Single-cell transcriptomics of mouse knee joints during embryonic and postnatal stages

Synovial joints are structures where two bones meet and essential for mobility. For example, knee joints allow flexion and extension in a single plan for stable walking motions, whereas phalangeal joints together with skeletal muscles facilitate fine motor movements in hands. Joint is enclosed in a capsule and lined by a synovial membrane inside. Articular cartilage covering bone surfaces provide a wear-resistant structure that reduces friction during movement. Meniscus acts like a shock absorber to avoid direct grinding between bone surfaces. Cruciate ligaments reinforce the integrity of a joint by mounting two bones in place and limit the degree of joint movement. All these complex structures with different shapes are fit within the joint capsule for purpose. We established lineage maps of individual joint lineages for human and mouse to understand the progenitors, transcriptional regulators, signaling cues and extracellular matrix (ECM) involved in each differentiation trajectory. We asked whether LGR5+ joint progenitor cells are presence in early stage and decline with maturation as in our findings of mouse, and whether LGR5 and COL22A1 demarcate the chondrogenic and ligamentogenic lineages in human.