Stem-Cell Research Unlocks the A.I. Barrier

-Sheyna Strommen for American Red Angus Magazine

Through artificial insemination, cattle producers throughout the world have been able to improve the quality and quantity of their herds by breeding cows to bulls with high genetic or phenotypic merit.

The limiting factor is: once a bull has stopped producing semen, the supply is exhaustible.

"That can be problematic for breeders who only learn about a bull's ability to produce exceptional traits - like high-quality, productive females - years after that bull is gone or has stopped producing semen," explained Dr. Kyle Caires, assistant professor and animal scientist at Berry College in Mount Berry, Georgia.

What if a bull's semen supply was everlasting even after his death? This is a question that has intrigued Caires ever since he was a young boy growing up on his family's cattle ranch on the Hawaiian island of Maui. It led him to Oregon State University where he earned a bachelor's of science degree in animal science and eventually to Washington State where he earned his Ph.D. in reproductive genomics.

Caires believes the possibility could revolutionize the cattle industry. And, thanks to research he and others are conducting at Berry College, the reality is closer than one may realize.

The answer lies in stem cell research - not the sometimes-controversial embryonic stem cells, but the adult stem cell population located in a male's reproductive system that continually produce sperm throughout his life.

Stem Cell basics

Stem cells, according to the Mayo Clinic, are the body's "raw materials," from which all other cells with specialized functions are generated. Under the right conditions in the body or in a laboratory setting stem cells can divide to form daughter cells with a more specific function, such as blood, brain or muscle cells.

In the case of Caires' research, spermatogonial stem cells (SSCs) deep within a male's testis either function to (1) make new stem cells (self-renewal) or (2) enter a process known as ‘differentiation' that leads to the production of sperm. The presence of SSCs underlies a major difference in reproductive physiology between males and females.

"Females are born with a defined number of eggs," explained Caires. Prior to birth, the stem cells within a female's ovary have differentiated themselves into oocytes. Thus, she has a set number of eggs throughout her lifetime. "Very few ovulate, while most are lost during each 21-day estrous cycle," said Caires, reiterating, "Very few ovulate."

By comparison, SSCs in males, while present at birth are maintained throughout adulthood. Caires explained that once puberty is attained, a male can maintain spermatogenesis (sperm production) for the better part of his life because of a balance between SSC self-renewal and differentiation.

The offspring of a single SSC can yield 1,096 spermatozoa, whereas in females, one primary oocyte will differentiate into one egg. "The cycle is constantly occurring and extremely prolific as a mature, healthy bull produces about 70,000 sperm per second," he said.

A male's testicles serve as the factory where this process takes place. Conventional wisdom tells us that the bigger the factory (scrotal circumference), the more sperm are produced and as a correlate, his daughters will be early maturing. But conventional wisdom may be a little misleading, Caires said. Some bulls with smaller scrotal circumferences may actually have longer testicles and more functional testis tissue, so the overall volume of the testicles should be considered.

Another misleading factor connected to scrotal circumference is subcutaneous fat. "During really good growing seasons, or with unlimited access to creep feed, fat can be deposited in the scrotum. This could potentially lead to poor estimates of reproductive merit and as a result of that fat infiltration, create a semi-heat stress condition that diminishes a bull's fertility." This is supported by the fact that bulls developed on high-energy rations have reduced fertility. "It takes 60-62 days for an spermatogonia to undergo all of the changes needed before spermatozoa are produced. Any impingement - heat stress or disease - could damage this sensitive process and put a bull outof-pocket for several months."

Unlocking the mysteries of SSCs may one day give ranchers better tools to not only predict bull fertility, but it may also lead to methods that produce an inexhaustible supply of semen.

Like Embryo Transfer, But for Sperm Production

For decades cattle producers have been able to freeze and later utilize semen and embryos with relatively good success. But again, once their supply is exhausted, it is gone.

A better understanding of the factors that promote SSC survival and development may allow producers to harvest these unique cells and transplant them into a new factory, thereby eliminating the supply conundrum.

Though it sounds like science fiction, researchers are coming closer to that reality each day.

Caires says the technique is successful in rodents, but cattle are more challenging as different hormones and factors regulate SSC development, not to mention the tremendous genetic variation between bulls. Mice strains often have inbreeding coefficients greater than 90 percent.

"We started having success in bulls only after we stepped away from what worked in rodents; we started from scratch. Bovine SSCs have been frozen in our lab and we feel this provides an opportunity for long-term genetic preservation," Caires said.

In cattle, the process begins with sterilizing a young, healthy recipient bull - eliminating his SSCs. Then, a testicular biopsy is harvested from a donor bull. Researchers isolate and culture the donor's SSCs in a laboratory to increase the number that will be transplanted into the recipient.

After SSC transplantation, the stem cells kick-start the recipient's reproductive factory and within several weeks, the recipient bull is producing sperm genetically identical to the donor. The donor bull may be young (pre-puberty), mature or recently deceased.

Caires and his team have, successfully isolated and worked with SSCs from bulls ranging from one day old to five years of age.

Practical Applications

Caires envisions several practical applications of the science.

"Without question, a better understanding of SSCs will provide insight towards boosting sperm production in bulls for the A.I. industry, and we have data to support that notion," he said. Caires has identified several genes important for regulating SSC function in bulls and he hopes this work will lead to new strategies to directly select for improved fertility traits, instead of factors that are merely associated with fertility, like scrotal circumference.

SSC transplantation can facilitate an endless semen supply, and accelerate sperm production in young bulls or after a debilitating event. The process can also extend the reach of bulls into areas where heat stress, terrain or insects have made it difficult for them to perform. If the process can work between breeds, ranchers could produce Red Angus-cross calves out of Brahman cows that were serviced by a Brahman recipient bull.

SSCs are the only adult stem cell population that transmits genetics to offspring, so they may also be a realistic means for genetic manipulation. Researchers could insert genes of interest into SSCs, transplant them, and produce sperm with the potential to create genetically modified livestock for agricultural and human biomedical applications.

"Realistically, those applications are several years away. The process needs more refinement before it will be a viable industry tool, but at this point, we have achieved 60 percent success with the technique in cattle," Caires concluded.