Ontogeny of Hematopoiesis

The current challenge in hematopoietic transplantation and regeneration therapies is acquiring and/or producing a reliable and plentiful source of hematopoietic stem cells (HSCs). Given that HSCs from bone marrow, peripheral, or umbilical cord blood undergo only limited/no expansion ex vivo, there is a high interest in understanding how the adult cohort of multipotent self-renewing HSCs are generated and expanded during embryonic development. The development of HSCs in vertebrate embryos begins in the major vasculature. HSCs are generated in a short window of developmental time starting at embryonic day E10.5 until E12 in the mouse embryo, and from gestational weeks four to six in the human embryo. The first HSCs, which are as potent as bone marrow HSCs in transplantation procedures, are generated in the aorta-gonad-mesonephros (AGM) region. HSCs are found in the major vasculature – aorta, vitelline artery, and umbilical artery – subsequent to the appearance of hematopoietic cell clusters closely associated with the lumenal walls of these vessels. The relationship of HSCs to these clusters and the identification of the precursors to HSCs have been recently established through genetic, phenotypic, and real-time imaging studies. Remarkably, HSCs and hematopoietic progenitors arise directly from a subset of endothelial cells (hemogenic endothelial cells) in a natural transdifferentiation event. They are made through a process called endothelial to hematopoietic cell transition (EHT). EHT and HSC generation is in part regulated through ventral-derived developmental signals and a group of pivotal (core) transcription factors, including Runx1 and Gata2. Conditional knockout strategies show that these transcription factors are required for the generation of vascular hematopoietic clusters and HSCs, suggesting a role in hematopoietic fate induction and/or cell expansion. Interestingly, whereas both Runx1 and Gata2 are required for HSC generation, only Gata2 remains essential in HSCs after their production. We are profiling hemogenic endothelial and HSCs by RNA sequencing so as to understand the complete genetic program that leads to generation of HSCs. These results will be discussed in the context of developmental signaling pathways (BMP4, Hedgehog, etc.) that appear to impact HSC generation and expansion, and the localized dynamic expression and function of Gata2 and Runx1 in vascular endothelial and hematopoietic cluster cells.