Multipotency of equine mesenchymal stem cells derived from synovial fluid

•Cartilage regeneration with cell therapy following arthroscopic surgery could be used in racehorses with intra-articular fractures (IAF) and osteochondritis dissecans (OCD).•In this study, mesenchymal stromal cells isolated from synovial fluid (SF) of horses were evaluated for multipotency.•The total number of SF-derived mesenchymal stromal cells reached >1 × 107 by the fourth passage.•SF-derived cells were strongly positive for CD44, CD90 and major histocompatibility complex (MHC) class I, and moderately positive for CD11a/CD18, CD105 and MHC class II. SF-derived cells were negative for CD34 and CD45.•SF-derived cells differentiated into osteogenic, chondrogenic, adipogenic and tenogenic lineages.•Chondrogenic induction in culture led to generation of a gelatinous sheet that was intensely stained with Alcian blue. Cartilage regeneration with cell therapy following arthroscopic surgery could be used in racehorses with intra-articular fractures (IAF) and osteochondritis dissecans (OCD). The aims of this study were to investigate the origin and multipotency of stromal cells in the synovial fluid (SF) of horses with intra-articular injury and synovitis, and to provide a new strategy for regeneration of lost articular cartilage. Mesenchymal stromal cells were isolated from SF of horses with IAF and OCD. Multipotency was analysed by RT-PCR for specific mRNAs and staining for production of specific extracellular matrices after induction of differentiation. The total number of SF-derived mesenchymal stromal cells reached >1 × 107 by the fourth passage. SF-derived cells were strongly positive (>90% cells positive) for CD44, CD90 and major histocompatibility complex (MHC) class I, and moderately positive (60–80% cells positive) for CD11a/CD18, CD105 and MHC class II by flow cytometry. SF-derived cells were negative for CD34 and CD45. Under specific nutrient conditions, SF-derived cells differentiated into osteogenic, chondrogenic, adipogenic and tenogenic lineages, as indicated by the expression of specific marker genes and by the production of specific extracellular matrices. Chondrogenic induction in culture resulted in a change in cell shape to a ‘stone-wall’ appearance and formation of a gelatinous sheet that was intensely stained with Alcian blue. SF may be a novel source of multipotent mesenchymal stem cells with the ability to regenerate chondrocytes.