Time-lapse imaging of thawed embryos
Our goal is to transfer just one embryo to avoid the risks of multiple pregnancy, and we can maintain great pregnancy rates with single embryos when the chance of implantation is extremely high. Transferring two embryos risks multiple gestation, with a 50% chance of twins. PFC is committed to single embryo transfer because this is what is safe for mother and baby.
When embryos are warmed for transfer, we look at them closely to make sure that they have not been damaged by the freezing and warming process. Embryos are preserved using a technique called vitrification, and survival rates from this procedure are above 98%. However, when these embryos are placed into the uterus of a patient, its difficult to get an implantation rate above 70%, even when we know that the chromosome number is normal. Some 30% of embryos fail to implant under apparently ideal circumstances.
Its tempting to suggest that these failures are due to some undiagnosed uterine factor, but in reality its likely that the embryos fail at least some of the time because of some inherent deficiency. In an attempt to figure what this failure is, and how it might be predicted, we have been watching embryo development after thawing with special time-lapse cameras. This project was developed by Sergio Vaccari, PhD, in our embryology lab and was started using embryos that patients had asked us to discard. We took chromosomally abnormal embryos, thawed them and placed them on the time-lapse cameras to see how they would develop.
We saw two things; first we observed that some embryos that appeared perfectly normal under the microscope continued to develop for 4-6 hours but then arrested in their development and died (see figure). And second, we saw that some embryos, particularly those of average quality or below, really struggled to hatch or escape from the shell that surrounds and protects embryos up to the point of implantation. For these later embryos, even when we made a small hole in the shell, using a procedure called Assisted Hatching (AH), embryos still had difficulty getting out.
Our conclusions from this project were that we may overestimate embryo survival after vitrification and that some embryos need help escaping from their shell at the point of implantation. This work won Sergio the top prize for new research at the 2015 Pacific Coast Reproductive Society meeting in Palm Springs.
The research is continuing by moving the project forward to look at embryos that are actually being transferred to patients. We are about to start offering patients the opportunity to have their embryos cultured under time-lapse cameras for a few hours before transfer, and the information we collect will be used to develop a model to predict viability. For example, we will measure the rate at which the cavity (or cyst) in the embryo expands after thawing to see if this predicts implantation. This cavity naturally deflates during the dehydration that is part of the freezing process. And refilling of the cavity involves all the cells on the outside of the embryo so if the cells are healthy, the cavity might be predicted to re-expand quickly. The imaging and measurements are all automated, and time-lapse is a routine method for watching embryos so we believe that there is no risk to the embryos in this study.
We are also making bigger holes in the shell before embryos are transferred to prevent any trouble that embryos may have with hatching. The shell is a non-living protein coat that surrounds an embryo and we can make very precise holes with a laser, to make a hole big enough for an embryo to get through in as little as 4 hours after warming.
Implantation rates of 70% are good, but we have to continue to explore why some embryos still dont implant even under what we see as ideal circumstances. We are hoping that imaging of embryos after warming will give us new insight on predicting implantation and allow us to continue to drive implantation rates higher.
- Joe Conaghan, PhD
PFC Laboratory Director
Figure: The thawed embryo is collapsed (A) and the blue arrow points to the shell. After 1 hour in culture (B) fluid begins to accumulate in the cavity (red arrow). Several hours later (C) the cavity has collapsed again and the embryo eventually disintegrates.