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Mammals may possess the ability to pause embryonic development before implantation, according to a new study in PLoS ONE. Many animal embryos are able to undergo diapause, a period of arrested development, before implanting in the uterus, but though only 2% of mammals were known to have this ability, reported Nature. Now, researchers in Italy and Poland have found that sheep embryos also have this capacity, which suggests that the trick, which can help delay development during stressful conditions, may be more widespread than previously realized.
The researchers introduced sheep blastocysts into the uteruses of mice whose ovaries had been removed. Mouse embryos will undergo a pause in development under these conditions, as the surge in estrogen required for implantation does not occur, making the mice’s uteruses are unreceptive to implantation. Sure enough, the sheep embryos similarly paused development, halting their growth and expressing a set of genes characteristic of diapause in mice. And there seemed to be no ill effects: the scientists successfully removed the embryos, restarted their development by placing them back in vitro, and got healthy lambs after implanting them into sheep surrogate mothers.
Grazyna Ptak, an embryologist at the University of Teramo in Italy, who led the work, hypothesizes that, if embryonic diapause is conserved among mammals, it might underlie the variation in implantation times in humans. She speculates that hormone variation caused by stress might cause some embryos to implant immediately, while others take up to 12 days. "Once the female is not stressed any more, the embryo will implant," Ptak told Nature.
Other researchers think this could also explain the difficulty in estimating due dates by calculating from last menstrual cycle, and suggest that hormone variation might be a more accurate predictor of embryo implantation.