A natural process called rheotaxis is used to measure sperm’s ability to swim upstream in the reproductive tract of cows
Testing the fertility and virility of breeding bulls can be expensive and time consuming.
Scientists at Cornell University in New York have developed a quick test to check if bulls have the right stuff.
The system is called the Rheolex method and it draws on the natural process called rheotaxis, in which the bull’s sperm swim upstream in the reproductive tract of cows. Basically, the stronger the sperm’s ability to swim against the tide, so to speak, the higher its fertility quality. The test focuses on that efficiency.
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“Assessing the quality of sperm in the semen of mammals is important since (for cattle) it provides valuable information to decide the fate of bulls,” said Mohammad Yaghoobi, doctoral student in the Department of Food Science, who led the study.
“Sperm has different physiological characters, like its motility or shape and release of some special chemicals from the head, long tail with membrane proteins on it, and DNA integrity, which is the intactness of the DNA strands in the sperm head.”
He said that each character in the sperm has been developed for a certain task in the fertilization process. Any problem in each of those parts will result in fertilization failure. There have been some tests developed to assess some of these characters like DNA integrity or chemical release from the head, but motility, or movement, parameters have been arbitrary so far and have failed to give any consistent explanation on fertility levels.
“We thought we could mimic this part of the female tract using microfluidic technology to exactly simulate what any sperm would undergo so that we could evaluate sperm response to fluid flow and tight channels similar to the female reproductive tract. This way we can see what nature is evaluating in a sperm (being) its ability to swim against the flow, which is called rheotaxis. We came up with a method to quantify this and named it RHEOtaxis quaLity indeEX (Rheolex).”
Working with the Cornell Nanoscale Facility, the scientists constructed the microfluidic device with tracts a little larger than human hair. The study platform measured the number of sperm during the rheotaxis route at varied flow rates.
“Suppose we have some channels and there are some shelters in the middle,” Yaghoobi said. “At first, the channels are empty, then we fill them with media (the fluid for culturing sperm). The sperm is injected inside the channels but since the flow does not go into the shelters no sperm will be dragged there. The sperm that can swim against the current will swim upstream and end up in the shelters.”
Thawed bull semen samples were tested in the device. They counted the number of sperm swimming into the shelters in five minutes via the quantification Rheolex method. The higher the Rheolex index, the higher the number of good-quality sperm with lower DNA fragmentation.
“Every sperm carries half of DNA (called haploid),” he said. “Some sperm might carry DNA, which has defects in it like fragments in their DNA double strand. These defects might end up going on to the egg after fusion with the sperm and the egg and then fertilization fails. In our experiments, it turns out those bulls that are showing higher rheotaxis ability have lower DNA fragmentation in their semen.”
He said that, as the shear rate (or flow rate) increases, the sperm that are highly mobile and have better strength will have the ability to reach the shelters while others will be washed away. However, not only does strength play a role but so does speed.
“If a sperm is swimming faster it is more powerful,” he said. “But another aspect of sperm motility alongside speed that would play a crucial role in fertilization is straightness in its path. If a sperm is swimming on a straight path with higher speed it can swim against the stream much more effectively and that is probably what would make it more likely for that sperm to reach the egg and have a chance at successful fertilization.”
He said that Rheolex proved that DNA integrity (the opposite of DNA fragmentation) is related to rheotaxis ability.
Translating the microfluidics science lab device into an efficient tool to use in the barn is still some way off but Yaghoobi is optimistic about the future of this technology. He considers Rheolex to be more dependable than conventional semen quality parameters, which he said fail to provide statistically significant predictions.
Computer-assisted sperm analysis ignores rheotaxis, the strenuous processes of sperm moving against the female’s biological stream.
“Our method gave us an edge to perform the experiments with much more reliability and ease. Fabricating micron-sized channels is not that hard but generating fluid flows of the order of one to 10 microlitres per hour is difficult.”
A microlitre is one millionth of a litre.
Not only does the Rheolex system set the stage for evaluating semen fertility and quality, but it also opens up IVF options.
“Rheolex proved that DNA integrity is related to rheotaxis ability,” he said. “If we select sperm based on rheotaxis ability, we can perform better in vitro fertilization (IVF) and hope for healthier babies born this way.”
He said that, for cattle, it will be a great method to evaluate semen before insemination. It will help farmers and ranchers choose the semen that has a better chance of fertilizing eggs leading to successful pregnancies, thus saving time and money in the breeding process with better outcomes in meat and milk production.
Development work continues and a patent is pending.
“We are now working on how to use the same concept in Rheolex and separate enough high-quality sperm for performing a successful IVF,” said Yaghoobi. “The results so far are promising. We even got better fertilization results compared to conventional methods of semen preparation. We have filed a patent for the system, and it is pending. Once patented, we can have it licensed and attract funding for large scale fabrication.”
The research was published in the journal Lab on a Chip.
