Research QuestionDoes metformin reverse endometriosis-associated infertility? DesignEndometriosis was induced by transplanting endometrial tissue fragments from B6CBAF1 mice into the same strain female recipients. Mice were divided into groups: Endometriosis-End (n=24), Sham-operated (n=12), Endometriosis with orally administrated metformin during 3 months (0,5mg/ml)-EndMet (n=21), and Sham-operated metformin-treated-ShamMet (n=16). Half of mice per group was used for fertility studies. Implant growth was monitored by ultrasound and serum estradiol, glucose, and cholesterol (total, LDL, HDL) levels were assessed. Fibrosis was quantified in Masson's trichrome-stained sections of eutopic-Eu and ectopic-Ec endometrial tissue by computer-assisted analysis. PCNA, CYP17a1, F4/80 and galectin-3 were analyzed by immunofluorescence and Western blotting; NFkB, GPX-1 and HO-1 only by Western blotting. Statistical significance was set at p<0.05. ResultsThe endometriosis model was successfully established. End animals showed lower fertility rates than sham-operated mice, whereas metformin treatment increased the number of fetuses per pregnant mouse, restoring fertility to control levels, besides reducing implant growth and vascularization. Ectopic endometrial tissue resembled eutopic counterpart but showed increased PCNA levels and fibrosis that decreased after metformin treatment. PCNA expression decreased in pregnant mice. Metformin diminished CYP17a1 expression in EuEnd and on the contrary upregulated F4/80, galectin-3, NFkB and antioxidant enzymes, HO-1 and GPX-1, in non-mated mice. ConclusionsThese results emphasize metformin's application, even in a subtherapeutic dose, to alleviate oxidative stress and to mitigate endometriosis lesions fibrosis in a murine model of endometriosis. The study highlights metformin's potential as a pharmacological intervention for improving infertility in endometriosis, reinforcing its promising contribution to reproductive health.