An embryo’s nucleus briefly goes soft after the first cell division, and that helps 2,000 genes switch on at once, a mouse study in Nature finds
Mouse embryos whose nuclei stayed stiff were much less likely to keep developing. The Japanese team hopes the finding could someday help IVF clinics pick healthy embryos, but it has not been tested in humans.
In the first days of life, an embryo’s nucleus, the compartment that holds its DNA, briefly turns soft and floppy, and that change helps about 2,000 genes switch on at the same time to start development, researchers in Japan reported in the journal Nature, according to a Kyushu University release Nov. 12.
The work was done in mouse embryos. Embryos whose nuclei did not soften were much less likely to keep developing, the team found. The researchers say that could eventually lead to better ways to judge which embryos are healthy in IVF, or in vitro fertilization, where eggs are fertilized in a lab. They have not yet tested whether the same thing happens in humans.
What happens
- Right after fertilization, the single-cell embryo’s nucleus is stiff.
- After the first division, at the two-cell stage, the nucleus becomes much softer and changes shape. A protein called lamin B1, which acts like scaffolding for the nucleus’s outer wall, is cleared away.
- The softer nucleus lets genes move into position near the nuclear wall, where they are read and switched on. In stiffened nuclei, that gene activity was blocked.
- By the four-cell stage, the nucleus is stiff again.
“We knew that the two-cell stage is a critical time in embryonic development,” said co-leader Kei Miyamoto of Kyushu University. The team also says the method could help study cancer, since some cancer cells have unusually soft nuclei that may help them spread.
How they measured it
To test stiffness without destroying the cell, the team built a tool from a glass tube thinner than the nucleus. The tube is inserted into the cell, gently sucks on the nucleus and measures how much it stretches. The study was co-led by Yuta Shimamoto of the National Institute of Genetics.
Source: Kyushu University, “The softening of an embryo’s nucleus after the very first division is critical for further development,” Nov. 12, 2026; Tanaka et al., “Transient Nuclear Softening for Zygotic Gene Activation in Mice,” Nature.