Engineers from the University of Toronto have discovered that human sperm can adapt their swimming style to their environment. While they usually gyrate in a three-dimensional, corkscrew-like motion, the team was the first to observe sperm slithering along a surface using a two-dimensional, snake-like motion. The discovery could offer a new way to select the fittest, highest-quality sperm to be used in infertility treatments.
Professor David Sinton (MIE) began studying the movement of sperm about six years ago, while he and his wife were seeking help conceiving. “We benefited from fertility treatments, and our story has a happy ending. The experience of infertility, however, was a real eye-opener.” he said. “I also wondered how a mechanical engineer could contribute to the treatment of infertility.”
Sperm selection is one of the aspects of infertility treatment where mechanical engineering can offer insight. Up to 90 per cent of human sperm in males with infertility have defects, e.g., poor motility, DNA damage or abnormal morphology such as malformed tails that cause them to swim in circles. In previous work, Sinton and his team used their expertise in microfluidics — understanding and manipulating the flow of fluids in very small spaces — to design tiny, fluid-filled channels through which they could ‘race’ sperm. They showed that the winners of this ‘sperm Olympic marathon’ tended to have higher DNA integrity, which in turn makes them more likely to produce a viable embryo.
But the marathon won’t be the only event in the sperm Olympics. Since then, the team has spent a lot of time observing sperm under the microscope to find other ways of selecting the fittest competitors. “You tend to see a lot of them near the walls of the channel,” said Sinton. “So the question has been, why are they there?”
To find out, graduate student Reza Nosrati (MechE PhD Candidate) collaborated with Professor Christopher Yip (IBBME, ChemE) and Amine Driouchi (Biochemistry PhD Candidate) on a technique called total internal reflection fluorescence (TIRF) microscopy. TIRF shows only what is happening within 100 nanometres of a flat surface. It was this method that allowed Nosrati to observe the sperm slithering in 2D for the first time.
“When you look at the TIRF microscopy images, you see that the head is actually aligned very closely with the surface,” said Nosrati, adding that they typically slithered for about one body length before returning to traditional 3D corkscrew motion. Nosrati measured the speed of both swimming modes and did calculations to compare their efficiency. “When human sperm swim in 2D, they are able to swim about 50 per cent faster,” he said.
Comparison of human sperm in 3D “corkscrew” mode versus 2D “slither” mode. (From “Two-dimensional slither swimming of sperm within a micrometre of a surface” by Sinton et al., published in Nature Communications)
Nosrati’s calculations showed that slithering is a more efficient motion when the fluid that the sperm are swimming through is thicker and more viscous. This fits with the environment in certain parts of the female reproductive tract. “It’s a favorable way to get through thick stuff,” said Sinton. “It seems to be a competitive advantage.” The work is published today in Nature Communications.
The team has more work to do before the finding can be applied. First, they have to figure out whether all sperm can do the slither motion, or just some of them. If slithering is indeed limited to a subset of sperm, they need to establish whether the ability to slither is correlated with desirable qualities, in the way that faster swimming is. But if it does work out, it’s not hard to imagine the team creating microchannels small enough that sperm could only get through by slithering. “We’re thinking of this like a field event,” says Sinton, returning to the Olympic analogy. “We could set up all types of different obstacles and see what gets through.”
The team is conducting such trials now. Increasingly common techniques for in-vitro fertilization — such as intracytoplasmic sperm injection, or ICSI — are designed for used with a single sperm cell. This makes finding that one-in-two-hundred-million winner all the more important.