A review of the promises and pitfalls of oocyte and embryo metabolomics
Highlights
► Embryo viability assessment with traditional evaluation is challenged. ► Novel approaches with genomic, proteomic or metabolomic analyses are on rise. ► Review on corresponding studies indicate metabolimic as promising technique. ► Ideal IVF outcome of a singleton healthy pregnancy can be improved by using these novel techniques.
Introduction
Since the establishment of IVF as a clinical treatment option, embryologists have sought to determine the best method of selecting embryos with the highest potential to establish pregnancy. Effective selection can also reduce the number of embryos that need to be replaced to achieve pregnancy. Although the pregnancy rate may be elevated with multiple embryo transfer, significant risk to mother and concepti accompanies multiple pregnancies and a general trend to transfer fewer embryos in a single cycle currently exists [1], [2].
Pregnancy rates after embryo transfer are dependent on the quality or developmental potential of selected embryos. The importance of embryo selection is thus further elevated when reduced numbers are transferred into a recipient. Traditionally, microscopic assessment of morphology provided a relatively simple, and somewhat reliable method for this purpose. However, it is accepted that there are numerous factors contributing to oocyte/embryo quality that are not necessarily reflected by morphology.
In recent years, there have been reports on different technologies that may provide additional information on the viability of embryos [3], [4], [5]. One of these is “metabolomics”, which in clinical embryology refers to the study of factors present in the culture medium that were secreted by the oocyte/embryo into its environment as a result of metabolic processes that occur within its cells, or due to leakage from cell damage. As part of the secretome (the total compliment of all the compounds that are secreted by the oocyte/embryo into its environment), the metabolome is easily collected non-invasively and analyzed for useful information about the “secret” life of the oocyte or embryo, which may not be indicated by morphology [6], [7]. Therefore, the study of preimplantation metabolomics provides a unique window into the cellular activities during a specific time frame of development. Furthermore, metabolomic analysis of follicular fluid can provide information about individual oocyte maturation and developmental potential (see [3] for review).
During preimplantation development, the major metabolic pathways utilized by the human embryo change as it progresses through early cell divisions. The best known is a switch in ATP synthesis from carboxylic acid to glucose metabolism around the time of compaction [8]. This will be discussed in more detail below. Understanding of such processes and their resulting products allows one to make predictions about secreted molecules that may be found in the environment of embryos. This serves as the founding principle of the study of metabolomics and its applications for embryo quality determination.
Pre- and post-maturation oocyte metabolism on the other hand is not as well understood, although significant progress has been made in the past few years [9], [10]. A recent study indicated definite differences in metabolomics between oocytes with good or poor developmental potential [11]. Furthermore, complex interactions between the oocyte and adjacent cumulus cells suggests that alteration of in vitro maturation medium by cumulus cells may be indicative of oocyte quality, although much improvement in human in vitro maturation rates is still required before routine clinical use can be implemented [12].
This review will focus on recent developments in oocyte and embryo metabolomics. Other techniques that have been employed for quality/viability assessment and selection procedures will be reviewed, the strengths and weaknesses of each will be discussed in light of practical application in a clinical setting. Table 1 summarizes the non-invasive techniques that have been used to measure biologically important chemicals in human embryo culture media, and the basic outcomes of each. Finally we will look at the future of metabolomics and the promises it holds for improvement of oocyte and embryo selection.
Section snippets
Light microscopy – the “traditional” approach
Selection of oocytes and embryos based on morphology using light microscopy is the traditional and most thoroughly studied method for embryo assessment and selection [13]. A wealth of information is available on both morphological parameters at every developmental stage and grading systems that are aimed at distinguishing the most developmentally competent oocytes or embryos. Specifics of criteria at each developmental stage is outside of the scope of this review, but details can be found in
Measurement of uptake and secretion of single or specific molecules in the culture medium
Metabolic pathways in cells require the uptake of certain substances as substrates, and an extensive list of products, of which many are secreted into the extracellular space, thereby altering the immediate environment surrounding the cells. This holds true for oocytes, culumus-oocyte-complexes and preimplantation embryos. Years of studies of these metabolic processes has equipped scientists with the knowledge of which agents should be expected to increase or decrease in the cellular
Recent advances in metabolomic grading of oocytes and embryos
Although the techniques used during analysis are essentially the same, oocyte and embryo BSM each presents its own challenges and implications. Probably the most distinct difference in the IVF lab is that embryos are cultured for several days in media drops, whereas mature oocytes are typically only kept in media for a few hours before insemination. They will therefore be discussed individually in light of these differences and progress that have been made in recent years.
Advantages of metabolomics as an embryo quality indicator in a clinical setting
The need for a rapid, accurate and practical method of determining oocyte and embryo viability has driven years of research aimed at finding the best technique for clinical application. Thus far, visual assessment of morphology by light microscopy is the only routinely used means to predict developmental and implantation potential. The remaining technologies, as outlined above, are mostly in the experimental phase, and few of them seem to have the practical potential to be implemented as a
Challenges of using metabolomics in a clinical setting
Despite currently being the most viable option of non-visual quality assessment, metabolomic profiling by spectroscopy of spent media has its challenges. Firstly, it requires installation and implementation of a system into a lab that does not specialize in the technologies. Thus, it requires extensive training of laboratory personnel and dedication to reap the benefits. There are also significant costs associated with both the initial installation and maintenance of the system. The added
The future of metabolomics in clinical oocyte and embryo quality assessment
There are currently many new and emerging technologies competing to be the next “novel application” in the IVF lab. Clinics are constantly looking to offer new services and techniques to patients to increase their chances of conceiving, and to set them apart. It is realistic to expect a non-subjective, qualitative grading system based on pregnancy outcomes to become routine in many labs in the next few years. The need for the ability to select embryos with the highest developmental potential
Conclusions
The available collection of publications on metabolomic profiling has been growing exponentially in the last decade. The progress is encouraging, and should this continue, we expect that metabolomic profiling by Raman and/or NIR spectroscopy will become the rapid, reliable and non-invasive technology that embryologists have been searching for. We expect that it will complement the current methods of oocyte and embryo selection and help to increase overall success rates for clinics that opt to
Conflict of interest
This is the conflict of interest statement for Zsolt Peter Nagy:
- 1)
I owned shares in the company of Molecular Biometrics
- 2)
I was on the Advisory Board of Molecular Biometrics
No other conflicts of interests exist for any of the other authors.
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2022, Research in Veterinary ScienceCitation Excerpt :Metabolomics examines small molecules (<1000 Da), the metabolites, which are the end products of cellular processes that are affected by the physiological state of a cell at any given time (Dumesic et al., 2015). In the case of ovarian cell types, their metabolites are found in a complex dynamic biological fluid that surrounds the developing oocyte: the so-called follicular fluid (FF) (Singh and Sinclair, 2007; Nel-Themaat and Nagy, 2011; Mariani and Bellver, 2018). The FF is formed from the transudation of theca and granulosa cells in the growing follicular antrum (Guerreiro et al., 2018) and contains a variety of molecules, such as steroid hormones, polysaccharides, proteins, reactive oxygen species (ROS), antioxidants, and metabolites (Mariani and Bellver, 2018).
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