To find out how big of a problem mosaicism is, the researchers developed a mathematical model using data from UF's IVF program and other programs to illustrate the different paths an embryo takes as it develops. While half of embryos turn out normal, with no mosaicism, the other half produce irregular cells as they divide.
Embryos that become mosaic after their cells divide twice typically stop progressing by the time they have accumulated eight cells. Too many of their cells have chromosomes that don't match and these mixed messages lead the embryo to stop developing, Kovalinskaia said.
Embryos with only one or two irregular cells still have a chance to develop normally though, she said. These embryos tend to not develop this abnormality until after their cells have divided a third time, typically the time when doctors perform preimplantation genetic diagnosis. This is where the problem with diagnosis occurs, researchers say.
"Our pathway concept may help to explain the observed outcomes during preimplantation diagnosis," said Kenneth Drury, Ph.D., director of the UF IVF and Andrology Laboratory and a clinical professor in the College of Medicine who was one of the researchers. "When we biopsy an eight-cell embryo, if we choose (the one) normal cell in an abnormal embryo, we may think that embryo is normal and transfer it.
"This occurs about 1 percent of the time, which means 99 percent of the time we will be accurate. Again the error is not due to a technical mistake but to natural errors occurring during early cell division in the embryo."
Extracting test cells before the embryo divides a third time could reduce the number of misdiagnoses because of mosaicism, the UF researchers suggest. Improving culture conditions could help too, Kovalinskaia said.
But the small margin of error shouldn't stop couples from having preim
Source:University of Florida