A proposed model of the paracrine action mediated by hCG that involves different cell types in the endometrium and that affects the implantation-stage embryo and endometrium. The pre-implantation embryo- and endometrium-derived CG may specifically act on endometrial surface epithelium alone (A) or may act on both the endometrial epithelial cells and stromal fibroblasts (B), resulting in up-regulation of a specific cohort of cytokines, chemokines and growth factors. Many of these factors (e.g., CCL3, CCL4, CCL5, CXCL10, FGF2, GMCSF, IFNG, IL-1b, IL-6, IL-13, LIF, PDGFb, TNF and VEGF) are reportedly embryotropic, some (e.g., IL-6 and LIF) are primarily secreted by endometrial epithelial cells, and a few (e.g., FGF2, GMCSF, INFG, IL-1b and VEGF) are secreted primarily by endometrial fibroblasts in response to hCG. In contrast, many of the secreted factors are known to regulate stromal fibroblasts (e.g., FGF2, GCSF, GMCSF, IFNG, IL-1b, IL-6, LIF, MIF, TNF and TRAIL), vascular physiology (e.g., CCL2, CXCL10, GMCSF, IL-1b, IL-6, LIF, MIF, PDGFb, TNF and VEGF) and glandular physiology (e.g., LIF, MIF, PDGFb, and TNF). Only a few of the factors (e.g., CCL2, IL-6, LIF and MIF) are secreted by isolated endometrial epithelial cells in response to hCG. Interestingly, many of these factors directly and indirectly regulate the functions of immunocompetent cells. A large number of reports [21, 32–53, 57–78, 80] and the results of the present study have been used to develop this model. BV (blood vessel), FB (fibroblast), GL (gland), IC (immunocompetent cells), PIE (pre-implantation embryo). There is no quantitative aspect to the length and thickness of arrows.