Development of the fetal bone marrow niche and regulation of HSC quiescence and homing ability by emerging osteolineage cells

Hematopoietic stem cells (HSCs) reside within a specialized niche where interactions with vasculature, osteoblasts, and stromal components regulate their self-renewal and differentiation. Little is known about bone marrow niche formation or the role of its cellular components in HSC development; therefore, we established the timing of murine fetal long bone vascularization and ossification relative to the onset of HSC activity. Adult-repopulating HSCs emerged at embryonic day 16.5 (E16.5), coincident with marrow vascularization, and were contained within the c-Kit⁺Sca-1⁺Lin^- (KSL) population. We used Osterix-null (Osx^-/-) mice that form vascularized marrow but lack osteolineage cells to dissect the role(s) of these cellular components in HSC development. Osx^-/- fetal bone marrow cells formed multilineage colonies in vitro but were hyperproliferative and failed to home to and/or engraft transplant recipients. Thus, in developing bone marrow, the vasculature can sustain multilineage progenitors, but interactions with osteolineage cells are needed to regulate long-term HSC proliferation and potential.