Warming the testicles only slightly can lead to male infertility

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OKAZAKI, Japan — The colder it is, the better it is when it comes to male fertility, a new study reveals. Researchers in Japan have found that even slight changes in the temperature of a man’s testicles can quickly lead to an inability to produce healthy sperm.

Spermatogenesis is the process by which sperm cells develop in a man’s testicles. The testicles contain many thin, tightly coiled tubes called the seminiferous tubules. Sperm develop in the walls of these tubules.

The new study found that at 34° Celsius (93.2° Fahrenheit), spermatogenesis is able to successfully produce healthy sperm. However, at temperatures between 37 and 38 °C (98.6 and 100.4 °F), this process severely breaks down.

Specifically, the process that separates homologous chromosomes into haploid sperm (meiosis) begins to malfunction. Damaged cells also begin to undergo cell death, leading to infertility.

Blue bars indicate germ cell types observed in testis explants cultured at indicated temperatures. *No germ cells survived at 40°C. (CREDIT: NIBB)

Heat is bad for humans and animals

The researchers note that several previous studies have examined the link between heat and impaired spermatogenesis in animals. These experiments involved moving the testes into an animal’s abdomen, where the body temperature is between 98.6 and 100.4°F.

However, these experiments could not control the actual testicular temperature. The new study used an organ culture of testicles to support full spermatogenesis inside an incubator. By culturing mouse testes at different temperatures, the team discovered that spermatogenesis fails in several stages when temperatures increase from 30°C (86°F) to 40°C (104°F).

“We did not expect such a delicate set of multiple temperature-dependent events to underlie this well-known phenomenon. This discovery could only have been achieved using an organ culture system” , says study leader Shosei Yoshida of the National Institute of Basic Biology in a Press release.

In terms of meiotic dysfunction, the ability to repair DNA double-strand breaks and pairing of homologous chromosomes collapses between 37 and 38°C. These two capacities are necessary for a good segregation of the chromosomes.

death of sperm
At 34°C (scrotal temperature), meiosis proceeds normally with all homologous chromosomes paired normally. At 37 and 38°C (core body temperature), spermatocytes show an aberrant chromosome pairing and undergo cell death. (CREDIT: NIBB)

“It was surprising that essential processes, such as meiosis, could be easily damaged at normal body temperatures. Due to the combined functions of the scrotum and checkpoint, only sperm developed at low temperatures fertilize eggs to generate generation. Key questions for future studies include the molecular mechanism of heat sensitivity and the biological importance of low temperatures in sperm production,” concludes Kodai Hirano, the lead contributor to this study.

The results appear in the journal Communications Biology.

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