The Role of β-Core Fragment hCG in Embryo Implantation and Early Pregnancy

Human chorionic gonadotropin (hCG) is a pregnancy biomarker, and five forms of this hormone are involved in female physiological regulation. β-core fragment hCG (bcf-hCG) is a fragment of hCG whose biological role in female reproduction has not been completely elucidated. This study aimed to investigate its role in embryo implantation and maintenance of a pregnancy-supportive environment. We analyzed the protein expression pattern of bcf-hCG in the intrauterine environment during early pregnancy by performing western blotting and immunohistochemistry with a monoclonal anti-bcf-hCG antibody. We performed a cell proliferation assay in the presence of bcf-hCG compared with intact hCG. We conducted an ex vivo study by performing intrauterine injection of bcf-hCG or intact hCG in mice. Endometrial thickness was measured using histological methods, and uterine gene and protein expression were analyzed following intrauterine injection of bcf-hCG. We evaluated the effect of bcf-hCG on embryo implantation in the uterus. bcf-hCG was highly abundant in the placenta and epithelial stromal glands of the uterine endometrium during early pregnancy and significantly induced proliferation of a stromal epithelial cell line. Intrauterine injection of bcf-hCG induced expression of specific genes and proteins, including homeobox A10, for embryo implantation and placental development. Upon embryo transfer, the implantation rate of bcf-hCG-treated embryos was higher than that of control embryos. In conclusion, bcf-hCG plays a role in the proliferation of glandular epithelial cells in the endometrium and placenta during early pregnancy. Therefore, bcf-hCG is an early-phase pregnancy biomarker that maintains the initial phase of pregnancy.