Researchers at Cornell University in Ithaca, New York, monitored the swimming activity of bovine sperm when exposed to biochemical cues from the environment. Sperm generally performed a symmetrical pattern of swimming with their flagellum (tail) and travelled in a straight line. However, exposure to agents which increased intracellular calcium, including caffeine, induced 'hyperactivation' – a state associated with asymmetrical beating of the flagellum and a circular swimming pattern.
'We think that hyperactivation modulates sperm's swimming behaviour as it ascends towards the fertilisation site… and as sperm responds to the biochemical factors present in the environment' said first author Meisam Zaferani, a graduate student in the lab of Professor Alireza Abbaspourrad. Therefore, this shift in motility, which is known to occur in the fallopian tube enables the sperm to search the area for the egg, encouraging successful fertilisation.
In the study, the team used custom-made microfluidic chambers to observe sperm motility in different chemical environments. Sperm had a tendency to hug the chamber walls as they swam in a straight line using their flagellum in a symmetrical fashion, similar to how they are understood to swim in the female reproductive tract. In the presence of an influx of calcium, the flagellum was shown to switch to asymmetrical beating causing the sperm to swim in a circular swimming pattern. Authors argued this would prevent the sperm from venturing into intrauterine 'dead ends'.
Our understanding of mechanisms governing sperm motility has advanced over recent years (see BioNews 1078 and 1102) now known to include factors such as temperature gradients, fluid flow and uterine wall architecture. However, it remains to be seen what governs this intracellular rise in calcium levels in living organisms including humans, as this new evidence is solely from an artificial in vitro context.
Nonetheless, as senior author Professor Abbaspourrad said, 'By understanding what determines the navigational mechanism… we may be able to use those cues to treat couples with infertility issues and select the best strategy for in vitro fertilisation'.
The study is published in the Proceedings of the National Academy of Sciences.