A new study from Cornell University reveals how sperm change their swimming patterns to navigate to the egg, shifting from a symmetrical motion that moves the sperm in a straight path to an asymmetrical one that promotes more circular swimming.
This change in behavior, called hyperactivation, enables the sperm to sweep the area once in the egg’s proximity, which improves the sperm’s chances of finding it.
By exposing the mechanisms involved, the study not only unravels a mystery of how the sperm navigates to the egg, but it also has implications for human in-vitro fertilization.
“By understanding what determines the navigational mechanism and the biophysical and biochemical cues for a sperm to get to the egg, we may be able to use those cues to treat couples with infertility issues and select the best strategy for in vitro fertilization,” said Alireza Abbaspourrad, the paper’s senior author.
The study, published in the Proceedings of the National Academy of Sciences, clarifies how millions of sperm travel through the female mammal’s reproductive tract, with only a handful eventually reaching the fertilization site. The sperm stay close to the side walls and swim in a straight line against a small amount of fluid that flows from the upper to lower part of the reproductive tract.
But once sperm reach the uterine junctional zone, enter the fallopian tubes and move towards the egg, an influx of calcium ions into their flagellum triggers hyperactivation and circular swimming. Without this shift, sperm could run into dead ends where they get stuck.