BDNF and endometriosis: Emerging studies have demonstrated that neurotrophins, as a family of nerve growth factors, including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5), are observed in endometriotic lesions, resulting in pelvic pain in endometriosis via neurogenic inflammation [10,11].