Rat synovial MSCs

All animal care and experiments were conducted in accordance with the ARRIVE guidelines and the institutional guidelines of the Animal Committee of Tokyo Medical and Dental University. Wildtype female Lewis rats (10 weeks of age, weighing 180–200 g) were purchased from Sankyo Labo Service Corporation, Inc. (Tokyo, Japan) (n = 40). Luciferase-expressing transgenic rats⁴⁷ were also used for in vitro and vivo imaging (n = 4). The rats were kept in an environmentally controlled animal facility under a 12 h light/dark cycle with food and water ad libitum. Synovium was harvested from the rats’ infrapatellar fat pads. The synovium was minced and digested with collagenase (Merck, St. Louis, MO, USA) for 3 h. Synovial nucleated cells were cultured for 7–10 days in α-minimum essential medium (α-MEM; Thermo Fisher Scientific, Carlsbad, CA, USA), 10% fetal bovine serum (FBS; Thermo Fisher Scientific), and 1% antibiotic–antimycotic (Thermo Fisher Scientific) at 37 °C under 5% CO₂. The resulting culture was collected and used as synovial MSCs^20,46,48.