1. Overview
The research team discovered that glassware used to manipulate and culture fertilized eggs in the fields of assisted reproductive technology (ART), livestock farming, and basic research contains toxic substances that interfere with the development of fertilized eggs (embryos). The team also identified the toxic substance as zinc, and developed a method to counter it by closely analyzing its effects on mouse, cow, and human fertilized eggs. The results of this research suggest that the unknown cause of reduced IVF outcomes might be due to the toxicity of glassware. It is hoped that this will lead to the development of safer and more effective IVF methods in the future. The researchers participating in this research include: Kazuo Yamagata (Professor, Graduate School of Biology-Oriented Science and Technology, Kindai University), Tatsuma Yao (Senior Researcher, Research and Development Center, Fuso Pharmaceutical Industries, Ltd.), Kazuki Kurimoto (Professor, Department of Embryology, Nara Medical University), Akira Funabashi (Professor, Department of Biosciences and Informatics, Keio University), Mikiko Tokoro (Researcher, Asada Institute for Reproductive Medicine, Asada Ladies Clinic), Satoshi Sugimura (Professor, Department of Biological Production, Tokyo University of Agriculture and Technology), and Takuya Yamamoto (Professor, Institute for the Advanced Study of Human Biology (ASHBi), Kyoto University).
2. Key Points
・Discovered that some glassware used in in vitro fertilization (IVF) inhibits the development of fertilized eggs.
・Identified that the toxic substance that inhibits development of fertilized eggs was "zinc" eluted from glassware.
・The findings of this study leads to the development of safer and more effective in vitro fertilization (IVF) methods.
3. Research Background
In the field of assisted reproductive technology (ART), livestock farming, academic basic research, in vitro fertilization (IVF) is performed in which eggs and sperm are removed from the body, fertilized and developed in culture medium, and then the fertilized eggs are transferred to the uterus. In the nearly 50 years since the first IVF baby was born in 1978, many technological developments have been made in fertilization methods, culture media, and other areas to improve the development rate of fertilized eggs and pregnancy rates. As a result of these technological developments, the birth rate in ART has risen significantly from around 5% in 1980 to around 30% today. In order to further increase the birth rate, it is necessary to improve the environment in which embryos are cultured. Many glass instruments are used in IVF operations. For example, glass-bottom dishes, which have a thin glass bottom that allows high-resolution observation of the embryos, are used as culture dishes. Filters containing glass fibers are used to remove impurities and bacteria from the culture medium, and thin glass pipettes are also used to manipulate the embryos. In addition to these, micropipettes used in intracytoplasmic sperm injection (ICSI) procedures and some containers for culture medium are also made of glass.
In previous research, the team developed a live-cell imaging technique to continuously observe the developmental process of embryos over a long period of time. This technique makes it possible to use large amounts of image data to numerically detect subtle differences in embryo development. During the course of the experiments, the team noticed that in some cases, the embryo development rate unexpectedly declines, even though multiple experimental conditions were kept consistent. After investigating the reason for this, the team found that the declines were caused by the glass-bottom dishes used for culture, and began their research based on the hypothesis that laboratory equipment, especially glassware, might contain some kind of toxicity that inhibits embryo development.
4. Content
The research team discovered that some of the glassware had leaked toxic substances that inhibited embryo development, and after analyzing the substances, the team found that the toxic substance was zinc. Zinc induced delayed development in mouse embryos, abnormal chromosome segregation, abnormal cytokinesis, and abnormal zygotic gene activation, and significantly reduced the formation of blastocysts.
Post-implantation, zinc-exposed embryos were associated with similar birth rates to embryos cultured without zinc, however, the birth weight increased by an average of 18%. Furthermore, zinc exposure affected the development of bovine and human embryos, with species-based variation in the strength of these effects. As a result of investigating ways to reduce the embryotoxic effects of zinc, the research team found: The embryo development rate and birth rate do not decrease in culture medium that had come into contact with glass, by adding chelating agent EDTA (ethylenediaminetetraacetic acid) to the culture medium at the appropriate time and concentration, or by thoroughly cleaning the glassware beforehand. This research highlights the importance of careful management of the equipment used in embryo culture, and also is expected to lead to the development of safer and more effective in vitro fertilization methods.
