In mock-ups of the female reproductive tract, bull sperm cluster in groups of two to four, which seems to help them swim upstream
When placed in a three-dimensional mock-up of the female reproductive tract, bull sperm – which are similar to human sperm – appear to team up in groups of two to four cells.When Tung and his team first noticed this clustering in their laboratory, they couldn’t understand why it was happening. “In biology, when [cells and structures] do something, they should probably get something out of it,” says Tung.
To resolve the mystery, the researchers injected 100 million fresh bull sperm into a silicone tube containing fluid that resembled cows’ cervical and uterine mucus – which has the consistency of melted cheese, says Tung. Then, they used a syringe pump to create two speeds of flow. When there was no flow, the clustered sperm swam in a straighter line than the individual sperm. In an intermediate flow, the clusters could swim upstream, whereas individual sperm couldn’t. When the flow was strong, the clustered sperm pushed through the oncoming current far better than individual sperm – which usually got swept away by the thick stream.In all these scenarios, there was never one “leader” sperm that was being supported by others in the cluster, says Tung.
The clusters probably serve an important role in the thick outflowing mucus in the vagina and cervix as well as in the uterus, where contractions push fluids in multiple directions, he says. Beyond that point, as the sperm reach the oviducts where fluids are thinner and less mobile, it is possible that they start swimming more individually., says Tung. Future analyses may include tests to see how well sperm cluster or to gauge the quality of female reproductive fluids.
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