Spacing of the Embryo in the Uterus is Disrupted by the Supine Position of the Body During the Peri-implantation Period in Mice

Abstract

Abstract. This study aimed to clarify the mechanism of the spacing of murine embryos along the metrial and antimetrial (MA) axis of the uterus using our newly developed experimental model. The model mice were produced by keeping mice in the supine position from the pre-implantation to implantation period. The starting points and periods of restraint of the mice in the supine position were set variously during the peri-implantation. Then, the position of the embryo was evaluated morphologically. In only one group (set in the instrument from the second day of pregnancy, Day 2, to Day 5), strong disruption of embryo spacing along the MA axis was observed. On the other hand, there was little abnormality in embryo positioning in the groups that were treated from Day 3 to Day 5 or from Day 3 to Day 6. These results suggested that determination of the position of the embryo in the MA axis is not related to duration of the experiment (2 days or 3 days), but is related to the starting time-point of the experiment, at Day 2 or Day 3. In conclusion, the period between Days 2 and 3 is critical for determination of the position of the embryo along the MA axis. Key words: Embryo implantation, Gravity, Mice, Spacing (J. Reprod. Dev. 56: [191][192][193][194] 2010) mbryo implantation is one of the most important events in mammalian reproduction [1]. This implantation consists of two major phenomena; the first is positioning of the embryo in the uterus before apposition of the blastocyst to the uterine luminal epithelium (LE), and the second is initiation of direct feto-maternal communication after completion of positioning (above "apposition"). In polytocous species, such as the mouse, precise positioning of individual embryos in the right place is required before the implantation window opens. The spacing of the mouse embryo is defined as follows: (1) the embryo aligns along the long axis of the uterine horn (LA axis) in equally spaced manner and (2) the embryo is localized exclusively at the anti-metrial side of the uterine horn the metrial and anti-metrial axis (MA axis).In mice, fertilization occurs at the ampulla of the oviduct. Fertilized eggs migrate to the uterine horn during the 2-to 16-cell stage at Day 2 of pregnancy (the day of the vaginal plug is defined as the first day of pregnancy, Day 1). The morula moves to the uterus around noon on Day 3. Then, the embryo develops to the blastocyst stage, and it completes spacing in the morning on Day 4. Finally, the blastocyst begins to attach to the LE [1][2][3].Leukemia inhibitory factor (LIF) is an essential factor for initiation of pregnancy after positioning of the embryo in mice [4]. LIF is involved in the process of implantation; it is involved in apoptosis of the LE and successive decidualization. The molecular pathway of LIF leading to successful implantation has gradually been revealed. In addition to LIF, some molecules have been suggested to be indispensable factors for the progress of implantation [4][5][6]. On the other hand, the mechanisms of embryo spacing ...