Pre-Implantation Bovine Embryo Evaluation—From Optics to Omics and Beyond

SIMPLE SUMMARY: With conventional breeding practices, even the best dairy/beef cows will likely produce no more than half-a-dozen calves in their lifetimes; a crime, would you not agree? In contrast, with embryo transfer technologies, a cow can pass her genetics to hundreds of calves through surrogate cows. As such, embryo transfer, particularly involving embryos produced by in vitro fertilization, also known as IVF, is being widely adopted. However, pregnancy rates from the transfer of IVF embryos remain unimpressive. Prior to transfer, embryos are evaluated under a microscope, and only high-quality embryos are transferred. Unfortunately, microscopy cannot accurately distinguish between high-quality and low-quality embryos. Many embryo evaluation methodologies have been tested to date to find a replacement for microscopy-based embryo evaluation, e.g., advanced microscopic systems and the evaluation of genetics and secreted proteins/molecules. The good news: many of them can accurately separate the wheat from the chaff. The bad news: many of them are expensive, kill embryos during evaluation, are too complicated, and/or are laborious and thus useless in the frontline, i.e., thousands of embryo production/transfer facilities. This article reviews the most common (and not so common) techniques that have been tested to date and provides insights about those that have the highest potential to replace microscopy-based embryo evaluation in-field applications. ABSTRACT: Approximately 80% of the ~1.5 million bovine embryos transferred in 2021 were in vitro produced. However, only ~27% of the transferred IVP embryos will result in live births. The ~73% pregnancy failures are partly due to transferring poor-quality embryos, a result of erroneous stereomicroscopy-based morphological evaluation, the current method of choice for pre-transfer embryo evaluation. Numerous microscopic (e.g., differential interference contrast, electron, fluorescent, time-lapse, and artificial-intelligence-based microscopy) and non-microscopic (e.g., genomics, transcriptomics, epigenomics, proteomics, metabolomics, and nuclear magnetic resonance) methodologies have been tested to find an embryo evaluation technique that is superior to morphologic evaluation. Many of these research tools can accurately determine embryo quality/viability; however, most are invasive, expensive, laborious, technically sophisticated, and/or time-consuming, making them futile in the context of in-field embryo evaluation. However accurate they may be, using complex methods, such as RNA sequencing, SNP chips, mass spectrometry, and multiphoton microscopy, at thousands of embryo production/collection facilities is impractical. Therefore, future research is warranted to innovate field-friendly, simple benchtop tests using findings already available, particularly from omics-based research methodologies. Time-lapse monitoring and artificial-intelligence-based automated image analysis also have the potential for accurate embryo evaluation; however, further research is warranted to innovate economically feasible options for in-field applications.