For the first time, scientists have image-mapped human sperms in 3D, directly observing its helical pattern while uncovering some rare statistics that govern their motion in the process.
In a study led by Aydogan Ozcan, assistant professor at UCLA School of Engineering, researchers successfully tracked 3D trajectories of more than 24,000 individual human sperms using unique, lens-free on-chip imaging. The holographic shadows of sperms from the imager gave the team the first direct look of the sperm’s helix pattern of motion.
“There was not a direct observation of a human sperm helix thus far. Of course people knew that sperms could swim in a helix but there was not a technique that could show a movie of that direct motion,” explained Ozcan. “This study shows the first direct observation of the trajectory of the helical sperm. It also reveals some of their extremely rare nature.”
According to the paper, published September 17 in Proceedings of the National Academy of Sciences, researchers were able to track more than 1500 sperms at once at a frame rate of 90 frames per second (1). Among the surprising statistics, the team found that helix patterns were only made by the micro-swimmers about 4-5% of the time. Additionally, almost 90% of them made right-handed helical patterns, leaving only around 10% conducting left-handed corkscrew patterns.
Previous efforts to track movements of microorganisms at such a high volume and speed have proved difficult because of the limitations of traditional lens-based microscopes, according to Ozcan.
“In a microscope with a lens, if the sperm is moving vertically compared to the observation plane, it quickly moves out of focus and you lose it. But with the holographic technique you can tolerate sperm that moves vertically and it doesn’t get out focus, so you can really look at a larger depth of field.”
To overcome the disadvantages of lens-based microscopes, the team developed a lens-free holographic on-chip imaging platform. Once in place, the sperms were placed at sub-millimeter distance to the active area of the silicon sensor chip. Then, using two different light sources, the researchers were able to capture holographic shadows of the sperms and reconstruct multicolor illumination-based holograms. The team could then plot the center of the head of the sperm and track the head of the sperm in 3D computationally, without the use of any lenses.
“Tracking was a challenge because there is so many of these shadows of sperms that you have,” said Ozcan. “In a given frame you have so many of these shadows but there are two different illumination sources. The vertical shadow of one sperm might overlap with the oblique shadow of another. So, you have track the same sperm and you can fail if you are not careful enough.”
Ozcan believes his lab’s technique could have a potential future application for high-throughput screening of different chemicals’ affect on the 3D trajectories of sperms. He says it could also be applied to track different micro-swimmers such as algae or micro-particles like dust and contamination in air.
For now, Ozcan’s lab is working to apply their research to animal farms or veterinarian services to help understand how sperm of different animals can be tracked in field conditions.
“I think that’s another idea where we could use some of the new science that we’ve generated in this paper for animal farms or veterinarians to have more powerful tools to understand semen qualities of animals. “
1. Su, T-W, L. Xue, and A. Ozcan. 2012. High-throughput lensfree 3D tracking of human sperms reveals rare statistics of helical trajectories. Proc Natl Acad Sci U S A