FLT1 and endometriosis: Previous studies report that proangiogenic (VEGFR1+) neutrophils are recruited to sites of hypoxia (77), and since menstruation is a hypoxic event (72), it is conceivable that the proangiogenic neutrophils in endometriosis retrograde effluent contribute to neoangiogenesis, endometrial cell survival, and ultimately lesion development.