May 29, 2022

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Celia physics illustrates the successful swimming capabilities of sperm

Scientists know that sperm can boast of their excellent swimming abilities, but they have not previously studied how they can be so active.

A recent study found that the “carpet” with small hairs in the fallopian tubes gives extra stimulation to the sperm, raising them.

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Now John Toner, a theoretical physicist at the University of Oregon, seems to have an explanation for how these hairs, called cilia, move fluids and tiny molecules in the body.

Small, hair-like bumps on the outside of cilia eukaryotic cells.

Toner developed a mathematical model that illustrates how cilia align particles to move in a fixed direction. The fluid flowing over the cilia helps the hair to curl in the same direction and push the molecules forward.

Toner and colleagues describe the equations in two new papers published in the Physics Review E and Physics Review letters.

Toner has been studying flow physics for many years and has previously developed equations that explain how hundreds of birds fly in the sky at once or how fish flocks swim in unison. He received the Lars Onger Award from the American Physical Society in 2020 for his work.

But a slightly different approach is needed to consider Celia’s problem. “In biological systems, many important processes take place on the surface where a solid meets a liquid,” Toner said. “I realized it was a completely different flow than I thought.”

Cilia not only activate the sperm, but also activate many important movements within the body, including the removal of mucus from the lungs.

This is an interesting challenge from a physical point of view, Toner explained, because the hair is placed at one end.

It turns out that the fluid moving on the cilia has a significant effect. This provides movement that helps align the cilia in one direction, creating a feedback cycle that pushes the fluid in a fixed direction. That is, the system stabilizes itself and adjusts.

“Individual hairs make small mistakes, but the whole fluid pulls them back in,” he said, noting that the toner was being pulled in a stream.

In the future, Toner will seek to improve his model to more closely reflect what appears in biological systems. He said, “We treated the liquid on top of Celia’s carpet as if it were at infinite depth.” But fluids in the body usually move through smaller channels. Determining the fluid depth in the sample will change the expected contacts between the cilia.