The Role of Neuroendocrine in Embryo Implantation

*Fenting Liu and Rong Li*

## **Abstract**

Neuroendocrine integration, an integration of the nervous system and endocrine system as its name implies, plays a critical role in the reproductive system. However, less progress has been made in the particular effects of neuroendocrine on embryo implantation. Recently, some significant knowledges have been gained on the regulation of neuroendocrine in embryo implantation. This chapter will summarize the current state of knowledges about the impaction of neuroendocrine on embryo implantation and discuss potential strategy to get higher pregnancy rate and to reduce recurrent implantation failure (RIF) possibility through modulating the neuroendocrine systems.

**Keywords:** neuroendocrine systems, embryo implantation, recurrent implantation failure, IVF-ET

#### **1. Introduction**

The success of mammalian pregnancy is mostly due to the smoothly embryo implantation into the maternal endometrium/decidua, which is a finely regulated process and requires the adaptation of maternal function to the needs of developing fetus. Human embryo implantation is a complex series of events involving blastocyst attachment, adhesion, and invasion into the endometrial tissue. Under normal circumstances, in order to ensure the consistency of this series, the blastocyst-stage embryo and the uterine endometrium must be collaborated during the "window of implantation" [1]. It has been long established that neuroendocrine is indispensable for mammalian reproduction [2], in which the neuroendocrine cell can synthesize and release various hormones into gonadal organ after receiving neuronal signal. Notably, the hypothalamus is the metronome of reproduction, and its main function is to receive neural signals from the brain and transform these neural signals into an endocrine output, the pulsatile release of GnRH, and other related factors. The pituitary gland, as a link between the brain and other neuroendocrine-related organs, is divided into two lobes. For the anterior lobe, it is related to the regulation of the hypothalamic-pituitary-gonadal axis (HPG axis), hypothalamic-pituitary-thyroid axis (HPT axis), and hypothalamic-pituitary-adrenal axis (HPA axis). For the posterior lobe, it is mainly about the regulation of the hypothalamic-neurohypophysis system [3]. Therefore, the description of an importance of the neuroendocrine to implantation from the aspects above is an enormous and necessary work.

Infertility affects millions of couples worldwide and its treatment has progressed immensely. As one of the most prevalent approaches of treating infertility, in vitro

fertilization and embryo transfer (IVF-ET) is always confronted with recurrent implantation failure [4], which is in the means of the situation when the transferred embryo repeatedly, failed to implant after IVF-ET [5]. A strong association between pregnancy rate following IVF-ET and recurrent implantation failure has been investigated [4], but less studies mention directly about the role of neuroendocrine in recurrent implantation failure. A thorough understanding of the processes governing human embryo implantation would be of significant benefit for the treatment of infertility. Hence, this review provides a summary of current empirical researches on the impacts of neuroendocrine aspects in implantation so that we more understand the maternal environment at the time of embryo implantation and might optimize it by altering neuroendocrine regulation to increase the success rate after IVF-ET.
