Mechanical and signaling mechanisms that guide pre-implantation embryo movement

Abstract

How a mammalian embryo determines and arrives at its attachment site has been studied for decades but our understanding of this process is far from complete. Using confocal imaging and image analysis, we evaluate embryo location along the longitudinal oviductal-cervical axis of murine uteri. Our analysis reveals three distinct pre-implantation phases: a) Embryo entry; b) Unidirectional movement of embryo clusters; and c) Bidirectional scattering and spacing of embryos. We show that unidirectional clustered movement is facilitated by a mechanical stimulus of the embryo and is regulated by adrenergic uterine smooth muscle contractions. Embryo scattering, on the other hand, depends on embryo-uterine communication reliant on the LPAR3 signaling pathway and is independent of adrenergic muscle contractions. Finally, we demonstrate that uterine implantation sites in mice are neither random nor predetermined but are guided by the number of embryos entering the uterine lumen. These studies have implications for understanding how embryo-uterine communication is key to determining an optimal implantation site necessary for the success of a pregnancy.

Disruption of Folate Metabolism Causes Poor Alignment and Spacing of Mouse Conceptuses for Multiple Generations

“…The occurrence of male and female embryos together in a single implantation site indicated dizygosity and suggested that twinning in the Mtrr gt mouse line likely resulted from poor blastocyst spacing at implantation. However, similar to controls, Mtrr gt/gt implantation sites were generally evenly spaced along the length of Mtrr gt/gt uteri (Figure 2C) rather than clustered together as is observed when there is a major uterine defect (Lu et al, 2013;Flores et al, 2020). Also, no more than one twinned implantation site was usually observed per Mtrr gt/gt litter (this study; Padmanabhan et al, 2013) indicating incomplete phenotype penetrance.…”

“…Mouse litters consist of 5-20 embryos that must be optimally spaced and oriented within the uterus for ideal growth. Spacing mechanisms are not well understood but likely involve mechanical forces generated by the uterus and cross talk between blastocysts and the uterine epithelium (Chen et al, 2013;Flores et al, 2020). The orientation of the developing mouse conceptus aligns with the antimesometrial-mesometrial axis of the decidual swelling, such that the placenta of each conceptus forms on the mesometrial side of the uterus (Smith, 1985).…”

Disruption of folate metabolism causes poor alignment and spacing of mouse conceptuses for multiple generations

“…The molecular mechanisms of blastocyst spacing are not well understood. In general, normal spacing of Mtrr gt/gt conceptuses along the length of the Mtrr gt/gt uterus was apparent (this study), potentially ruling out a mechanical defect in the uterus (Flores et al, 2020). Indeed, the low frequency of twinning in the Mtrr gt mouse line implies an embryo-specific mechanism since a maternal effect would affect a greater number of conceptuses within one litter.…”

“…Indeed, excessive intrauterine fluid disrupts embryo spacing and infrequently leads to twinning due to abnormal interaction between the uterine epithelium and blastocysts (Lu et al, 2013). Typically, blastocyst spacing in the mouse uterus is a highly regulated event requiring mechanical processes (e.g., muscular contractions, ciliary movement) and molecular signalling between the uterus and blastocyst (Shi et al, 2014;Flores et al, 2020). Also, inhibitory signalling between neighbouring pre-implantation littermates might transpire.…”

“…Mouse litters consist of 5-20 embryos, each form their own placenta on the mesometrial side of the uterus (Smith, 1985) and must be equally spaced within the uterus for ideal growth. Spacing mechanisms are not well understood but likely involve mechanical forces generated by the uterus and cross talk between blastocysts and the uterine epithelium (Flores et al, 2020). After implantation, the ICM will form the embryo proper, with the progenitor cells of the cardiovascular and nervous systems among the first to develop (McDole et al, 2018).…”