Physiological and pathological angiogenesis in endometrium at the time of embryo implantation.

Embryo establishes contact with the endometrium during implantation. Proper endometrial vascular development and maintenance at the time of embryo implantation is crucial for successful pregnancy. Vascular development at the maternal-embryo interface can be regulated by various cell types, of which uterine natural killer (uNK) cells play an important role. Abnormal angiogenesis and uNK cell number/function may lead to reproductive failure, particularly in women with recurrent miscarriage (RM) and women with recurrent implantation failure (RIF) after IVF-ET treatment, which are the important clinical hurdles in reproductive medicine to overcome. In this review, we aim to discuss the current knowledge of physiological angiogenic processes and the pathological angiogenesis at the time of implantation, as well as the possible mechanism and potential treatment.