Having a child is the dream of many infertile couples. Thousands of children are born through in vitro fertilization (IVF). IVF means that the sperm and eggs of mammals complete the fertilization process in an artificially controlled environment outside the body. However, when a sperm and an egg are fertilized in a test tube, the resulting embryo is often defective. In a study published in the journal PNAS on January 4, researchers at the Research Center of the University Hospital of Montreal (CRCHUM) in Canada discovered an important element for understanding "how these abnormal phenomena occur in developing embryos."

　　Greg FitzHarris, a professor at the University of Montreal and researcher at CRCHUM, pointed out: “About half of the embryos used for fertility treatment have some cells with the wrong number of chromosomes. These so-called “mosaic” embryos are considered to be of poor quality. Clinics choose not to transfer them to women's uterus. By studying mouse embryos, we found that in the developing embryo, defective cells divide and continue through a mechanism."

　　In mice, normal oocytes (eggs) contain 20 chromosomes, while in humans they contain 23 chromosomes. The abnormal number of chromosomes in cells, called aneuploidy, is a well-known problem in reproductive biology. FitzHarris said: "In eggs and embryos, aneuploidy is usually related to infertility. However, the reason why aneuploidy cells appear in embryos has always been mysterious. Using cutting-edge microscope technology, we The small satellite-like structures next to the main nucleus can be distinguished, called micronuclei. By monitoring how these cells containing micronuclei divide, we have observed that the genetic material from the micronuclei is only passed on to daughter cells. This indicates that micronuclei are produced. Aneuploidy, which in turn leads to mixed embryos."

　　This is a discovery obtained in mice, but the same mechanism may exist in humans. Choosing the best embryo is the key to successful in vitro fertilization. In the fertility clinic, a morphological examination of the embryo is performed three or five days after conception and before it is transferred to the woman's uterus. In order to check whether it is a mixed embryo, a procedure is sometimes performed to remove embryonic cells and perform genetic analysis. This method is complicated, expensive and invasive. Jacques Kadoch, medical director of the Assisted Reproductive Clinic at the Health Center of the University of Montreal (CHUM), points out: "This discovery is important because if future studies show that the same phenomenon exists in humans, it can provide early non-invasive detection of aneuploidy. "

　　The issue of separating mixed embryos is controversial. Some clinicians believe that defective embryos should not be used. Some other doctors believe that mixed embryos can produce healthy babies, indicating that the embryos can repair themselves naturally. However, abandoning the debate about whether to use mixed embryos and a better understanding of the mechanisms involved in embryonic development are crucial and open up a whole new field of research. FitzHarris believes: "We want to understand how to help embryos develop normally, because ultimately we want to increase the chances of success for infertile couples who rely on medically assisted reproduction methods."

　　In IVF, only 30% to 50% of transferred embryos will lead to pregnancy. Therefore, choosing the best embryos and ensuring that they are healthy on the first day of conception is a major issue for doctors and infertile couples. challenge.