A Groovy Cervix Helps Sperm Swim

While the labyrinthine vaginas and spiralling penises of ducks have become famous (at least on the Internet), the genitals of other species, though perhaps less flamboyant, record remarkable evolutionary stories of their own. In mammals, the female reproductive tract has to balance between providing a pathway for sperm to reach and fertilize the egg and protecting itself against invasion by pathogens. In a paper appearing in PNAS, researchers from Cornell University and the Huazhong University of Science and Technology show how the female genital's biophysical traits accomplish both tasks at once.

The team focused on a series of microgrooves lining the inner surface of the cervix in several mammals, including cows and humans. Sperm have been seen within the grooves in cows, though the cilia lining the grooves beat towards the vagina, creating a current that the sperm have to swim against. The researchers wanted to understand how this combination of grooves and current affected the efforts of sperm — or parasites — to swim up the through cervix.

To find out, they made bull sperm swim through a "microfluidic device". The sperm were put in at one end, and a flow control at the other end allowed the researchers to create a current for them to swim against. The middle of the device consisted of a series of channels, some of which had microgrooves designed to mimic the cervix and some of which were smooth. By measuring how the sperm fared in channels with our without grooves, the team could evaluate the effect of the structures in the cervix; by switching on and off the flow, they could test the effect of a current.

In addition to testing sperm, the researchers also investigated the performance of a protozoan parasite, Tritrichomonas foetus, which is transmitted into the vagina from the bull's foreskin. T. foetus causes infertility in cows, and a similar pathogen, Trichomonas vaginalis, affects around 170 million humans annually, causing preterm births and low birth weights. While sperm are equipped with a single flagellum at the rear, T. foetus has three flagella in the front and a fourth that comes out of the front but folds over the body to point backward. Together with differences in shape, this means that sperm and T. foetus swim very differently — a fact which the cervix has evolved to take advantage of.

When a swimming sperm bumped into a microgroove's sidewall, it stayed against the wall and swam along it. By contrast, T. foetus bounced away from the wall within a few seconds, if not immediately. The difference comes from the mechanics of their swimming motion. Sperm are "pusher" swimmers. Their flagellum pushes them forward, but also makes them twist at the same time. T. foetus, on the other hand, swims in a motion called "run-and-tumble" — it moves forward when the flagella all beat in sync, but tumbles about randomly when they beat asynchronously.

The difference becomes even more pronounced in the face of a backwards flow. Fluid dynamics create an area of reduced flow near the wall; since sperm swim close to the wall, they can take advantage of this and make their way upstream. By contrast, T. foetus doesn't say near the wall, and so ends up swept away by the current. In other words, the combination of microgrooves and a gentle current act together to make it easier for sperm to reach the egg (despite swimming upstream) while keeping pathogens out. Fantastic! (Also, it's a great example of why biologists should pay more attention to female genitals.)

Ref
Tung, C. et al. Microgrooves and fluid flows provide preferential passageways for sperm over pathogen Tritrichomonas foetus. PNAS Eary Edition. (2015) doi: 10.1073/pnas.1500541112

Image credits
The image is the logo of the Groovy project and is distributed under a CC-BY-SA license via Wikimedia Commons.