Identification and specification of the mouse skeletal stem cell.

How are skeletal tissues derived from skeletal stem cells? Here, we map bone, cartilage, and stromal development from a population of highly pure, postnatal skeletal stem cells (mouse skeletal stem cells, mSSCs) to their downstream progenitors of bone, cartilage, and stromal tissue. We then investigated the transcriptome of the stem/progenitor cells for unique gene-expression patterns that would indicate potential regulators of mSSC lineage commitment. We demonstrate that mSSC niche factors can be potent inducers of osteogenesis, and several specific combinations of recombinant mSSC niche factors can activate mSSC genetic programs in situ, even in nonskeletal tissues, resulting in de novo formation of cartilage or bone and bone marrow stroma. Inducing mSSC formation with soluble factors and subsequently regulating the mSSC niche to specify its differentiation toward bone, cartilage, or stromal cells could represent a paradigm shift in the therapeutic regeneration of skeletal tissues. Single-cell RNA-sequencing (C1 platform) of FACS-sorted mouse skeletal stem, progenitor, and stromal populations (mSSC, CSP, Thy+, and 6C3+).骨骼组织如何由骨骼干细胞分化而来？本研究针对高纯度的出生后小鼠骨骼干细胞（mouse skeletal stem cells, mSSCs）及其向骨、软骨及基质组织分化的下游祖细胞，绘制了骨、软骨与基质的发育图谱。随后，我们对这些干/祖细胞的转录组进行分析，以挖掘可指示mSSC谱系定向分化潜在调控因子的独特基因表达模式。本研究证实，mSSC微环境因子可作为强效的成骨诱导因子，且数种特定组合的重组mSSC微环境因子可在原位激活mSSC的遗传程序，即便在非骨骼组织中也能诱导软骨、骨及骨髓基质的从头形成。通过可溶性因子诱导mSSC生成，并后续调控mSSC微环境以指定其向骨、软骨或基质细胞的分化方向，或可成为骨骼组织治疗性再生的范式革新。本研究对经荧光激活细胞分选（Fluorescence-Activated Cell Sorting, FACS）分离得到的小鼠骨骼干细胞、祖细胞及基质群体（mSSC、CSP、Thy+及6C3+），采用C1平台开展了单细胞RNA测序（single-cell RNA-sequencing）。