells in vitro from pluripotent cells for therapeutic purposes. Recent studies have verified that HSCs develop through a hemogenic endothelial intermediate. Using Ncx1−/− (sodium/calcium exchanger) mouse embryos that are unable to initiate heartbeat and circulation, we showed that multipotential myelo-lymphoid HSPCs can be generated in the embryo proper, the yolk sac and the placenta. This implies that hemogenic endothelium extends more broadly than previously thought, spanning from the major intra-embryonic arteries to extra-embryonic hematopoietic tissues. In order to understand how hemogenic endothelium is established, we defined genomewide target genes for Scl/Tal1, a bHLH transcription factor that initiates hematopoietic specification from mesoderm. Our studies indicate that Scl governs the divergence of multiple mesodermal fates, activating major hemato-vascular transcription factor networks required for the establishment of hemogenic endothelium and generation of HSCs, as well as repressing regulators of competing mesodermal fates. Imbalance in these mesodermal networks in Scl-deficient embryos results in complete loss of hematopoietic cells, impaired establishment of hemogenic endothelium and profound cardiac defects with disorganized endocardium and ectopic activation of myocardial and mesenchymal transcriptional networks. These studies reveal a much broader role for Scl than previously anticipated and delineates Scl as a master regulator of mesoderm specification that coordinates proper development of both the blood and circulatory systems. The intimate relationship between the development of these mesodermal organ systems was also evidenced through studies using the heartbeat deficient Ncx1−/− embryos, which revealed that blood flow is essential for the emergence of HSCs from hemogenic endothelium in the placenta and in the embryo. Interestingly, the development of the lineage-restricted progenitors in the yolk sac was not affected, thus providing a unique opportunity to investigate the mechanisms that regulate HSC development specifically. We show that in the absence blood flow, hemogenic precursors are unable to be released from hemogenic endothelium to circulation and accumulate in placental vasculature. Our studies suggest that shear forces and changes in oxygen tension prompted by circulating red cells are required for suppressing endothelial signaling pathways, releasing adherens junctions in hemogenic endothelium and completing the emergence of HSCs. These studies emphasize the highly dynamic nature of the embryonic hematopoietic microenvironments and pinpoint the requirement of the earliest embryonic blood cells for proper HSC development.

Disclosures:
No relevant conflicts of interest to declare.