For several years now the topic of mosaic embryos as sparked a great deal of debate throughout the medical community, as well as certain organizations, and even a few political venues. In technical terms, it is a type of embryo which contains normal as well as abnormal cells. In the In Vitro Fertilization process, these embryos are not considered for further development or transfer to an otherwise healthy uterus.
A growing body of research has found that somewhere around 20 percent of all embryos that are produced through in vitro fertilization process classify as being the mosaic. Screening and identification of these embryos are typically done through a special test Preimplantation Genetic Screening test or PGS. It is a critical step in assessing which fertilized embryos will be selected for implantation into the woman’s uterus.
New research into the fertility processes of nonhuman primates shed new light on the frequency of abnormal cells and genetic structures in embryos. This might provide new hope for couples seeking IVF treatment or those who are already engaged in infertility treatments.
How Frequent Are Mosaic Embryos?
It’s important to keep in mind that not every embryo that is produced in the IVF process contains the 46 perfectly healthy chromosomes. There are some embryos that have more, as well as others which have fewer. This relatively common abnormality is known as aneuploidy, and there are some research studies that estimate it can occur in as many as 80% of all human embryos.
For some couples, this percentage can be even greater. There have indeed been couples where a single IVF cycle produced only mosaic embryos.
The unfortunate trend with aneuploidy is that it has been linked in many cases to an increased risk of IVF failure, as well as miscarriage issues, certain genetic disorder, and birth defects. Due to this, mosaic embryos that are found during the Preimplantation Genetic Screening test that have both normal and abnormal cells, are not considered as ideal candidates for IVF transfer.
For many couples who produce only mosaic embryos, it could signal the end of the IVF process. Especially, if they have only been approved by their medical provider for a single IVF cycle. This can be emotionally distressing and leave the couple yearning for answers.
Fortunately, recent research published in the journal Genome Research by specialists at the Oregon National Primate Research Center at OHSU, in Portland, Oregon, offers new hope for couples seeking infertility treatments. The study was headed up by Shawn L. Chavez, Ph.D. It represented the first confirmation of a non-human primates mosaic embryos with the ability to adapt to their abnormalities as well as persist in development. In several of these instances, it even resulted in a positive IVF outcome.
The processed the researchers used employed sophisticated time-lapse imaging integrated with single-cell sequencing techniques. This gave them the ability to precisely track the development of mosaic embryos produced by a rhesus macaque. The researchers were able to identify a unique relationship that can develop between mosaic embryos and two other important biological processes. Specifically, it was related to cell fragmentation and the process of blastomere exclusion.
Genetic Adaptation May Be Possible In Mosaic Embryos
After IVF transfer, in utero large cells are formed as a fertilized egg starts to divide. This is known as blastomeres, which can potentially break down into smaller cellular fragments. In the past problems that developed in this stage of development often lead to excluding that embryo from the uterine transfer.
Through their special imaging process, the researchers found that both the blastomeres and the cellular fragments started to act like garbage cans within the embryo. While the cells carrying critical DNA continue to divide, the embryo itself appears to be able to naturally identify the blastomeres that carry genetic abnormalities. It then actively works to prevent them from developing any further.
By the time the embryo reaches the stage where it would naturally implant into the uterus, the abnormal cells, and segments of DNA have been excluded from the rest of the embryo. This suggests that even a previously imperfect IVF embryo could be considered for transfer to the uterus while still possessing the ability to divide. It could offer new hope to couples trying to conceive via IVF, yet only end up with a high number of mosaic embryos.
Other researchers involved in the OSHU study noted how this discovery could have a very positive impact on the IVF processes for humans in the future. It could also have the potential to open up new techniques for testing the efficacy of human mosaic embryos.
Of course, the ideal selection of embryos with normal chromosome development will remain the preferred option. It certainly carries the highest probability of resulting in a successful pregnancy. Yet, couples whose first, or only IVF cycle, yields only mosaic embryos, there might be a way to identify ones that are potentially viable for transfer. The early findings from this study do seem to suggest that in some of these scenarios these embryos can potentially result in a normal pregnancy.
The Potential For Future Research
Future research studies into this cellular phenomenon will need to implement the same type of live-time lapsed cellular imaging to shed further light on the relationship between aneuploidy, and cell fragmentation in the blastomere exclusion process that occurs within the early stages of a developing embryo.
The hope is that the overall results of these future tests will be similar to what was seen in the rhesus macaque study published by the Oregon National Primate Research Center at OHSU. While mosaic embryos are considered to be a contender for selection at the start of the IVF process they might still be viable for producing a successful pregnancy and a healthy fetus.
While this research is still in its early stages, it does provide a ray of hope. Further studies may also be able to yield greater insights on the specific cellular mechanisms to identify in mosaic embryos.
Source – OHSU