A couple in their thirties visits a fertility clinic to learn why they haven't been able to have a child—only to discover that the experts don't know either. The husband produces a more-than-adequate supply of sperm, the wife ovulates regularly, her tubes are normal; both of them are healthy and fit. The diagnosis? Idiopathic infertility.
Conception is much more complicated than the sperm-meets-egg-makes-baby model that most of us learned in high school, says Sandi Staros, a doctoral candidate in physiology who has been studying reproductive biology in cows for the past six years. For instance, the first sperm to meet the egg don't fertilize it. Imagine the film clips you've seen of conception: millions of wriggling sperm swimming fanatically toward the egg. This "vanguard of sperm," this first wave, may jostle and bump and vie for position, but they never include the one, the prom king of the sperm dance. "They may physically trigger the egg," Staros says, "but no one knows."
Another mystery is the process called capacitation. Says Staros, "Some sperm stay behind in the oviduct. In a cow, the sperm are sequestered there for 12 hours. In a human, they stay in the oviduct for at least 8 hours, and sometimes up to 24. Conception never happens at the time of sex." In the oviduct, some sperm undergo capacitation: the bulbous sperm head, called the acrosomal cap, is modified so that when it attaches to a binding site on the egg, the sperm can transfer its genetic material to the egg. Yet if more than one capacitated sperm fertilizes the egg, a condition known as polyspermy occurs, and the genetic excess causes the egg to die. The egg has a way to prevent polyspermy—by "hardening"—but it's not foolproof. Polyspermy does occur; no one knows how often, since it causes a cow—or a human—to spontaneously abort.
At Penn State's Dairy Breeding Research Center, Staros is studying ways to increase fertilization in cows, with the hope that this knowledge can one day be applied to human infertility problems. She focuses on a protein called EAP, short for estrus-associated protein. EAP was identified in 1990 by then-graduate student Robyn Gerena, who discovered that it was only present in a cow's oviduct during estrus, the period of time when most mammalian females are able to conceive. Taking Gerena's research a step further, Staros is examining whether or not EAP increases fertility and enhances embryo development. Since human females produce a protein similar to EAP, called oviductin, Staros's and Gerena's work could have far-reaching applications.
But bovine EAP is hard to come by. Cows have to be surgically fitted with tubes that collect oviductal fluid on a daily basis. The amount of the protein derived daily from the fluid is so minuscule, about an eyedropper's worth, that it took Staros two years to accumulate enough to test.
Trying to mimic the process that takes place in the oviduct, Staros places a bovine egg in a glass petri dish and adds sperm that have already undergone capacitation. Without EAP, the fertility rate is 53 percent; when the protein is added, the rate increases to 73 percent. Why? Staros has several theories. If you were to look at an egg under a microscope, it appears to be wrapped in a fine mesh netting. This is the zona pellucida, which is made up of intertwined proteins and contains large numbers of binding sites for the sperm. "One possibility is that EAP increases the number of binding sites," says Staros, "which increases the potential for fertilization." Yet she has also found that the number of sperm that bind to the egg is lower when EAP is present, which means that EAP could somehow decrease the chances for polyspermy to occur. Or perhaps EAP causes the egg to become fertilized earlier, which would trigger the egg to "harden," again preventing polyspermy.
Staros's next step is to find out EAP's effect on the embryo's genetic makeup. "It is possible that this protein causes certain genes to be turned on in the embryo," she says. "I don't think this protein is the key to fertilization, since an egg can be fertilized without it. However, we know that EAP facilitates in vitro fertilization, which in humans doesn't have a high success rate. It may be that in vitro fertilization is missing some important proteins found in the body. By knowing how EAP functions in cows, we could then look at how similar proteins function in humans."
Sandi Staros is a Ph.D. candidate in physiology, the College of Agricultural Sciences, Dairy Breeding Research Center, University Park, PA 16802; 814-865-5896; staros@psu.edu. She received the John O. Almquist Graduate Student Award for her work in reproductive biology. Her adviser is Gary Killian, Ph.D., professor of reproductive physiology, Dairy Breeding Research Center, 865-5894; lwj@psu.edu. Robyn Gerena, Ph.D., is now a research associate at the DBRC; rlg8@psu.edu. Their work is funded by the USDA.