Definitive Hematopoiesis In the Mammalian Embryo Prior to HSC Formation.
Authors: McGrath Kathleen E , Frame Jenna M , Koniski Anne , Kingsley Paul D , Palis James 
Published: October 2019

Abstract
Abstract 1599
The ontogeny of hematopoiesis in mammalian embryos is complicated by the requirement for functional blood cells prior to the emergence of hematopoietic stem cells or the bone marrow microenvironment. In the murine embryo, transplantable HSC are first evident at embryonic day (E) 10.5 and the first few HSC are found in the fetal liver hematopoietic environment by E12.5. However, two overlapping waves of hematopoietic potential arise in the yolk sac before E10.5. The first “primitive” wave produces progenitors from E7.25 to E8.5 with primitive erythroid, megakaryocyte and macrophage potentials. The resulting primitive erythroid cells mature within the circulation and support embryonic growth past E9.5. At E8.5, a second wave of hematopoiesis begins in the yolk sac and generates definitive erythroid and multiple myeloid progenitors that are the proposed source of the hematopoietic progenitors seeding the fetal liver before HSC colonization. We have identified a cell population displaying a unique cell surface immunophenotype in the E9.5 yolk sac that contains the potential to form definitive erythroid cells, megakaryocytes, macrophages and all forms of granulocytes within days of in vitro culture. Furthermore, all definitive hematopoietic colony-forming cells (BFU-E, CFC-myeloid and HPP-CFC) in the E9.5 yolk sac have this immunophenotype. These erythro-myeloid progenitors (EMP) are lineage-negative and co-express ckit, CD41, CD16/32 and Endoglin. Interestingly, this is not an immunophenotype evident in the adult bone marrow. Other markers that have been associated with HSC formation (AA4.1, ScaI) or with lymphoid potential (IL7R, Flt3) are not present on these cells at E9.5. Consistent with the lack of lymphoid markers, we also do not observe short-term development of B-cells (CD19+B220+ expressing Rag2 RNA) in cultures of the E9.5 sorted EMP, while bone marrow Lin-/ckit+/ScaI- cells do form B-cells under the same conditions. Clonal analysis of sorted EMP cells revealed single cells with both erythroid and granulocyte potential, similar to the common myeloid progenitors in adult bone marrow. Though these EMP are enriched at E9.5 in the yolk sac, they are also found at low levels in the fetal blood, embryo proper and placenta, consistent with their entrance into the circulation. By E10.5, EMP were most highly enriched in the newly formed fetal liver. Additionally by E12.5, a time when the first few HSCs are detected in the fetal liver, we find active erythropoiesis and granulopoiesis in the liver and the first definitive red blood cells and neutrophils in the bloodstream. Therefore, we believe the yolk sac definitive progenitors' fate is to populate the fetal liver and thus provide the first definitive erythrocytes and granulocytes for the embryo. The differentiation of embryonic stem cells (ES) and induced pluripotent stem cells (iPS) cells into mature cells types offers the hope of cell-based therapies. Analysis of differentiating murine ES cells reveals overlapping waves of primitive and definitive hematopoietic colony forming potential. We demonstrate the appearance of an EMP-like (ckit+/CD41+/FcGR+) population coincident with the emergence of definitive hematopoietic progenitors during murine ES cell differentiation as embryoid bodies. We have confirmed with colony forming assays that definitive hematopoietic potential is associated with this immunophenotypic group. Our studies support the concept that blood cell emergence during ES cell differentiation closely mimics pre-HSC hematopoiesis in the yolk sac.

Disclosures:
No relevant conflicts of interest to declare.