Generation of HSCs in the embryo and assays to detect them.

The precise temporal and spatial emergence of hematopoietic stem cells (HSC) in the murine embryo has been somewhat controversial largely due to differences in the assays utilized to demonstrate HSC repopulating ability. One strategy is to determine where and when one can first detect HSC that engraft upon transplantation into lethally irradiated adult mice. However, knowing that the primary sites and patterns of hematopoiesis change during ontogeny, an alternative strategy is to select transplantation models where the recipient subjects more closely mirror the stage of development of the donor cells. In this regard, perhaps the most relevant assay to determine the presence of HSC activity in the early embryo is to transplant the donor cells in utero into recipient embryos. Other recipient models that may permit engraftment of embryonic cells include the use of submyeloablated or genetically HSC deficient newborn mice. Additional informative strategies have included co-culturing embryonic tissues that appear to lack HSC activity, with stromal cells derived from different developmental sites of hematopoiesis to induce HSC emergence, followed by transplantation as a means to determine which embryonic tissues possess HSC potential. This review will highlight some of the various transplantation assays used to identify HSC from embryonic tissues.