SPP1 and endometriosis: Hypoxia-induced angiogenesis in endometriosis is multifaceted, with HIF-1α expression postulated to increase a number of angiogenic factors, including vascular endothelial growth factor A (VEGF-A), leptin, IL-8, cysteine-rich protein 61 gene (CYR61), osteopontin (OPN), and fibroblast growth factor 9 (FGF9) [137,138,139].