Fertility research: Sperm perform as if they were in a peloton bike race

The physics of how a sperm travels its way to an egg in mammals such as humans and livestock is not well understood.

Scientists have found that the tendency for sperm to clump together as they make their way upstream through the thick, elastic-like fluid in the female reproductive system is more than just random behaviour.

Consider cyclists gathering in a “peloton” formation to avoid wind resistance.

In a series of experiments using cattle sperm, researchers have found biological benefits to sperm working together that may have implications for fertility studies.

Despite the popular notion that the fastest, fittest male reproductive cell wins the fertilization race, research has shown that sperm often cooperate together to navigate the female reproductive system in a wide variety of mammalian species.

A new study published in Frontiers in Cell and Developmental Biology offers some compelling reasons behind newly identified cluster behaviour.

Previous research by a team led by scientists from North Carolina State University and Cornell University discovered for the first time that sperm assemble naturally without sticking together when swimming in a viscous, elastic fluid.

This is the type of fluid that sperm encounters migrating through the cervix and uterus into the oviduct where the egg is fertilized. The term viscoelasticity refers to both thickness and elasticity — think melted cheese.

However, separate sperm teams do not outperform single swimmers, as they do in other examples of group behaviour. For example, the head of a woody mouse’s sperm has a hook that actually connects it to other sperm, attaching hundreds to thousands in a kind of sperm train faster than a single sperm.

Go against the tide

The researchers wanted to know the potential biological benefits of this seemingly bizarre behavior on a scale and in an environment that isn’t easy to study – specifically, the streams of viscous, elastic fluid that flows through narrow channels in the female reproductive system.

In a series of experiments using cattle sperm (a good model for human diversity) and a microfluid device to mimic the physical parameters of the female organ, they observed how sperm collected in a viscous elastic fluid react to different flow scenarios.

They found three potential biological benefits to sperm aggregation, based on the strength of the current against which the sperm must travel.

First, in the absence of outflow, staphylococcal sperm seem to change direction less and swim in a more straight line. As against a light to medium flow, the cluster sperm is more uniform, like a group of fish going upstream. Finally, at high physiological flow rates, there appears to be safety in numbers against drift due to strong flow.

“In general, I would say that identifying advantages of motion that are not related to improving speed is not routine, and therefore important. Dr. Chih Kwan Tong, co-author and associate professor of physics at North Carolina A&T State University, noted, in some ways, to open up ways A new sperm performance assay.

Fertility needs physics

Dr. Tong, as a physicist, said he was particularly interested in the protection dynamics at play when the flow is greater. “This might resemble peloton formation in cycling, although the fluid mechanics of sperm is drastically different from that of motorcyclists. We would definitely like to know more about this.”

Watching sperm swim is not just a science sport. A better understanding of the physics of how sperm travel through the complex female reproductive system to fertilize an egg may have implications for infertility treatments and beyond.

“In the long term, our understanding may provide a better choice of sperm used for intervention such as in vitro fertilization or other assisted reproductive technologies,” said Dr. Tong.

“This may be needed because these methods usually bypass some or all of the selection mechanisms found in the female system and lead to less positive outcomes.”

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Source: Frontiers in Cell and Developmental Biology