Scientists from the Institute of Immunology and Physiology of the Ural Branch of the Russian Academy of Sciences and Ural Federal University have found that elevated temperatures not only impair sperm quality (concentration and motility) and lower testosterone levels, but also activate mast cells in the male reproductive organs, which may in turn affect reproductive function. The findings, based on experiments with rats, have been published in the journal Pathophysiology.

"Traditionally, research on male infertility has focused on sperm cells — their functional characteristics and genetic features. However, growing evidence suggests that the microenvironment within the testicles plays an equally important role in supporting normal spermatogenesis and testosterone synthesis," says Boris Yushkov, Head of the Laboratory of Immunophysiology and Immunopharmacology at the Ural Branch of the Russian Academy of Sciences.

Mast cells release biologically active substances that can alter the testicular microenvironment. Under normal conditions, testicles function at a temperature 2–4 °C below core body temperature. The experiments demonstrated that exposure to 43 °C in rats activated mast cells and stimulated their migration into the testicles and epididymides.

"Mast cells are among the cells that respond to heat stress. Under elevated temperatures, they can change the normal microenvironment in the testicles through the substances they release, which may potentially lead to disruptions in male reproductive function," explains Ali Sadek, co‑author of the study and a PhD student at UrFU Department of Biology and Fundamental Medicine from Syria.

The impact of elevated temperature depends on both its intensity and duration. In most cases, the effects are reversible: a single visit to a sauna is unlikely to cause persistent issues in healthy individuals. However, in conditions associated with chronic testicular overheating — such as cryptorchidism or varicocele — as well as in certain occupational settings involving prolonged and regular heat exposure, significant changes in spermatogenesis may occur, notes Yulia Khramtsova, Senior Researcher at the Laboratory of Immunophysiology and Immunopharmacology and Associate Professor at the Department of Biology and Fundamental Medicine at UrFU.

The researchers also point to lifestyle factors that may contribute to reduced male fertility: a sedentary lifestyle, obesity, tight underwear and clothing, occupational heat exposure, and pathological changes in the testicles.

"Approximately 30 % of male infertility cases are classified as idiopathic — meaning their exact cause is unknown. Infertility affects around 17.5 % of couples worldwide, and in Russia this figure may reach 20 %," adds Ali Sadek.

The findings could help inform the development of new approaches to managing fertility‑related issues associated with heat stress. In the long term, the researchers plan to investigate strategies for maintaining a healthy testicular microenvironment, preserving beneficial cellular interactions, and mitigating the negative changes caused by various stressors.

Anastasia Pyankova