No country has yet modified its laws to take advantage of that latitude.Įmbryo models are generally made from embryonic stem cells, “pluripotent” cells derived from early embryos that can develop into every tissue type in the body. It now recommends that the 14-day limit on human embryo research be relaxed on a case-by-case basis if a good scientific case can be made for extending it. Recognizing the new potential for finding out useful information about how human embryos develop post-gastrulation, the International Society for Stem Cell Research revised its guidelines in 2021. Hanna thinks the technology could also work with human embryos and could perhaps grow them for many weeks-if the aims of the science justified the project responsibly and the law did not forbid it. Using their rotating bioreactor, in which the embryos were sustained in a nutrient solution and an atmosphere with precisely controlled oxygen and carbon dioxide levels, the team grew mouse embryos for 12 days, half of the full gestation period for mice. Hanna and his coworkers showed in 2021 that they could grow natural mouse embryos in vitro far beyond gastrulation. In 2016, however, Zernicka-Goetz’s team at Cambridge and the developmental biologist Ali Brivanlou at Rockefeller University and his colleagues showed that they could grow IVF mouse embryos all the way up to the gastrulation stage, using a soft polymer gel matrix as a kind of uterine surrogate. For decades, it was a comfortable restriction, since human embryos generally stopped growing in vitro after only five to six days, around the stage when they would normally implant in the uterine lining. This 14-day rule was subsequently implemented in the guidelines of the International Society for Stem Cell Research, which are widely followed by scientists worldwide. In 1990, following reports from the US Department of Health, Education and Welfare and the UK Warnock Committee years earlier, many countries decided that the formation of the primitive streak at 14 days should mark the limit for how long human embryos could be sustained in vitro. A central furrow called the primitive streak develops as the precursor to the spinal column, defining the nascent body’s central axis of bilateral symmetry. The cells begin to specialize into the tissues that will form the nerves, internal organs, gut, and more. As the developmental biologist Lewis Wolpert put it, “It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.” That is when the rather featureless blob of embryonic cells starts to fold and rearrange itself to acquire the first hints of body structure. That two-week point is when one of the most crucial stages of development occurs, called gastrulation. You’d need to be an expert to distinguish these living entities from real mouse embryos at a comparable developmental stage. After about eight days, it was possible to make out the central axis that would, in a normal embryo, become a spinal column, along with the bulbous blob of the nascent head and even a primitive beating heart. Teams led by Magdalena Zernicka-Goetz at the University of Cambridge and by Jacob Hanna at the Weizmann Institute of Science in Rehovot, Israel, both made them from mouse stem cells and grew them in rotating glass bottles filled with nutrients, which acted like a kind of crude artificial uterus. Meanwhile, other research groups showed last year just how far these embryo models made from stem cells can develop toward whole organisms. None of the pregnancies lasted more than a few days, however, before spontaneously terminating. In three of the monkeys, the models successfully implanted in the uterus and continued to develop. Then they transferred the models into eight female monkeys. The multi-institutional team of researchers, led by Zhen Lu at the State Key Laboratory of Neuroscience in Shanghai, grew the embryo models in vitro to a roughly nine-day stage of development, making them equivalent to what is called a blastocyst in normal embryos. Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
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