Signaling and transcriptional networks governing late synovial joint development

Background: During synovial joint development, cavitation marks the end of the emergence of new cell types and the onset of the consolidation of cell type specific programs. However, the transcriptional programs that regulate this crucial stage prior to joint maturation are incompletely characterized. Gdf5-lineage cells give rise to the majority of joint constituents such as articular cartilage, meniscus, ligament, and tendon. Therefore, to explore pre-maturation of the synovial joint, we performed single cell RNA-Seq analysis of 1,306 Gdf5-lineage cells from the murine knee joint at E17.5. Results: Using computational analytics and in situ hybridization, we identified nine sub-states contributing to articular cartilage, meniscus, cruciate ligament, synovium, lining, and surrounding fibrous tissue. We identified a common progenitor population that is predicted to give rise to ligamentaocytes, articular chondrocytes, and lining cells. We further found that while a large number of signaling pathways orchestrate the differentiation of this progenitor to either ligamentocytes or to lining cells, only continued FGF signaling guides these cells to a default chondrocyte fate. Conclusions: Our single cell transcriptional atlas is a resource that can be used to better understand and further study synovial joint development and the reactivation of embryonic programs in diseases such as osteoarthritis.