Embryo vitrification left a mark on mitochondria in the adult mouse heart

After embryo vitrification, mitochondria in the left ventricles of adult mice performed less effectively during energy production and at maximum respiratory capacity

On 15 August, the authors posted a preliminary research paper, or preprint, describing an experiment with four groups of mouse embryos: embryos produced by natural fertilization or IVF, either fresh or vitrified. In adult mice that developed from vitrified embryos, mitochondria from the left ventricle consumed oxygen more slowly during ATP production and at maximum respiratory capacity. Both IVF and vitrification increased hydrogen peroxide (H₂O₂) production per unit of oxygen consumed.

Vitrification preserves an embryo at a very low temperature. The embryo is cooled rapidly, then warmed before transfer to the uterus. In the United Kingdom, frozen embryo transfers accounted for 48% of all IVF cycles in 2024.

The authors obtained blastocysts, embryos several days after fertilization, either naturally or through IVF. In each branch of the experiment, some embryos were transferred to the uterus while fresh, while others were first vitrified and warmed. Recipient females carried all four groups. The IVF group combined fertilization outside the body with subsequent embryo culture.

Changes were already detectable in the blastocysts. IVF and vitrification reduced mitochondrial membrane potential, the electrical gradient that helps produce ATP, and depleted glutathione, a substance that helps cells neutralize oxidants. Vitrification also increased the proportion of cells showing signs of programmed cell death and reduced the proportion of the inner cell mass, which gives rise to the organism.

The heart obtains most of its energy from mitochondria. In these organelles, oxygen helps convert nutrients into ATP, a molecule that carries energy. In the adult offspring, the researchers isolated mitochondria from the left ventricle and measured oxygen consumption during ATP production and at maximum respiratory capacity. Vitrification reduced oxygen consumption under both conditions. In the IVF group, respiration supported by fatty acid substrates was lower. In both branches of the experiment, more H₂O₂ was produced per unit of oxygen consumed.

Inside mitochondria, electrons pass through a chain of protein complexes, and complex III is one part of this chain. Its activity was lower after both procedures. When the researchers blocked complex I, the differences in H₂O₂ production remained. After they blocked complex III, the differences disappeared. This experiment identifies electron transfer through complex III as a shared point of vulnerability.

Cryobiologists have proposed comparing experiments according to the biological system that was preserved and the test subsequently used to assess its function. This approach is described in the biostasis classification. In this study, the authors measured mitochondrial respiration in the left ventricles of adult mice that developed from transferred embryos.

In this mouse model, vitrification altered both the state of the early embryo and mitochondrial function in the left ventricle during adulthood.