Dissecting human skeletal stem cell ontogeny by single-cell transcriptomic and functional analyses

Human skeletal stem cells (SSCs) have been discovered in fetal and adult bones. However, the spatiotemporal ontogeny of human SSCs during embryogenesis has been elusive. Here we map the transcriptional landscape of human embryonic skeletogenesis at single-cell resolution to address this fundamental question. We found remarkable heterogeneity within human limb bud mesenchyme and epithelium, as well as the earliest osteo-chondrogenic progenitors. Importantly, embryonic SSCs (eSSCs) were found in the perichondrium of human long bones, which self-renew and generate osteochondral lineage cells, but not adipocytes or hematopoietic stroma. eSSCs are marked by the adhesion molecule CADM1 and highly enrich FOXP1/2 transcriptional network. Interestingly, neural crest-derived cells with similar phenotypic markers and transcriptional network were also found in the sagittal suture of human embryonic calvaria. Taken together, this study revealed the cellular heterogeneity and lineage hierarchy during human embryonic skeletogenesis, and identified distinct skeletal stem/progenitor cells that orchestrate endochondral and intramembranous ossification.