Background: Many studies have reported that inflammation and oxidative stress are involved in the pathogenesis of polycystic ovary syndrome (PCOS). Bone marrow mesenchymal stromal cells (BM-MSCs) have anti-oxidant and anti-inflammation properties. In this study, we investigate the beneficial effect of stem cell therapy on folliculogenesis in mice with induced PCOS Methods: Mouse model of PCOS was performed through daily injection of testosterone enanthate (1 mg/100 g/body weight subcutaneous (s.c).) for a period of 5 weeks. Naval Medical Research Institute (NMRI) mice (21 days old) were divided into three groups: control, PCOS and PCOS + BM-MSCs. BM-MSCs were labeled with Hoechst 33342 (0.5 mg/mL) and then injected into the mice (10 6 /animal, via the tail vein) at 1 and 14 days after PCOS confirmation. Mice were humanely killed at 2 weeks after last transplantation. Ovarian stereological studies were done. Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), testosterone, interleukin (IL)-6 and tumor necrosis factor (TNF)-a serum levels were measured. The levels of malondialdehyde (MDA) and total antioxidant capacity (TAC) in serum were analyzed. Apoptotic index for ovarian follicles was assessed using Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL). CD31 expression in ovarian vessels was assessed with the immunohistochemistry. Results: There was a significant increase in the total volume of ovary, cortex, number of antral follicles, volume of oocyte and zona pellucida thickness, and there was a significant decrease in the primary and preantral follicles number in the PCOS + BM-MSCs group compared with the PCOS group. There was a significant increase in the serum level of FSH and TAC and a significant decrease in the serum level of testosterone, LH, MDA and percentage of TUNEL-positive apoptotic cells in the PCOS + BM-MSCs group in comparison with the PCOS group. Discussion: BM-MSC transplantation improves folliculogenesis in mice wi
