A recent study shows that a one-day-old human embryo consists of two apparently identical cells, but one actually contributes more to the body's organs and tissues. This discovery may help improve the success rates of in vitro fertilization (IVF).
Published in the journal Cell, the study reveals that the fertilized egg's first division predisposes subsequent cells toward different fates, laying the foundation for development into a complex fetus. Ali Brivanlou, a developmental biologist at Rockefeller University in New York City, called the study "a major advance" and said its clinical significance will become clearer as research progresses.
Origins of Asymmetry
Researchers long believed that all cells in a mammalian fertilized egg were identical until the embryo reached 16 cells, with differentiation beginning only later in development. After all, a fertilized egg can split into two separate embryos after several cell divisions and still produce identical twins.
However, in a 2001 paper, developmental biologist Magdalena Zernicka-Goetz of the California Institute of Technology reported that the first two cells of a mouse embryo are actually different. One cell primarily forms the fetus, while descendants of the other mainly form the yolk sac.
Years of Research
Zernicka-Goetz had long wondered whether the same phenomenon occurs in humans. She obtained 54 fertilized eggs from an IVF clinic before they had completed their first division. The researchers allowed the eggs to divide in the laboratory and labeled one blastomere with a fluorescent protein, enabling them to track how each cell's descendants developed.
After four to five days in culture, the embryos began forming distinct structures. Analysis showed that most cells in the structure that forms the fetus came from the faster-dividing blastomere, while descendants of the slower-dividing cell primarily developed into the yolk sac.
Brivanlou was surprised by the degree of asymmetry but considered it logical because the human body ultimately becomes extremely complex. "The deeper we look, the more we appreciate that life is a series of successive symmetry-breaking events," he said.
Causes of Asymmetric Development
It remains unclear what causes this asymmetry. In mice, the point where the sperm enters the egg affects how the egg divides. Zernicka-Goetz believes that other factors, such as chromosome organization within the egg, may also affect this balance.
Understanding which cells are more likely to form the fetus could enable IVF clinics to screen embryos more effectively and identify those most likely to result in a successful pregnancy. "If we can understand where the vulnerabilities are during this period, some miscarriages could be prevented," she said. Although it is difficult to predict how this early asymmetry affects the body later, its effects may be long-lasting.
News | Scientific Breakthrough: Human Embryos Form the Body Through Asymmetric Cell Division
A recent study shows that a one-day-old human embryo consists of two apparently identical cells, but one actually contributes more to the body's organs and tissues. This discovery may help improve the success rates of in vitro fertilization (IVF).
Published in the journal Cell, the study reveals that the fertilized egg's first division predisposes subsequent cells toward different fates, laying the foundation for development into a complex fetus. Ali Brivanlou, a developmental biologist at Rockefeller University in New York City, called the study "a major advance" and said its clinical significance will become clearer as research progresses.
Origins of Asymmetry
Researchers long believed that all cells in a mammalian fertilized egg were identical until the embryo reached 16 cells, with differentiation beginning only later in development. After all, a fertilized egg can split into two separate embryos after several cell divisions and still produce identical twins.
However, in a 2001 paper, developmental biologist Magdalena Zernicka-Goetz of the California Institute of Technology reported that the first two cells of a mouse embryo are actually different. One cell primarily forms the fetus, while descendants of the other mainly form the yolk sac.
Years of Research
Zernicka-Goetz had long wondered whether the same phenomenon occurs in humans. She obtained 54 fertilized eggs from an IVF clinic before they had completed their first division. The researchers allowed the eggs to divide in the laboratory and labeled one blastomere with a fluorescent protein, enabling them to track how each cell's descendants developed.
After four to five days in culture, the embryos began forming distinct structures. Analysis showed that most cells in the structure that forms the fetus came from the faster-dividing blastomere, while descendants of the slower-dividing cell primarily developed into the yolk sac.
Brivanlou was surprised by the degree of asymmetry but considered it logical because the human body ultimately becomes extremely complex. "The deeper we look, the more we appreciate that life is a series of successive symmetry-breaking events," he said.
Causes of Asymmetric Development
It remains unclear what causes this asymmetry. In mice, the point where the sperm enters the egg affects how the egg divides. Zernicka-Goetz believes that other factors, such as chromosome organization within the egg, may also affect this balance.
Understanding which cells are more likely to form the fetus could enable IVF clinics to screen embryos more effectively and identify those most likely to result in a successful pregnancy. "If we can understand where the vulnerabilities are during this period, some miscarriages could be prevented," she said. Although it is difficult to predict how this early asymmetry affects the body later, its effects may be long-lasting.