Using a high-speed camera capable of recording over 8,000 frames in one second, and a microscope stage with a piezoelectric device to move the sample up and down at an incredibly high rate, they were able to scan the sperm swimming freely in 3D.
Illustration of the sperm tail moving rapidly in 3D. Image credit: polymaths-lab.com
“Human sperm roll as they swim, much like playful otters corkscrewing through water to swim forwards,” said Dr Gadelha, head of the Polymaths Laboratory at Bristol’s Department of Engineering Mathematics and an expert in the mathematics of fertility.
“The otter-like spinning of human sperm is however complex: the sperms’ rapid and highly synchronized spinning causes the tail to rotate around the swimming direction, drilling into the fluid with helical waves.”
“The centreline of the 3D tail shows traveling waves of changes in chirality, known in mathematics as perversion. This is similar to the changes in spiraling direction we see in plant tendrils. We now know that these perversion waves are correlated to the spinning movement of the sperm’s head.”
Computer-assisted semen analysis systems in use today, both in clinics and for research, still use 2D views to look at sperm movement. We can now see that 2D projections of sperm swimming can introduce inaccuracies in how we see the tail’s movement. Novel use of 3D microscope technology combined with mathematics may provide fresh avenues for unlocking the secrets of human sperm swimming.
“With over half of infertility caused by male factors, understanding how the human sperm tail moves is fundamental to developing future diagnostic tools to identify unhealthy sperm,” adds Dr Gadelha, whose work has previously revealed the biomechanics of sperm bendiness and the precise rhythmic tendencies that characterize how a sperm moves forward.
* NOTE: The researchers have conducted further checks following concerns about some of the mathematical conclusions in the corresponding peer-reviewed paper. Although every aspect of the experiment and analysis of the work are correct, the interpretation that the flagellar beat is asymmetric cannot be concluded with confidence. Thus, whether the sperm tail beats asymmetrically remains an open question. This was communicated as soon as it became apparent to the journal, Science Advances, which is conducting its own review. The University of Bristol and its researchers are committed to the integrity of scientific research and welcome input and debate from the scientific community which contribute to the advancement of science.
Source: University of Bristol