In vitro cultures of oocytes within preantral follicles are promising research areas for producing viable eggs in assisted reproductive technology. While previous studies have successfully matured eggs from cultured preantral follicles, denuded undeveloped oocytes, sometimes collected with these follicles, are typically considered unviable. Furthermore, effective methods for culturing these denuded oocytes to produce viable eggs have yet to be established.
Researchers at Shinshu University have created a technique for lab-culturing mature oocytes from denuded oocytes. This discovery marks a huge achievement in fertility research and has great promise for resolving important issues in reproductive science.
A study led by Assistant Professor Kanako Morohaku at Shinshu University, published on August 21, 2024, in Biology of Reproduction, presents a novel method for producing viable, embryo-forming eggs from denuded oocytes taken from preantral follicles in mice. Preantral follicles are an earlier developmental stage than the more commonly used antral follicles in assisted reproductive technology (ART).
In this study, the researchers isolated denuded oocytes and granulosa cells (GCs) and cultured them in a U-shaped dish for eight days. This method allowed the formation of granulosa cell-oocyte complexes (GOCs), which helped the oocytes interact with supporting cells again.
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The researchers also investigated the effects of oxygen concentration on oocyte growth by culturing the GOCs under both high (20%) and low (5%) oxygen levels, based on previous findings suggesting oxygen levels influence oocyte quality. This approach could help overcome challenges in using preantral follicles for ART.
In their study, the researchers found that low oxygen levels significantly improved the development of granulosa cell-oocyte complexes (GOCs) and promoted the formation of transzonal projections crucial for oocyte growth. After culturing the GOCs on a collagen-coated membrane for four more days, the oocytes matured into fertilizable eggs.
The team then fertilized these eggs and transferred 126 embryos into female mice, resulting in six live pups. This marks the first successful production of offspring from preantral follicle-derived denuded oocytes cultured in vitro, a significant milestone in reproductive science.
This finding has implications that go beyond fertility treatments for humans. According to the researchers, the strategy may help large animals like pigs and cattle, whose ovaries have a distinctive shape that makes typical egg maturation techniques less efficient.
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Additionally, the approach could aid in conserving endangered species by enabling ex vivo egg production, offering a new way to preserve genetic resources.
The researchers are optimistic that their innovative method for culturing denuded oocytes will be a valuable advancement in assisted reproductive technology (ART). Prof. Morohaku highlights that ex vivo egg production is crucial for human infertility treatments and livestock and endangered species. This technique could improve genetic resource management, breeding, and the creation of animal models to study germ cell development.
The method has special potential for animals like pigs and cattle, whose ovaries have a special shape that makes traditional culturing methods less successful. The group thinks that their research could greatly advance reproductive biology, opening up new avenues for treating infertility and aiding in the preservation of threatened species. Their work aims to advance human reproductive health and genetic diversity preservation in the animal kingdom.
Journal Reference:
- Tomohiro Kohama, Ikuo Tomioka, Kanako Morohaku. In vitro production of viable eggs from undeveloped oocytes in mouse preantral follicles by reconstructing granulosa cell–oocyte complexes. Biology of Reproduction. DOI: 10.1093/biolre/ioae155