ABOVE: This image shows tissue derived from a single human skeletal stem cell. Bone is in yellow, blue indicates cartilage, and marrow is pictured in red.

CHAN AND LONGAKER ET AL.

Three years after its discovery of skeletal stem cells in mice, the same research team has identified the human version of this precursor to bone, cartilage, and stroma, the bone marrow’s support cells. In a study published today (September 20) in Cell, the authors show that these skeletal stem cells are both self-renewing and multipotent.

“For many years there’s been this debate about a true human skeletal stem cell. This study unequivocally demonstrates that it’s there and that it is self-renewing,” says Richard Oreffo, a stem cell biologist at the University of Southampton in the UK who did not participate in the work. “There’s still a lot to do, but this is a tremendous step forward for the field.”

Michael Longaker, a craniofacial surgeon at Stanford University, tells The Scientist that one of the problems he faces in the operating room is having enough bone to do the reconstructions his patients need. In the search for a source of more tissue, he and colleagues identified the mouse skeletal stem cell in 2015, and in the current study focused their attention on finding the human equivalent.

The researchers started with a human fetal femur and sorted the nonhematopoietic cells from blood precursors. They then sequenced individual, nonblood cells’ RNA. Based on the expression profiles of cells located in areas of active growth in the fetal bone, they found a suite of four proteins—PDPN, CD146, CD73, and CD164—the presence or absence of which the team hypothesized could define skeletal stem cells. None of these markers were shared with mouse skeletal stem cells.

To test their hypothesis, the authors used fluorescence-activated cell sorting to isolate groups of cells positive for PDPN, CD73, and CD164 and negative for CD146. Both in culture and when transplanted underneath the outer layer of the kidney in adult mice—a system that serves as a kind of in vivo incubator—these cells were capable of regenerating themselves indefinitely and differentiating into cartilage, bone, and stroma.

Notably, these cells do not become fat cells, which differentiates them from mesenchymal stem cells, a term that some researchers have used interchangeably with skeletal stem cells in the past, but that the authors write likely contain a mix of distinct stem cell types.

Isolating a skeletal stem cell in fetal bone is one thing, but the researchers wanted to check whether they could gather these cells from more accessible sources. They determined that skeletal stem cells were present in both damaged and undamaged adult human femurs. Plus, with the application of certain factors, the scientists could derive human skeletal stem cells from blood cells and from adipose stroma—the nonfat, nonvascular cells that physicians collect during liposuction.

A comparison of skeletal stem cells from each of these sources using single-cell RNA sequencing confirmed that while they all had similar gene-expression profiles, the fetal and induced pluripotent stem cell–derived cells were more like each other than they were to those from adult bone or adipose stroma.

“This is a really valuable paper and a next step in the process of unraveling the stem and progenitor populations that are present in bone marrow,” says George Muschler, an orthopedic surgeon at the Cleveland Clinic who was not involved in the study. The authors have “added several new tools—particularly this PDPN marker—to the set of markers that we as scientists can use to find cells of interest,” he adds.

Linda Sandell, who studies osteoarthritis at Washington University in St. Louis and did not participate in the work, hesitates at the designation of the cell identified in the study as a stem cell, given that it can only give rise to three types of cells. Nonetheless, she says, by defining the markers for different subpopulations of progenitor cells, the authors have provided a starting point to identify cells that only become cartilage, a finding that could be a boon for treating diseases such as osteoarthritis, in which the protective cartilage at the ends of bones wears down.

“If tissue engineering and regenerative medicine is going to go in the direction of growing and using cells that can be expanded in the laboratory, these tools are really extraordinarily important in helping people start from a controlled environment,” says Muschler. “Having better tools that narrow the range and variation in the starting materials . . . could have important implications on cell manufacturing in the future for therapies.”

Longaker agrees that the findings could open the door to regeneration of the aging skeleton. “In plastic surgery we want to replace like with like, so if you have the skeletal stem cell or the bone-forming progenitor, use that for bone and cartilage for cartilage,” he says.

C.K.F. Chan et al., “Identification of the human skeletal stem cell,” Cell, doi:10.1016/ j.cell.2018.07.029, 2018.