Optics Does That?: Sex-enriched Bovine Semen

Dr. David Appleyard’s official title is principle biosystems engineer at Genus PLC, but that doesn’t really do justice to the unusual nature of his job. The product that Genus offers is sex-enriched bovine semen, which allows their customers assurance that their cattle’s offspring has a better than 85% chance of being female, something that is especially important for dairy farmers who need female cattle to produce milk.

“What does this have to do with optics?” you might be asking. A lot, actually. The process that Genus uses to create their product is heavily dependent upon a number of optical tools. And Appleyard is part of the team that designed the process and built the instruments that Genus uses to enrich bovine semen, by selecting for sperm cells of the desired sex. As he describes it, “I helped create the technology used to make sexed bull semen for use in dairies or other cattle operations. And so, day-to-day, I’m an engineer. I get to look at really interesting problems related to biology, optics, electrical engineering, computer science, data, and try and use my skillset to make a better product, come up with a good solution, and keep our production floor and manufacturing plant running as smoothly as possible.”

Core to the process that Appleyard helped create is the fact that mammalian sperm cells carry either the genes for creating female offspring or the genes for creating male offspring, but not both, although there are rare instances where this is not the case. "In the mammalian species, the sperm defines the sex of the offspring," says Appleyard. "And so, what we're doing is looking at every single sperm cell in an ejaculate and trying to identify if it is going to give a male or female offspring. And then we're using a really interesting optical-based methodology to enrich the sample so that we get rid of all of the unwanted cells from either male or female, so that our final product is highly enriched for one or the other sex."

Inside the lab at Genus PLC.

Battle of the sexes

This enrichment process targets a key difference between female and male sperm cells: the quantity of DNA that they carry. "Female" sperm carry an X chromosome, whereas "male" sperm cells carry a Y chromosome. And X chromosomes are larger than Y chromosomes, which means that female sperm cells carry 3.8% more DNA. At the beginning of the process, the sperm cells are stained with a special dye (called a minor groove intercalating dye) that binds evenly to the DNA inside of the cells. This means that 3.8% more dye binds to the female sperm cells. When illuminated with 355 nm light the dye fluoresces, and the female sperm cells emit 3.8% more fluorescence than male sperm cells, allowing for them to be differentiated.

But there is a complication due to the shape of the cells. Bovine sperm cells are shaped “like a pancake with a tail,” says Appleyard, and their shape creates lensing effects. This means that the rotation angle of the sperm cell with respect to the fluorescence detector can change the apparent brightness of the cell; an edge-on measurement will yield a different measurement from one taken observing the flat side of the cell. A random orientation of the cells during measurement obscures the 3.8% difference between male and female cells. So, to get accurate and consistent measurements, the cells are preprocessed using microfluidics to create a train of cells that are oriented in the same direction with a uniform rotation, which helps to eliminate noise in the measurement due to orientation.

With all the sperm cells stained and uniformly oriented, they are then passed into the measurement portion of the flow cytometer. "In the simplest sense," says Appleyard of the Genus Sex Semen (GSS) instrument, "it gets cells to line up in a single-file line, so that they can be run in front of a laser beam, which is going to light up that dye, which we're going to then measure with some sort of fluorescence detector." These measurements are then plotted, and he notes that "cell fluorescence really clusters based on the sex of the cells. You're going to see a cluster that's about 3.8% brighter than the other cluster, and there you are starting to see that separation between male and female."

Frozen in time

With the sex of the cells identified, a pulsed laser is then used to eliminate the unwanted cells. “We use a small pulsed laser, and we actually slice the cells that we do not want in our final product into two pieces with a little pulse of energy,” says Appleyard. These immobilized cells remain in the product and through field studies, their presence has been shown to have no negative effects on conception. Once processed, the final product is stored in straws, which are frozen in liquid nitrogen where they can be stored nearly indefinitely until they are purchased and defrosted for artificial insemination.

Appleyard’s initial task for his first several years working at Genus was to help design and build the GSS instrument. Today, Genus has factories in Madison, Wisconsin, where Appleyard is based, and also the UK, India, and Brazil. Currently, the GSS instrument can produce product with a minimum sex-skew of 85% female. Appleyard’s current task is finding ways to mature the technology and improve product quality by increasing the sex-skew and making the instrument more precise, efficient, and cost effective, all of which benefits Genus’s customers.

Speaking of customer satisfaction, Appleyard commented that “One of the things that’s been really cool with the technology is that it has a broader impact to very, very small farmers, people who may have only one or two cattle,” for whom yielding female offspring that can produce milk is very important.

So now you know. Sex-enriched bovine semen: optics does that.

Optics Does That is looking for more stories! Do you, or does someone you know, have an interesting or unusual application of optics that you use at work or elsewhere? Please send us an e-mail and tell us about it: emilyp@spie.org.