Abstract
Skeletal stem and progenitor cell populations are crucial for bone physiology. Characterization of these cell types remains restricted to heterogenous bulk populations with limited information on whether they are unique or overlap with previously characterized cell types. Here we show, through comprehensive functional and single-cell transcriptomic analyses, that postnatal long bones of mice contain at least two types of bone progenitors with bona fide skeletal stem cell (SSC) characteristics. An early osteochondral SSC (ocSSC) facilitates long bone growth and repair, while a second type, a perivascular SSC (pvSSC), co-emerges with long bone marrow and contributes to shape the hematopoietic stem cell niche and regenerative demand. We establish that pvSSCs, but not ocSSCs, are the origin of bone marrow adipose tissue. Lastly, we also provide insight into residual SSC heterogeneity as well as potential crosstalk between the two spatially distinct cell populations. These findings comprehensively address previously unappreciated shortcomings of SSC research.
Highlights
Harnessing the regenerative potential of skeletal stem cells (SSCs), as the therapeutic answer to osteoporosis, osteoarthritis, or fracture nonunions, is a long sought-after goal
Postnatal long bones harbor SSC subtypes with exclusive adipogenic potential Using fluorescence-activated cell sorting (FACS), we have previously identified an osteochondral SSC (CD45-Ter119-Tie2-CD51+Thy16C3-CD105-) that gives rise to a bone cartilage and stromal progenitor (BCSP) that in turn generates more lineage-restricted osteochondrogenic and stromal lineages (Chan et al, 2015)
In agreement with the original reports, we found that osteochondral SSC (ocSSC)/BCSPs are enriched in micro-dissected non-marrow fractions of femurs while perivascular SSC (pvSSC) and the committed adipogenic progenitor cell (APC) are more evenly distributed but accumulate at the ends of long bones where coincidentally bone marrow adipose tissue (BMAT) first appears (Figure 2A and Figure 2—figure supplement 1A; Chan et al, 2015; Scheller et al, 2015; Ambrosi et al, 2017)
Summary
Harnessing the regenerative potential of skeletal stem cells (SSCs), as the therapeutic answer to osteoporosis, osteoarthritis, or fracture nonunions, is a long sought-after goal. We and others have relied on fluorescence-activated cell sorting (FACS) to prospectively isolate cells with unique surface marker profiles drawn from the selective expression of a broad panel of surface proteins in freshly isolated single-cell suspensions from enzymatically dissociated skeletal tissue (Sacchetti et al, 2007; Tormin et al, 2011; Chan et al, 2015; Ambrosi et al, 2017; Chan et al, 2018) Using this approach, we have identified a mouse and human SSC with a defined lineage hierarchy of downstream progenitor cell populations and their translational value that revealed novel targets for bone and cartilage repair (Murphy et al, 2020; Ambrosi et al, 2019; Tevlin et al, 2017). We investigated ocSSCs and pvSSCs in mice to confirm their bona fide stem cell properties and show that they are unique SSC types that are molecularly and functionally distinct
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