Mild experimental increase in testis and epididymis temperature in men: effects on sperm morphology according to spermatogenesis stages.

Plain-language summary

This study set out to find which stage of sperm production is most sensitive to mild heat. Five healthy volunteers wore a specially designed underwear that held the testicles up against the body, which keeps them warmer than usual but still below core body temperature, for about 15 hours a day for 120 days in a row. Semen was collected on set days chosen to catch sperm at known stages of development. The share of abnormally shaped sperm rose by day 34 of heating, stayed high for the whole heating period, and was still high 45 days after the heating stopped. It returned to starting values 73 days after the heating stopped. The pattern of abnormalities pointed at the later stages of sperm formation.

Key findings

• The share of abnormally shaped sperm rose by day 34 of heating and stayed high for the rest of the 120 days.
• The abnormalities were still present 45 days after the heating stopped.
• Sperm shape returned to starting values 73 days after the heating stopped.
• The pattern pointed at the later stages of sperm formation rather than the earliest ones.
• This was five men, so the timeline is the useful part and the size of the effect is not something to lean on.