Why do embryos fragment and how does it affect the chance of success?
Fragmentation is very common in human embryos and occurs in embryos that grow inside the body, as well as those grown in the IVF laboratory. In the first few days of life, human embryos increase their number of cells as each cell splits into 2 new identical cells. Occasionally, as the cells split into two some of the contents of the cells (the cytoplasm) are not recaptured into a new cell, but are left free inside the embryo. This cellular debris is termed as “fragmentation”. Unfortunately the reason that this occurs in human embryos is still very poorly understood, several mechanisms have been suggested by various researchers but as yet there is no definitive answer to this question. It does also appear to be embryo specific, as one patient may produce some embryos that are fragmented and some that are not and It can also vary for the same patient from cycle to cycle.
However what we do know from our own clinical data, is that the results with fragmented human embryos are very difficult to predict. Depending upon the degree of fragmentation, good pregnancy rates can still be achieved with these embryos, but sometimes the pregnancy rates are reduced, compared to a similar stage embryo without fragmentation. At Care, we have carried out research to look at the patterns of fragmentation and try to work out from the patterns if we can predict which embryos can achieve a pregnancy and which cannot. From our work, there does not seem to be a clear type of fragmentation that is more successful than another. We think that the reason for this, is that it might depend which parts of cytoplasm “escape” from the embryo and end up as fragments.
Within the embryo are many key cellular structures, as well as chromosomes that contain the genetic information required by the embryo, that are all vital for continued normal embryo development. If one of these vital components is expelled from the cells and ends up as part of a fragment it can no longer take part in the progression of the embryo. In this case the likelihood of successful implantation may be affected. However if the fragments are composed only of cytoplasm without any key structures, then the fragmentation may not affect or reduce the chances of success.
Unfortunately as the chromosomes and cellular structures in the embryo are so small, we cannot see them using a standard microscope and so we cannot tell in the laboratory, what the fragments are composed of. Therefore our only choice is to transfer the embryos and see what the outcome is. In a lot of cases the outcome is positive.
From additional research work carried out at CARE we have been able to show that fragmented embryos can go on and produce genetically normal blastocysts and so we know that these embryos are often capable of achieving a pregnancy. The best option if a whole cohort of embryos is fragmented, is to transfer the best embryo available, as it will still have a reasonable chance of success.