In mammalian reproduction, sperm have a tough task: like trout heading upstream, they have to swim against a current through a convoluted female reproductive tract in search of the unfertilized egg.
The research finds that, in the presence of a gentle fluid flow, the biophysics of the female reproductive tract—in particular, the grooves that line parts of it—critically guide sperm migration without aiding the migration of pathogens.
The study, published online in the Proceedings of the National Academy of Sciences, also shows that sexually transmitted pathogens that infect humans and cattle with trichomoniasis are swept safely away under the same fluidic conditions.
The findings point to the co-evolution of sperm motility and the female reproductive tract, and could lend new insight into fertility treatments by shifting focus to the physics of the interaction of sperm with the female reproductive tract.
Sperm are “pusher microswimmers.” With an asymmetrical body and one rear flagellum, sperm propel themselves forward while rotating like a screw. In an earlier paper in Physical Review Letters, the researchers found that the sperm’s upstream swimming ability relates to this flagellar pushing motion coupled with hydrodynamic interaction with surfaces.
For the new study, the researchers applied the idea. First author Chih-kuan Tung, a physicist and postdoctoral associate at Cornell University, led the design of a microfluidic device that mimics the biophysical environment of the two junctions in the female reproductive tract, the cervix, and the junction between the uterus and oviduct, or fallopian tube.
They modeled the bovine tract, which is structurally similar to that of humans in this regard. Tung used data provided by the group of Susan Suarez, professor of biomedical sciences in the College of Veterinary Medicine, which studies how sperm move through the female tract.
The device, about 4 centimeters long, was carved with 10-20 micron-wide grooves that are found in the female reproductive tract.
They found that when sperm swam against a current similar to the downward current found in the female reproductive tract, the sperm tended to enter microgrooves and swim through them against the direction of the flow.
The microgrooves shielded them from the larger flow, which is how they use the microgrooves to help them along their journey.
In contrast, bovine pathogens called Tritrichomonas foetus didn’t enter the microgrooves, and thus were washed away by the fluid flow. T. foetus, which has a counterpart, Tritrichomonas vaginalis that infects humans, is a puller microswimmer, because it mainly uses its front flagella to pull itself forward.
“Often fertility treatments are focused on the chemicals that will help sperm swim better,” says Mingming Wu, associate professor of biological and environmental engineering. “This is a paradigm shift into thinking more about fluid flow and physical forces.”
Ever since reading a paper in 1989 by June Mullins and Richard Saacke of Virginia Tech, in which they described the presence of microgrooves filled with sperm in the cervixes of cows, Suarez had been interested in addressing the question of whether microgrooves provide guiding pathways for sperm. The development of microfluidic technology enabled her to address this question.
“Finding that the trichomoniasis pathogens did not enter these grooves was also exciting, because it illustrates how males and females co-evolve to facilitate fertilization while reducing infection by sexually transmitted pathogens,” says Suarez.
The work ties in with Wu’s other research interest in the microenvironment of tumor cells—a growing field that employs not only genetics and chemotaxis of tumor metastasis, but also the physical cues that allow tumor cells to grow, change, and migrate.
The National Institutes of Health have supported the work, as has the Cornell Nanobiotechnology Center, which the National Science Foundation funds.
Source: Cornell University