Sperm change how they swim in response to different conditions in the vaginal tract. The ability to adjust to these conditions could be a measure of sperm health. If this is shown in future research, the most adaptable sperm could one day be selected for fertility treatments to maximise the chance of conceiving.
“The female reproductive tract is a complex environment that plays a critical role in influencing sperm migration behaviour to ultimately select high-quality sperm for natural fertilisation,” says Farin Yazdan Parast at Monash University in Melbourne, Australia.
After vaginal intercourse, mucus secretion into the fallopian tubes intensifies, she says. This stimulates fluid movement in the vaginal tract, making it more viscous.
Previous studies into how sperm change their swimming styles to navigate these tracts separately considered factors such as the sperm’s shape and the viscosity and flow of the fluid they swam through, says Yazdan Parast. To better understand the collective conditions sperm face, she and her colleagues wanted to study these factors together.
The researchers filled a container with a mucus-like liquid, similar to that in the vagina, which they could manipulate to be more or less viscous.
They then attached a bull sperm cell to a tiny chamber in the container to observe how it changed the way it moved its tail as the team altered the viscosity of the liquid. Bull sperm have a similar structure and swimming pattern to human sperm, but it is easier to gain regulatory approval to study them, says Yazdan Parast. The experiment was repeated for nine other bull sperm cells.
The team imaged the sperm’s movements using a technique called high-resolution dark-field microscopy, finding that the more viscous the liquid was, the larger the up and down motions made by the sperm’s tails.
The tails beat fastest when the environment around them most resembled the area upstream of the vaginal tract, when sperm start to approach an egg. Sperm that can’t adapt to these conditions are probably less likely to fertilise an egg, says Yazdan Parast.
With further research, the findings could help to improve fertility treatments in people. “By replicating the relevant conditions in sperm selection processes, we may have the potential to identify and isolate sperm that display optimal swimming behaviour under conditions similar to those encountered within the female reproductive tract,” she says.
Several devices already exist to sort sperm according to their quality, says Meurig Gallagher at the University of Birmingham, UK. Before making a new, improved one, we need to “properly understand what characteristics make a ‘good’ sperm, and studies like these are important steps”, he says.