Sperm discovery may improve hog breeding

Sugar slows the maturation rate of sperm in pigs, which could extend its storage time inside the sow’s reproductive tract

In a variety of mammals, birds, reptiles and amphibians, after mating sperm is stored in a portion of the female reproductive tract called the sperm reservoir.

This reservoir in the mammalian lower oviduct, known as the isthmus, regulates sperm function and extends the cells’ viability and lifespan, which are traits necessary for fertility in species like swine.

But a breeding problem is that on occasion semen deposits and the sow’s ovulation are not always well-synchronized for fertilization to happen.

For some pig farmers, artificial insemination (AI) has become an accepted management procedure for selective breeding. But sometimes achieving success can be difficult, especially at the start of an AI program because of the variability of ovulation timing.

A recent study at the the University of Illinois’ College of Agricultural, Consumer and Environmental Sciences has revealed that a natural sugar slows the maturation rate of sperm in pigs. This allows for the possibility of extending its storage time inside the sow’s reproductive tract. That, in turn, could increase the chance of successful fertilization when cues trigger the sperm’s gradual release toward the ampulla, the fertilization site. Sperm normally remains viable for a day or two in the oviduct, a tube that connects the uterus with the ovaries.

“We knew there was something about the oviduct that was increasing sperm lifespan, but we didn’t know what it was exactly,” said David Miller, professor in the department of animal sciences, in the news release. “In this study, we discovered the molecules of the oviduct that bind sperm and increase their life are complex sugars called glycans.”

The research team screened more than 400 sugars for their ability to hold sperm. They isolated the sugars from the pig oviduct and applied the complex sugars to beads mimicking the oviduct lining in the laboratory. Compared with other sugars, the glycan-treated beads bound more sperm.

“We have some evidence of what they do and don’t do,” said Miller. “What we do know is that they block the influx of calcium that normally happens as sperm fertilizes. But we don’t know exactly how the suppression of calcium is activated. And we don’t know what that suppression of calcium causes in the sperm’s lifespan. We are really interested in that and studying that now. We have some early evidence but it’s pretty preliminary. There seem to be other outcomes of binding sugars that are unrelated to calcium as well. This is a work in progress.”

Miller said the 400 sugars tested are just a fraction of all the sugars in nature, estimated at around 20,000 different kinds and types.

“We found out glycans were delaying the normal influx of calcium into sperm,” said Miller. “Normally, calcium slowly comes into sperm as they mature, and that triggers them on their differentiation pathway. (This pathway) is the process of the ability of the sperm to fertilize an egg. You could say it is the development of the fertilizing ability of the sperm.”

However, he said that when sperm are bound to glycan sugars, the process stops the calcium movement inside the cells. In this way, the glycans are blocking the differentiation pathway and making the sperm live longer.

Sperm storage in the oviduct appears to delay capacitation and lengthens sperm lifespan, which increases the opportunity for egg cells to be fertilized. While the research showed that sperm would bind to specific glycans, it remains unknown whether glycan-binding alone could lengthen sperm lifespan.

This research could make it possible to conduct fertility tests for sperm using glycans in the lab. This would allow for the possibility of isolating the sperm whose lifespan did not increase when exposed to glycan sugars. They would likely be less fertile and could therefore be discarded in favour of the sperm responsive to the sugar influence.

Miller said that it might be possible to introduce supplementary glycans in the oviduct during AI to create a larger reservoir of viable sperm and increase the potential for fertilization.

The press release stated that the results also extend the scientists’ understanding of fertility across other animal species. Miller has done research to show that a similar sugar binds and extends longevity in bovine sperm, and he is currently looking for genetic similarities in sperm storage organs among a variety of animal groups.

While conditions among species may be slightly different, he said that as far as they know, the principles are maintained across cows and pigs.

The research study was recently published in the journal PLOS ONE.

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