Fig. 8.

Proposed model of hCG stimulation of the EG-VEGF/PROKR1 and PROKR2 system in human placenta during the first trimester of pregnancy. A Illustration of placental villi with the new physiological regulation between hCG and EG-VEGF. hCG released from the syncytiotrophoblast layer will activate EG-VEGF and PROKR2 expression in the ST layer in an autocrine manner and PROKR1 expression in the cytotrophoblast layer in a paracrine manner. B Summary of the proposed model according to which PROKR1, expressed by cytotrophoblasts, is only increased in response to high hCG levels. In contrast, EG-VEGF and PROKR2, which are mainly expressed by differentiated syncytiotrophoblasts, are increased in response to low hCG levels