Sperm may be tiny, but they have strong preferences—especially when it comes to temperature. In mammals, including humans, sperm are built to thrive in slightly cooler conditions than the body’s normal temperature. Yet, their final destination—the female reproductive tract—is noticeably warmer.
This difference in temperature raises a key question: how do heat-sensitive sperm survive the warmer environment and still succeed in fertilizing an egg?
Scientists at Washington University School of Medicine in St. Louis have found a surprising answer. Their research shows that the warmer temperatures inside the female body are not a problem for sperm—in fact, they are a trigger. When sperm encounter this heat, a special signal is activated that changes how they move. They go from swimming gently to making powerful, thrashing motions that help them reach and penetrate the egg.
This intense movement, known as the sperm’s hyperactive state, is crucial for fertilization. The discovery could lead to new treatments for infertility and may even help in the development of male contraceptives.
The key player in this process is a protein found in all mammals called CatSper. This protein helps control the sperm’s movement. For years, scientists believed that CatSper was triggered by the chemical environment in the female body—specifically pH levels and, in some primates, the hormone progesterone. But this didn’t explain why most mammalian sperm do not respond to progesterone.
Temperature became the focus. Unlike mammals, birds don’t have cooling systems for their reproductive organs and also lack the CatSper protein, suggesting a strong link between temperature and sperm activation.
To test this, scientists used tiny tools usually used to study brain cells. They watched how individual sperm reacted when the temperature around them rose above 38°C (100.4°F). At that point, CatSper kicked in, and the sperm changed from smooth swimming to wild, twisting movements needed to fertilize the egg.
This study reveals a built-in temperature switch in sperm that activates just in time to help them complete their mission.
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