Researchers in the UK found that sperm with a reinforced outer coat around their tails were better able to swim through high viscosity fluid, similar to cervical mucus.
'During the sperm selection process, IVF clinics don't use a highly viscous liquid to test for the best sperm as until now it was not clear whether this is important,' said study author Dr Hermes Gadêlha at the University of York.
'Our study suggests that more clinical tests and research are needed to explore the impact of this element of the natural environment when selecting sperm for IVF treatments.'
Dr Gadêlha and his colleague Professor Eamonn Gaffney at the University of Oxford created models of virtual sperm using features from the tails of human, mammal and sea urchin sperm.
They compared human and mammalian sperm with sea urchin sperm, since unlike mammals, sea urchins fertilise their eggs outside their bodies in open water. While sperm from mammals and sea urchins share similar flexible tail cores, mammalian sperm seem to have a reinforced outer tail structure.
'Using our virtual sperm model, we created a "Frankenstein" sperm by adding and removing features of the tails from different species so that we could identify how each one functions," wrote Dr Gadêlha in an article for The Conversation. 'We also increased the liquid viscosity for the various sperms to swim through.'
They found that sperm with a reinforced outer coat made more powerful, rhythmic swimming strokes when travelling through high viscosity fluid. However, in low viscous fluids these sperm thrashed around.
In contrast sperm with sea urchin-like tails buckled in high viscosity fluids.
These findings demonstrate that these structural differences are the key adaptations that allow the sperm to swim successfully through different fluid thicknesses.
Sperm selection is a crucial step in the IVF process. However, Dr Gadêlha notes that using 'sperm count' in the clinic is a 'poor predictor of fertility'.
On average, of the 55 million sperm released in one ejaculation only 15 manage to make it through the female reproductive tract. In addition to battling thick cervical mucus - which is 100 times thicker than water, sperm must swim through powerful uterine contractions, acidic environments, and survive attack from the immune system. The tail plays a crucial role in propelling the sperm through this environment towards the egg.
Previous studies have found other attributes of sperm tails that may help determine their swimming success, such as additional scaffolding (see BioNews 954) inside the tail and a corkscrew-like structure (see BioNews 939) at the tail tip.