Signaling and transcriptional networks governing late synovial joint development.

Synovial joints are complex organs composed of specialized tissues whose coordinated formation is essential for proper joint function. The cellular composition and regulatory programs governing late synovial joint development are less well characterized as compared to earlier stages of joint development. Here, we generated single-cell transcriptomic data of Growth differentiation factor 5 (Gdf5)-lineage cells from the embryonic mouse knee joint at embryonic day 17.5 and integrated these data with those from earlier developmental stages to infer transcriptional state progression across joint development. We identified nine distinct Gdf5-lineage populations corresponding to major joint tissues, including articular chondrocytes, superficial lining cells, ligament-tenogenic progenitors, synovial fibroblasts, and progenitor populations, and we validated their localization within the embryonic joint. We found a meniscus-associated progenitor state characterized by low expression of canonical chondrocyte markers and high expression of pleiotrophin (Ptn), distinguishing it from conventional articular chondrocytes. In addition, we found that Col22a1 and tetraspanin 15 (Tspan15) mark a transcriptionally and spatially distinct superficial lining cell population. To identify potential regulatory mechanisms underlying articular chondrocyte development, we reconstructed dynamic gene regulatory networks along the progenitor-to-chondrocyte trajectory, revealing a transition from a common regulatory state to cell type-specific regulatory program at later stages. In summary, our study provides data that improves our understanding of the developmental transcriptional programs that contribute to the major tissues of the synovial joint.