Is human chorionic gonadotropin directly involved in the regulation of human implantation?

The regulation of human implantation is not fully understood. hCG as one of the earliest embryonal signals may be a major regulator in the parakrine embryo-endometrial communication. The expression of full-length hCG/LH-receptor mRNA could be demonstrated in human endometrium throughout the follicular and secretory phase of the menstrual cycle. In contrast, in early pregnancy decidua only truncated variants could be detected. To investigate direct effects of hCG on the human endometrium, an intrauterine microdialysis device was developed to measure parakrine mediators within the uterine cavity in vivo. Using this system, hCG was applied in the secretory phase and the endometrial response was evaluated. The administration of hCG (500 IU/ml) provoked a significant inhibition of intrauterine IGFBP-1 and M-CSF, while LIF, VEGF and MMP-9 were significantly stimulated. Taken together there appear to be multiple direct effects of hCG on the endometrium that precede the classical endocrine role of the hormone.