Embryo quality is the ability of an embryo to perform successfully in terms of conferring a high pregnancy rate and/or resulting in a healthy person. Embryo profiling is the estimation of embryo quality by qualification and/or quantification of various parameters. Estimations of embryo quality guides the choice in embryo selection in in vitro fertilization.
In general, embryo profiling for prediction of pregnancy rates focuses mainly on visual profiles and short-term biomarkers including expression of RNA and proteins, preferably in the surroundings of embryos to avoid any damage to them. On the other hand, embryo profiling for health prediction puts more focus on the genome, and where there is a risk of a genetic disorder it more often involves cell sampling from the embryo for preimplantation genetic diagnosis.
Prediction of pregnancy rates
Microscopy
Embryo quality is mainly evaluated by microscopy at certain time points using a morphological scoring system. This has shown to significantly improve pregnancy rates. Assessment of morphological features as a reliable non-invasive method that provides valuable information in prediction of IVF/intra cytoplasmic sperm injection (ICSI) outcome has been frequently used as an soring system of the embryo quality. The parameters for evaluation at day 2-3:
Number of cells and division rhythm: The optimal number of cells is 4 at day 2 and 8 at day 3 (A quality). In day 3 9-10 cells is B, >=10 is C (suboptimal) and <=4 is D (barely implant). A normal division rate is to double cell number each 24 hours. A higher rate implies chromosomal abnormalities and a lower rate entails possible embryo arrest (it is dying).
Fragmentation: happens due to cell apoptosis and can be quantified by the % of the embryo total volume eccupied by fragments. Fragments are cytoplasm fractions without nuclei.
Cells symmetry and size: it is normal that all blastomeres had same or similar size in embryos with 2, 4 or 8 cells, while for the rest of embryos, a certain variety in cells size is normal. When the number of cells is impaired, if all of them have the same size, it is considered asymmetrical. Those embryos with one big size blasomere is considered abnormal and is associated with high rate of polyploidy.
Multinucleation: multinucleated blastomeres on day 2 and day 3 is associated to a lower implantation rate. These embryos often are mosaics or with aneuploidy. It is more related to abnormalities on day 2 than on day 3.
Cytoplasm aspect: the presence of vesicles on day 3 is considered a sign of embryo genome activation and, therefore, of good prognosis. The presence of vacuoles is a sign of bad prognosis.
Time-lapse microscopy is an expansion of microscopy wherein the morphology of embryos is studied over time. As of 2014, time-lapse microscopy for embryo quality assessment is emerging from the experimental stage to something with enough evidence for broader clinical use. Studies using the EmbryoScope (tm) time-lapse incubator have used several indicators for embryo quality, such as direct cleavage from 1 to 3 cells, as well as the initiation of compaction and start of blastulation. Also, two-pronuclear zygotes (2PN) transitioning through 1PN or 3PN states tend to develop into poorer-quality embryos than those that constantly remain 2PN.
Molecular analysis
Molecular analysis can be performed by taking one of the cells from an embryo. The analysis can vary in extent from a single target biomarker to entire genomes, transcriptomes, proteomes and metabolomes. The results may be used to score embryos by comparing the patterns with ones that have previously been found among embryos in successful versus unsuccessful pregnancies:
Transcriptome profiling
In transcriptome evaluation, gene expression profiling studies of human embryos are limited due to legal and ethical issues.
Gene expression profiling of cumulus cells surrounding the oocyte and early embryo, or on granulosa cells, provides an alternative that does not involve sampling from the embryo itself. Profiling of cumulus cells can give valuable information regarding the efficiency of an ovarian hyperstimulation protocol, and may indirectly predict oocyte aneuploidy, embryo development and pregnancy outcomes, without having to perform any invasive procedure directly in the embryo.
In addition, microRNA (miRNA) and cell-free DNA (cfDNA) can be sampled from the vicinity of embryos, functioning as transcriptome-level-markers of embryo quality.