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Squishiness Can Indicate Embryo Viability, Stanford Researchers Find


A team of bioengineers and physicians has found that the squishiness of an hour-old fertilized egg can predict its viability, a metric that could lead to safer, more successful IVF pregnancies.

By Bjorn Carey

Selecting the right fertilized embryo for in vitro fertilization (IVF) might be as simple as choosing a ripe piece of fruit: Just give it a squeeze. If it’s too firm or too soft, it might not be good. The sweet spot in between is just right.

A study by Stanford University bioengineers and physicians discovered that measuring the rigidity of an hour-old fertilized egg can predict its viability more accurately than current methods at this early stage. The technique, published in Nature Communications, has the potential to vastly increase the success rate of single-egg IVF and, consequently, could improve the prognosis for both mothers and babies.

Current IVF embryo screening is a relatively qualitative procedure. Scientists start by fertilizing an egg with sperm. Five or six days later, once the embryo has reached the 60- to 100-cell blastocyst stage, they evaluate the embryo’s morphology and the rate at which cells have been dividing. The best-looking embryos that have been dividing at the “best” rate are then selected for transfer.

Testing squishy embryos

Plucking a few cells from the blastocyst for genetic testing can increase the odds of choosing a successful embryo. However, this invasive procedure can stress the embryo, even though the sample is taken from cells that will eventually form the placenta.

In either case, the results are still uncertain, and the roughly 70 percent failure rate means that doctors typically implant multiple embryos into a mother’s womb in the hope that one will take hold. This can often lead to complications.

“A lot of twins are born because we don't know which embryos are viable or not, so we transfer several at one time,” said Livia Yanez, a lead author of the study and a PhD student in bioengineering in David Camarillo's lab at Stanford. “This can increase the risk of neonatal mortality and cause complications for babies and the mothers.”

“We wanted to develop a mechanical test that could ascertain embryo viability well enough that doctors could implant just one embryo and have a very good feeling that it would be viable,” she said of the work, which was funded by a seed grant from Stanford Bio-X.

Following a tip from the Stanford Medicine Fertility and Reproductive Health Services team, that some eggs are squishier than others, the researchers began investigating whether this had something to do with the fertilized egg’s ultimate development potential.

Using a pipette, the researchers applied a small amount of pressure to mice eggs an hour after fertilization and recorded how much each egg deformed. They placed the embryos in a standard nurturing liquid and reexamined them at the blastocyst stage. The eggs that had provided a certain degree of “push back” at the earlier stage were more likely to produce healthy-looking, symmetrical embryos at the blastocyst stage. The researchers built this data into a predictive computer model that, based only on the egg's squishiness after fertilization, can now predict with 90 percent accuracy whether a fertilized egg will grow into a well-formed blastocyst.

Next, they transferred the embryos to mother mice. Embryos that were classified as viable based on their squishiness were 50 percent more likely to result in a live birth than embryos that were classified as viable using conventional techniques.

“Although cancer and other diseases involve stiff tumors or tissues, our colleagues have been surprised that we can gain so much information from this simple little mechanical test,” said Camarillo, an assistant professor of bioengineering at Stanford. “It is still surprising to think that simply squeezing an embryo the day it was fertilized can tell you if it will survive and ultimately become a baby.”

The researchers repeated the experiment with fertilized human eggs and found that the rigidity assessment could predict with 90 percent accuracy whether the embryo would reach a healthy blastocyst stage. They are ramping up to test embryo viability in IVF patients.

The researchers are still investigating why squeezing a fertilized egg can provide such powerful information. So far, they have found that in the human embryos that were either too firm or too soft, groups of genes that play key roles in repairing DNA, managing cell division, and aligning chromosomes during replication were under- or misexpressed. In addition, they found a down-regulated gene that is linked to egg hardening, which normally occurs after the sperm penetrates the egg to prevent additional sperm from entering. This could hint at why nonviable embryos are unsuccessful, Yanez said.

From a clinical perspective, once confirmed, the benefit is immense in that it could give us a proxy of viability of the embryo in the blink of an eye, and from that information, we can adjust the patient's cycle in order to improve success.

Additional co-authors on the study include Jinnuo Han of the Stanford School of Medicine and Renee Reijo Pera, formerly of the Stanford School of Medicine and currently at Montana State University.

Reprinted by permission of Stanford News Report

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