For many livestock species, artificial insemination (AI) is standard, but it can be tricky to achieve success the first time, thanks to variability in ovulation timing across the herd.
Sperm remain viable for a day or two once they reach the oviduct -- the tube connecting the uterus with the ovaries -- in pigs and cattle. The amount of time sperm can be stored in the oviduct has a direct bearing on AI success; if ovulation happens just outside that window, the effort and expense of AI have to be repeated.
A new University of Illinois study identifies a naturally occurring sugar that slows the maturation of sperm in pigs, opening up the possibility of extending sperm storage time within the female reproductive tract and increasing the chances of successful fertilization through AI.
"We knew there was something about the oviduct that was increasing sperm life span, but we didn't know what it was, exactly," said David Miller, professor in the University of Illinois department of animal sciences and co-author on the PLOS One study. "In this study, we discovered the molecules of the oviduct that bind sperm and increase their life span are complex sugars called glycans."
After screening more than 400 sugars for their capacity to hold sperm, Miller's team had an inkling that glycans were a candidate for pigs. To confirm, they isolated various sugars from the pig oviduct and applied them to beads in the laboratory, mimicking the oviduct lining.
Compared with other sugars, the glycan-treated beads bound more sperm, they said, noting that it was not just the physical act of slowing down sperm that increased their life span.
"We found out glycans were delaying the normal influx of calcium into sperm," Miller said. "Normally, calcium slowly comes into sperm as they mature, and that triggers them on their differentiation pathway, which makes them capable of fertilization, but binding to these immobilized sugars actually stops that calcium movement inside the cells. So, in a sense, the glycans are blocking their differentiation pathway and making them live longer."
Miller said he sees several potential applications for this discovery. For example, it might be possible to conduct a fertility test for sperm using glycans in the lab. Sperm whose life span did not increase when exposed to glycans would likely be less fertile and could be discarded. It might also be possible, someday, to introduce supplemental glycans in the oviduct during AI to create a larger reservoir of viable sperm.
The results also extend scientists' understanding of fertility across animal species. Miller has done research to show that a similar sugar binds and extends longevity in bovine sperm, and he's currently looking for genetic similarities in sperm storage organs among a wide variety of animal groups. Nature may use the same mechanisms to lengthen the life span of sperm after mating in several species.
Besides Miller, other researchers involved in the research include Sergio Machado, Momal Sharif, Govindasamy Kadirvel and Nicolai Bovin. The research was supported by the National Institutes of Health and the Agriculture & Food Research Initiative of the U.S. Department of Agriculture's National Institute of Food & Agriculture.