Updated project metadata. Equine Osteoarthritis (OA) is a heterogeneous, degenerative disease of the musculoskeletal system with multifactorial causation, characterised by joint metabolic imbalance. Mesenchymal stromal cell therapy (MSC) is a form of regenerative medicine that utilises MSC properties to repair damaged tissues. MSCs have been described as acting via the paracrine signalling of secreted factors. Despite its wide use in veterinary clinical practices, the exact mechanism of action of MSCs has not been fully characterised. Here, we have characterised synovial fluid extracellular vesicles (EVs) from control, osteoarthritic and MSC treated animals in order to gain insights into this mechanism. An in vivo, carpal osteochondral fragment model of equine OA was used for this study. Six horses underwent surgical intervention. All horses enrolled in the study were female trotter horses between the ages of 4 and 7. The contralateral limb of each horse served as a sham control. 69 synovial fluid samples were collected via aseptic arthrocentesis at day 0, 18, 21, 28, 35, and 70. Allogenic mesenchymal stromal cell therapy from male donors was injected into the OA afflicted joint after day 18. Synovial fluid (200µl) was Hyalaronidase treated (1µg/ml) and EVs were isolated using differential ultracentrifugation. EVs were characterised in collaboration with Nanoview Biosciences, whereby the Exoview human tetraspanin assay was used. EV concentration, surface marker identification, fluorescent microscopy and tetraspanin colocalization analysis was performed using pooled samples reflecting experimental groups - control, OA and OA including MSCs - across time.