Cytoplasmic velocity in polar trophectodermal cells reflects embryonic ability to implant in vitro

The standard protocols used in clinics to assess quality of embryos are still unable to reliably assess the embryo's ability to implant and develop full term. On the other hand, being able to predict the implantation potential of a given embryo appears to be crucial, as implantation abnormalities are one of the major causes of reproductive loss in mammals. Notwithstanding the differences in the implantation mechanism between different species, the functional trophectoderm is prerequisite in this process. In the present study we investigated whether the velocity of cytoplasmic movement in mural and polar TE cells of mouse blastocysts can be used to assess the embryo implantation potential. The cytoplasmic velocity was measured using time-lapse imaging in E3.5 and E4.5 mouse embryos and analyzed using Particle Image Velocimetry. We found that it decreases during late preimplantation embryo development and is higher in polar than in mural trophectoderm cells of mouse blastocysts. Our data indicate that cytoplasmic movement velocity was only minimally affected by alterations in the keratin cytoskeleton, despite its established role in determining the biomechanical properties of trophectoderm cells. Furthermore, cytoplasmic movement velocity was not affected by either maternal or postovulatory aging, both of which are known to impair the developmental potential of embryos. However, using an outgrowth assay, we showed that the velocity of cytoplasmic movement in polar, but not mural, trophectodermal cells in E4.5 blastocysts reflects the embryo's ability to implant in vitro. Therefore, analysis of cytoplasmic speed may support evaluation of the embryo quality.