Haematopoiesis is the process whereby mature blood cells of distinct lineages are produced from mulitpotent haematopoietic stem cells (HSCs). Despite decades of study, the origin of these HSCs during mammalian embryogenesis has remained unknown. Reporting in Immunity, Cumano and colleagues show that, in mice, HSCs are generated in an intraembryonic region, termed the splanchnopleura (Sp), and not from the extraembryonic yolk sac (YS) blood islands.

During vertebrate embryogenesis, haematopoietic cells first appear within the extraembryonic YS. As the bone marrow and fetal liver (the main haematopoietic organs in mammals) require an input of exogenous haematopoietic precursors to generate differentiated progeny, it was suggested that HSCs originate in the YS and later migrate to the fetal liver. However, the intraembryonic Sp region has also been shown to have haematopoietic activity.

The differentiation potential of haematopoietic precursors from the YS and Sp, separated from mouse embryos before circulation, has been investigated previously by the authors. These experiments established that lymphoid potential is restricted to precursors of intraembryonic origin. In transplantation experiments using normal, irradiated mice as recipients, neither precursors from the YS or from Sp were able to reconstitute an adult haematopoietic compartment. As this might have been due to technical limitations of the system used, the authors developed a new organ culture and cell-transfer system to investigate this further. The protocol involved culturing the Sp and YS, again separated before the onset of blood circulation, for 4 days (to ensure a sufficient number of HSCs were available), before transplanting the precursors into recombinase activating gene (Rag)−/− or Rag−/− crossed with common γ-chain (Ragγc−/−) mice. Rag−/− mice lack T and B cells (so reducing possible competition between donor and recipient precursors) and Ragγc−/− mice additionally lack natural killer cells (which might kill haematopoietic precursors).

Ragγc−/− mice injected with YS cells generated donor-derived myeloid cells, but this reconstitution was transient, and after 3 months donor-derived progeny were no longer observed. No lymphocytes were ever detected in these mice. Therefore, YS cells cannot generate lymphocytes, but can provide short-term myeloid reconstitution. By contrast, after injection of Sp-derived cells into Ragγc−/− recipients, donor-derived myeloid cells, as well as B and T cells were generated. These myeloid and lymphoid cells were still present 8 months after injection, indicating that Sp-derived percursors can provide long-term reconstitution of recipient mice.

The authors conclude that the only cells capable of adult long-term haematopoiesis are from the Sp region. So, during mouse embryogenesis, HSCs are generated intraembryonically and do not derive from the YS.