Embryonic myeloid cells orchestrate niche cell homeostasis critical for the establishment of the definitive HSC pool

The lifelong replenishment of blood cells relies on the function of definitive HSCs that migrate to the bone marrow during development. This process is tightly controlled by the bone marrow microenvironment. Embryonic macrophages emerge before the onset of definitive hematopoiesis, seed into discrete tissues and contribute to specialized resident macrophages throughout life. However, the functional impact of embryonic macrophages on HSCs or the niche remains unknown. Here, by taking advantage of a lineage tracer mouse tool we show that bone marrow macrophages consist of two ontogenetically distinct cell populations from embryonic and adult hematopoiesis. Mice lacking embryonic myeloid cells have decreased HSC numbers in the bone marrow accompanied by an increase of stem cells in the liver of neonate mice. The emergence of HSCs from embryonic sources is unperturbed because pre-HSC and HSC numbers are normal, suggesting a key role for embryo-derived myeloid cells in orchestrating HSC trafficking around birth. We show here that the establishment of a normal cellular niche space in the bone marrow critically depends on embryonic myeloid cells that are important for the development of mesenchymal stromal cells, but not other non-hematopoietic niche cells, providing evidence for a specific role for embryo-derived myeloid cells in the establishment of a normal niche environment pivotal for HSC homing.