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Fertility treatments are becoming more advanced, including through cryopreservation, or the freezing of eggs and sperm. However, studies involving animal sperm often use a lactose solution containing chicken egg yolk and OEP, a detergent containing sodium lauryl sulfate. The egg yolk carries the risk of microbial contamination and high batch variability, while the detergent has the potential to damage oocytes during in vitro fertilization.

These issues with current sperm cryopreservation solutions motivated a team of researchers at Kyoto University and RIKEN to see if they could find a better alternative. They set out to develop a more reliable, chemically defined cryopreservation solution for rat sperm that does not contain egg yolk or OEP, which they then evaluated for its IVF efficacy.

"We were particularly interested in whether rat sperm could be cryopreserved without egg yolk, which has been used for many years in conventional sperm preservation solutions," says first author Kohtaro Morita.

The team first performed cryopreservation using only the lactose solution, without the egg yolk or OEP, and examined sperm motility after thawing, helping the researchers determine the optimal lactose concentration. They then added OptiPrep, which is known to be effective in protecting sperm during cryopreservation, and found that it improved sperm motility after thawing.

The researchers subsequently added various cryoprotectants in different combinations and concentrations to identify the formulation that yielded the highest rat sperm motility.

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