They found that of the 242 embryos, 100 were able within five or six days to form normal-looking blastocysts about the same proportion that would be expected to be successful in normal pregnancies. Because they had tracked the embryos' development so closely, they were then able to go back and identify three specific parameters collectively associated with successful blastocyst formation: the duration of first cytokinesis (the last step of a period in the cell cycle called mitosis in which the cell physically divides), the time between first and second mitoses, and the synchronicity of the second and third mitoses. All of these events occur as the embryo progresses from one cell to four cells within the first two days after fertilization.
"It completely surprised me that we could predict embryonic fate so well and so early," said Reijo Pera. If an embryo's values fell within certain windows of time for the three predictive parameters, that embryo was more than 90 percent likely to go on to develop successfully into a blastocyst.
When the researchers looked at the gene expression profiles of individual cells from the embryos, they found that, as had been previously shown, the embryos at first express only genes from the maternally derived egg. By roughly the third day (the eight-cell stage) they begin to express genes specific to embryonic development, and the relative proportion of embryonic to egg genes increases steadily during the next few cell divisions.
Surprisingly, however, they found that not all cells in an embryo are behaving identically: While some cells may be expressing mostly maternal genes, others in the same embryo are churning out mostly embryonic genes.
Similarly, not all cells in an embryo are dividing in synchrony: The researchers found embryos in which some cells were dividing on sc
|Contact: Krista Conger|
Stanford University Medical Center