Chan et al. identify and prospectively isolate a progenitor cell population derived from mouse fetal limb skeletal elements that forms an ectopic haematopoietic stem cell (HSC) niche in vivo. When transplanted under the adult mouse kidney capsule, the cells recruit host-derived blood vessels, produce donor-derived ectopic bones, and generate a marrow cavity populated by host-derived long-term reconstituting HSCs. Niche generation was inhibited by the suppression of factors involved in endochondral ossification. This paper describes an in vivo assay of haematopoietic stem cell (HSC) niche formation. A population of progenitor cells is sorted from fetal bones, and when transplanted under the adult mouse kidney capsule, the cells recruit host-derived blood vessels, produce donor-derived ectopic bones, and generate a marrow cavity populated by host-derived long-term reconstituting haematopoeitic stem cells. Little is known about the formation of niches, local micro-environments required for stem-cell maintenance. Here we develop an in vivo assay for adult haematopoietic stem-cell (HSC) niche formation1,2. With this assay, we identified a population of progenitor cells with surface markers CD45-Tie2-αV+CD105+Thy1.1- (CD105+Thy1-) that, when sorted from 15.5 days post-coitum fetal bones and transplanted under the adult mouse kidney capsule, could recruit host-derived blood vessels, produce donor-derived ectopic bones through a cartilage intermediate and generate a marrow cavity populated by host-derived long-term reconstituting HSC (LT-HSC). In contrast, CD45-Tie2-αV+CD105+Thy1+ (CD105+Thy1+) fetal bone progenitors form bone that does not contain a marrow cavity. Suppressing expression of factors involved in endochondral ossification, such as osterix and vascular endothelial growth factor (VEGF), inhibited niche generation. CD105+Thy1- progenitor populations derived from regions of the fetal mandible or calvaria that do not undergo endochondral ossification formed only bone without marrow in our assay. Collectively, our data implicate endochondral ossification, bone formation that proceeds through a cartilage intermediate, as a requirement for adult HSC niche formation.

Bone-related maladies are a major health burden on modern society. Loss of skeletal integrity and regeneration capacity through aging, obesity, and disease follows from a detrimental shift in bone formation and resorption dynamics. Targeting tissue-resident adult stem cells offers a potentially innovative paradigm in the development of therapeutic strategies against organ dysfunction. While the essential role of skeletal stem cells (SSCs) for development, growth, and maintenance of the skeleton has been generally established, a common consensus on the exact identity and definition of a pure bona fide SSC population remains elusive. The controversies stem from conflicting results between different approaches and criteria for isolation, detection, and functional evaluation; along with the interchangeable usage of the terms SSC and “mesenchymal stromal/stem cell (MSC)”. A great number of prospective bone-forming stem cell populations have been reported with various characteristic markers, often describing overlapping cell populations with widely unexplored heterogeneity, species specificity, and distribution at distinct skeletal sites, bone regions, and microenvironments, thereby creating confusion that may complicate future advances in the field. In this review, we examine the state-of-the-art knowledge of SSC biology and try to establish a common ground for the definition and terminology of specific bone-resident stem cells. We also discuss recent advances in the identification of highly purified SSCs, which will allow detailed interrogation of SSC diversity and regulation at the single-cell level.