Abstract
Conception rates for transferred bovine embryos are lower than those for artificial insemination. Embryo transfer (ET) is widely used in cattle but many of the transferred embryos fail to develop, thus, a more effective method for selecting bovine embryos suitable for ET is required. To evaluate the developmental potential of bovine preimplantation embryos (2-cell stage embryos and blastocysts), we have used the non-invasive method of optical coherence tomography (OCT) to obtain live images. The images were used to evaluate 22 parameters of blastocysts, such as the volume of the inner cell mass and the thicknesses of the trophectoderm (TE). Bovine embryos were obtained by in vitro fertilization (IVF) of the cumulus-oocyte complexes aspirated by ovum pick-up from Japanese Black cattle. The quality of the blastocysts was examined under an inverted microscope and all were confirmed to be Code1 according to the International Embryo Transfer Society standards for embryo evaluation. The OCT images of embryos were taken at the 2-cell and blastocyst stages prior to the transfer. In OCT, the embryos were irradiated with near-infrared light for a few minutes to capture three-dimensional images. Nuclei of the 2-cell stage embryos were clearly observed by OCT, and polynuclear cells at the 2-cell stage were also clearly found. With OCT, we were able to observe embryos at the blastocyst stage and evaluate their parameters. The conception rate following OCT (15/30; 50%) is typical for ETs and no newborn calves showed neonatal overgrowth or died, indicating that the OCT did not adversely affect the ET. A principal components analysis was unable to identify the parameters associated with successful pregnancy, while by using hierarchical clustering analysis, TE volume has been suggested to be one of the parameters for the evaluation of bovine embryo. The present results show that OCT imaging can be used to investigate time-dependent changes of IVF embryos. With further improvements, it should be useful for selecting high-quality embryos for transfer.
Highlights
Bovine embryo transfer (ET) has been widely used to produce calf in combination with other reproductive technologies, such as in vitro fertilization (IVF)
Embryos for transfer are usually selected by observation under a conventional optical microscope at the time of transfer, and embryo quality is subjectively assigned as one of the codes according to the International Embryo Technology Society (IETS) standards for embryo evaluation [5, 6]
We have recently developed a technique for the 3D imaging of bovine blastocysts and used it to evaluate 22 parameters including the volumes of the inner cell mass (ICM), TE, zona pellucida (ZP) and blastocoel of an embryo [36]
Summary
Bovine embryo transfer (ET) has been widely used to produce calf in combination with other reproductive technologies, such as in vitro fertilization (IVF). We have recently developed a technique for the 3D imaging of bovine blastocysts and used it to evaluate 22 parameters including the volumes of the ICM, TE, zona pellucida (ZP) and blastocoel of an embryo [36]. We used this technique to compare the characteristics of embryos that did or did not develop to term in order to identify the parameters associated with successful ET. We have tried to obtain 3D images of early-stage bovine embryos
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