Human sperm propel themselves through viscous fluids using their whip-like tails, in defiance of Newton's third law of motion, according to a new study.
Kenta Ishimoto, a mathematician at Kyoto University, and his colleagues studied non-reciprocal interactions in sperm and other swimming microscopic biological elements, to learn how they slide through materials that should, in theory, resist their movement.
When Newton devised his famous laws of motion in 1686, he sought to explain the relationship between a physical body and the forces acting on it using some precise principles that turned out not necessarily to apply to the microscopic cells through which viscous fluids wriggle.
Newton's third law can be summarized as follows: "For every action, there is an equal and opposite reaction." This indicates a certain symmetry in nature where opposing forces work against each other. In the simplest example, the collision of two marbles of equal size as they roll on the ground results in their force being transferred and bouncing based on this law.
However, nature is messy, and not all physical systems are bound by these symmetries.
So-called non-reciprocal interactions appear in unruly systems consisting of bird flocks, molecules in fluids, and sperm, where they exhibit asymmetric interactions with the animals behind them or with the fluids surrounding them, creating a loophole for equal and opposite forces to circumvent Newton's third law.
Ishimoto and his colleagues analyzed experimental data on human sperm and also modeled the movement of the green algae, Chlamydomonas, in which both swim using thin, flexible flagella that protrude from the cell body and change shape or deform to propel the cells forward.
High-viscosity fluids usually dissipate flagellar energy, preventing sperm or single-celled algae from moving much. However, somehow, the flexible flagellum can push these cells without eliciting a response from their surrounding environment.
The researchers found that the sperm's tails and flagella have "strange elasticity," allowing these flexible appendages to move without losing much energy to the surrounding fluid.
But this strange elastic property did not explain the propulsion resulting from the wave motion of the flagellum. Therefore, from modeling studies, the researchers also derived a new term, the eccentric modulus of elasticity, to describe the internal mechanics of the flagellum.
“From simple solvable models to biological flagellar waveforms of Chlamydomonas cells and sperm cells, we have studied the individual curvature modulus to decipher non-reciprocal internal interactions within the material,” the researchers concluded.
The team adds that the findings could help design small self-assembling robots that mimic living materials, while modeling methods could be used to better understand the basic principles of collective behaviour.
The study was published in the journal PRX Life.
Discover one of the underlying causes of infertility in men
Millions of couples around the world suffer from infertility, and half of the cases occur in men. In 10% of them, little or no sperm are produced.
Now, new research conducted by the Storrs Institute of Medical Research, in collaboration with the Wellcome Center for Cell Biology at the University of Edinburgh, sheds light on what can go wrong in sperm formation, leading to potential theories about possible treatments.
"One important cause of infertility in males is that they can't produce sperm," said Scott Hawley, a researcher from the Storrs Institute for Medical Research. "And if you know exactly what's wrong, there are emerging technologies now that may give you a way to fix it."
The study, published in the journal Science Advances, may help explain why some men do not produce enough sperm to fertilize an egg.
In most sexually reproducing species, including humans, an important retinal bridge-like protein structure must be built correctly to produce sperm and egg cells.
The team, led by former postdoctoral researcher Katherine Bellmare, discovered that in mice, a single, very specific point change in this bridge led to its breakdown, leading to infertility, thus providing insight into human male infertility due to similar problems with meiosis.
Meiosis, the cell division process that gives rise to sperm and eggs, involves several steps, one of which is the formation of a large protein structure called the synaptonemal complex. Like a bridge, the complex holds pairs of chromosomes in place allowing the necessary genetic exchanges to occur that are necessary for the chromosomes to properly separate into sperm and eggs.
“One of the major contributing factors to infertility is meiotic defects,” Bellmaier said. “To understand how chromosomes separate into reproductive cells correctly, we are really interested in what happens right before that when the synaptonemal complex forms between them.”
Previous studies have examined several proteins that make up the synaptonemal complex, how they interact with each other, and identified different mutations associated with male infertility.
The protein that the researchers studied in this study forms the section-containing bridge networks found in humans, mice and most other vertebrates, suggesting that it is important for the complex.
Modeling different mutations in a potentially crucial region of the human protein enabled the team to predict which ones might disrupt the protein's function.
The researchers used a precise gene-editing technique to introduce mutations in one of the major synaptonemal complex proteins in mice, allowing them, for the first time, to test the function of key regions of the protein in living animals. Only one mutation, predicted from modeling experiments, was verified to cause infertility in mice.
The researchers said: "We are talking about microsurgery here. We focused on a very small region of a single protein in this giant structure that we were quite sure could be an important cause of infertility."