Osaka Metropolitan University achieves efficient oocyte collection in rats — Contributing to laboratory animal reduction

A research group led by Professor Takehito Kaneko and Researcher Yuki Nakagawa from the Graduate School of Veterinary Science at Osaka Metropolitan University (OMU) announced their success in improving the efficiency of oocyte collection in representative rat strains that show poor response to superovulation protocols. By reconsidering the timing of ovulation induction in hormone-induced superovulation methods, they were able to induce ovulation of 2-3 times the usual number of oocytes. They also confirmed that fertilization capacity was normal. The results are expected to contribute to reducing the number of laboratory animals while being applicable to rare animal species and strains where oocyte collection is difficult. The findings were published in Heliyon.

Humans typically ovulate one oocyte at a time, whereas mice and rats ovulate approximately 10 at a time, but hormone-induced superovulation methods can cause ovulation of 2-3 times as many oocytes at once. However, some animal species and strains are known to show poor response to hormone administration, making it impossible to obtain sufficient oocytes. Therefore, this study aimed to clarify the cause and develop methods for efficiently obtaining oocytes from these strains.

The research used a rat strain (BN: Brown Norway) that shows poor response to superovulation induction methods using the gonadotropic hormones pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG). BN rats are representative laboratory rats that were also used for whole genome sequencing, but they normally produce few offspring. It had been reported that even with PMSG and hCG superovulation induction methods, only an average of a few oocytes could be recovered.

In superovulation induction methods, ovulation is induced by administering hCG 48 hours after PMSG administration. For example, 37 oocytes can be recovered from Wistar rats commonly used in research and 50 from F344 rats. In contrast, only an average of 7 oocytes were actually obtained from BN rats.

To investigate the cause, the research group observed the ovaries of BN rats during ovulation, which revealed that follicular maturation was insufficient. When the hCG administration time was delayed to 72 hours to allow sufficient follicular maturation, the number of ovulated oocytes increased dramatically to an average of 43. Follicular maturation was confirmed to have progressed during this period. However, when delayed to 80 hours, the number of ovulations decreased.

When the fertilized oocytes obtained were transplanted into rat oviducts to produce offspring, 46% developed into normal offspring at the typical rate, confirming that the fertilization capacity of the oocytes was also normal.

It became clear that strains with poor response to superovulation induction methods do not actually have poor responses, but rather have insufficient follicular development, resulting in slow oocyte development and fewer ovulations.

Kaneko commented: "Ovulation mechanisms in mammals are complex and much remains unknown. This research has revealed that even within the same species, there are differences in follicular and oocyte development. We hope to apply these findings to human infertility treatment and artificial breeding of endangered species with low birth rates in zoos. Our laboratory is also engaged in artificial breeding research for the conservation of endangered species. There are many endangered species among rodents, particularly in Okinawa, and we expect that these findings can be applied to them."

Journal Information
Publication: Heliyon
Title: Importance of the eCG-hCG injection interval for superovulation, fertilization, and embryonic development in rats
DOI: 10.1016/j.heliyon.2025.e43619