Serotonin is present in the SF of patients with RA and can promote the gap formation in joint vasculature. (A) Serotonin concentrations were measured in SF of RA and OA patients using an ELISA serotonin kit (n = 27). **P = .0012. (B) Serotonin suffices to promote prompt formation of gaps in joints. Diluent (PBS) or serotonin (20 mg/kg) was injected intravenously in mice before administration of 0.45-μm-diameter fluorescent microspheres. The fluorescence was portrayed using an in vivo imaging system 45 minutes after the microsphere injection (left). The radiant efficiency quantifications in the ankle joints for control (diluent) and serotonin injections are presented on the right. ***P < .0001; *P = .024.