(A) The skeletal stem cell hierarchy in mice beginning with a self-renewing mouse Skeletal Stem Cell (mSSC) differentiating to lineage-restricted bone, cartilage, and stromal cells through a Bone, Cartilage, Stromal Progenitor (BCSP) cell8. (B) The skeletal stem cell hierarchy in humans beginning with a self-renewing human Skeletal Stem Cell (hSSC) differentiating to lineage-restricted bone, cartilage, and stromal cells through a Bone, Cartilage, Stromal Progenitor (BCSP) cell9.