Fig. 3.

Possible human decidua-embryo interaction indicated by in vitro observation. Despite of difficulty in obtaining human specimen during normal peri-implantation stage, a number of studies have utilized in vitro co-culture system to investigate signaling communication between the decidual cells and the embryo. Fertilized eggs secrete interleukin-1β (IL-1β) and growth factors such as IGF-II and HB-EGF, which are indicated to regulate decidual cellular development, in line with expression of IL-1RI and type 2 IGFR in the endometrial cells. Signaling for decidual expression of Toll-like receptors (TLRs) is unknown. Soluble factors from the embryo but detected in maternal peripheral blood, such as human chorionic gonadotropin (hCG), preimplantation factor (PIF), and soluble form of HLA-G (sHLA-G), may have systemic effects such as promoting ovarian progesterone production, balancing cytokines and chemokines secretion in the periphery. Decidual cells also express growth factors such as G-CSF, and cytokines and chemokines such as IL-6, IL-8, and CXCL1. At least some of them are considered to be under the control of signaling from the embryos