Successful pregnancy in mammals depends on a series of crucial events, including fertilization, embryo development, embryo implantation, and decidualization. During the pre-implantation period, the embryo enters the uterine cavity and becomes a blastocyst with healthy development and differentiation for implantation to the uterus. In addition to the healthy development and differentiation of the blastocyst, the uterus must be in a receptive state. In mice, uterine receptivity is divided into 3 stages: pre-receptive (neutral), receptive, and non-receptive (Tu et al., 2014). In mice, the uterus becomes receptive only on the night of the 4th day of pregnancy (the day of implantation), on the 5th day, the uterus becomes non-receptive and does not respond to the presence of blastocyst. In mice, implantation occurs at the anti-mesometrial side of the luminal epithelium, which is the opposite side of the blood vessel-rich mesometrium (Ye, 2020). After the attachment, cells of the adluminal epithelium in mice and humans undergo apoptosis, so the embryo is embedded in the stroma (Zhang et al., 2013). Moreover, embryo implantation initiates the decidualization process, that includes the differentiation of stromal cells to decidual cells. This cellular differentiation process is a prerequisite for a successful pregnancy (Ramathal et al., 2010).

The main focus of this study was to investigate the potential roles of apelin/apelin receptor during the peri-implantation period in mice. Apelin was first purified from bovine stomach extracts in 1998 (Tatemoto et al., 1998). Apelin is synthesized as 77 amino acids pre-proapelin (Reaux et al., 2001) and undergoes enzymatic degradation into fragments such as apelin-36, 31, 28, 19, 17, 16, 15, 13, and 12 which was summarized in a review (Antushevich and Wojcik, 2018). The apelin receptor (APJ) is a member of the G protein-coupled receptors (GPCRs) and was defined as the orphan receptor until apelin was discovered (Reaux et al., 2001). The physiological roles of the apelin/APJ system were shown in many events such as the cardiovascular system (Galanth et al., 2012), immune response (Horiuchi et al., 2003), stress response (Newson et al., 2013), fluid regulation (Takayama et al., 2008), food intake (Sunter et al., 2003), and angiogenesis (Kalin et al., 2007). Also, apelin is an essential angiogenic factor for normal blood vessel development and endothelial cell proliferation (Cox et al., 2006). The apelin-APJ signal is necessary for embryonic angiogenesis and is upregulated throughout tumor angiogenesis (Kalin et al., 2007). Apelin activates nitric oxide synthase (NOS), causing endothelium-dependent vasodilation and triggering nitric oxide release from endothelial cells (Zhou et al., 2016), Nitric oxide (NO) must be at the optimal level for embryo implantation (Ota et al., 1999). Apelin is known to be expressed in the human placenta (Cobellis et al., 2007, Telejko et al., 2010). The presence of apelin in the human placenta points out the importance of this peptide for pregnancy. However, the role of the apelin/APJ system in embryo implantation and decidualization processes is not known.

We hypothesized that the apelin/APJ system has a role in uteri during the peri-implantation period in mice. In this study, we determined the localization and expressions of apelin and APJ in peri-implantation period mouse uteri and implantation sites via immunohistochemistry and Western blot techniques, respectively.