Researchers at the University of Colorado Anschutz Medical Campus conducted a new study that found stress-induced changes in sperm motility happen after a stressful event, not during it, and that these changes actually enhance sperm performance. The work, published today in Nature Communications, provides fundamental insights into how stress affects the reproductive process, with the potential to improve foetal development outcomes.

Globally, semen quality has declined significantly during the last five decades, correlating with a variety of environmental stresses. This new study examines the effects of stress on sperm's ability to move through a female's reproductive system and fertilise an egg, a process known as motility. The researchers focused on extracellular vesicles (EVs), which are tiny particles produced from the male reproductive system and contribute to sperm growth and maturation. Interestingly, the study discovered that changes in these EVs occurred after the stressful event had gone rather than during the event itself.

"Our findings show a significant, time-dependent increase in sperm motility following perceived stress, which aligns with previous studies on changes in microRNA in human sperm," said Tracy Bale, PhD, the study's lead author and the Anschutz Foundation Endowed Chair in Women's Integrated Mental and Physical Health Research at the Ludeman Centre at CU Anschutz. "This timing, where sperm function improves after stress, might be evolutionarily beneficial to increasing birth rates, particularly following challenging times like those experienced during the COVID pandemic."

The study used both human and animal models and discovered that stress-induced EVs increased sperm motility and mitochondrial respiration—an important process for producing the chemical energy required to power cellular functions.

First author Nickole Moon, PhD, a student at CU Anschutz, used this illustration to convey the findings: "Imagine you have a car that is straining to climb a high incline. Stress on the engine causes the car to lose efficiency. However, with a bit more gas, you can improve overall performance and drive more smoothly. With the appropriate tweaks, cells, like your car, become more efficient under stress.

The fact that equivalent results were observed in both people and animal models shows that the stress-induced increase in sperm motility could be a species-wide coping mechanism. This research may have broader consequences for reproductive health.

While the study focused mostly on men, the researchers emphasised the relevance of understanding how stress affects both partners during the conception process. They are particularly interested in how stress-induced alterations in sperm motility may influence foetal development, especially brain development.

"The impact of stress on germ cells, fertility, and the mechanisms underlying the transmission of parental stress experiences across generations are not well understood," said Neill Epperson, MD, professor and chair of the CU Department of Psychiatry, who worked on the study. "By continuing to recruit participants for these trials and conducting thorough stress evaluations in conjunction with the work taking place in the lab, we aim to gain a deeper understanding of how past stressors may affect future offspring."

Bale emphasised the significance of raising awareness about the effects of stress on reproduction, stating, "As stress is a common part of daily life, understanding its effects on reproduction and development will be essential for improving fertility and addressing broader ecological impacts on endangered species."

The research team is preparing additional studies to gain a deeper understanding of the transmission of stress information to EVs and its impact on fertilisation. They will also investigate how these alterations may affect brain development, and they will soon begin a trial to test their model and learn more about the interaction between EVs and sperm in seminal fluid.

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