Male infertility can affect animals as well as people. When a prize stallion fails to impregnate its mare, or a champion bull can't sire a cow, that spells bad news for their human breeders. More often than not, the hang-up in humans is an absence or paucity of sperm cells.

A little mouse shall lead these male domestic species back from sterility to paternity. Research veterinarian Ina Dobrinski, an assistant professor of large-animal reproduction at the University of Pennsylvania in Philadelphia, relates how: "Spermatogenesis is a highly organized testicular process that generates virtually unlimited numbers of sperm during adulthood. We have established complete spermatogenesis," she continued, "by grafting testis tissue from newborn mice into mouse hosts. Our results indicate that this approach is applicable to diverse mammalian species, starting with pigs and goats."

Dobrinski is senior author of an article in today's Nature, dated Aug. 15, 2002. Its title: "Sperm from neonatal mammalian testes grafted in mice." She counted two salient findings apparent in her paper. "The first: Complete spermatogenesis can occur in a different host species. And the second major finding is that we got complete spermatogenesis from newborn tissue.

"Starting at the beginning of 2001 and probably finishing this spring," Dobrinski recounted, "what we did was take testicular tissues from newborn animals - mice, pigs, goats. We prepared very small fragments of this immature testis tissue, about 1 millimeter in diameter, and in our first experiment transplanted them just under the skin of these immunodeficient nude mice. Then we just let the mice go, and over time recovered the injected pieces of tissue. We monitored their growth, which expanded to 4 to 8 mm over 10 weeks. We also measured reproductive hormone levels in the rodents' blood. Then we examined them histologically, and eventually isolated the spermatozoa."

Unlimited Spermatozoa By The Million

"As the Nature paper reports," Dobrinski continued, "we could show that the tissue developed normally, as would be expected if left in the donor animal, and eventually produced normal functional sperm. We did the same thing in two larger domestic species, newborn piglets and neonatal goats. We recovered these little pieces of testis tissue and transplanted them - first from pig, then from goat - into other mice. More than 60 percent of these grafts survived and produced working sperm. For example, from a 0.75-gram porcine graft, we isolated 112 X 10⁶ - or 112 million sperm, comparable to that in intact pig testes. Goat testis xenografts," Dobrinski added, "also yielded high titers of sperm, 67 X 10⁶, or 67 million sperm per gram.

"Our records indicate," Dobrinski observed, "that this approach is applicable to diverse mammalian species.

"People," she recalled, "have tried to get spermatogenesis to occur in vitro but it didn't go all the way from stem cell to the spermatozoon. We think our system, by transplanting small pieces of tissue - using the mouse as a bioincubator - gives us a powerful tool to study spermatogenesis in different species.

"We tried before," Dobrinski went on, "to transplant isolated germ cells. That was successful in rodents - hamsters, mice, rats - but not in more evolutionarily distant species, when they were transplanted to mice. We didn't isolate the stem cells on purpose because in experiments where we tried this before that didn't work, probably because we had taken the stem cells away from their surroundings. In this case, we purposely left the stem cells in their little microenvironment and transplanted the whole piece of tissue, including the seminiferous tubules, the way the testis is organized with the Sertoli cells.

"Grafting fresh or preserved testis tissue offers an invaluable tool for conserving endangered species or valuable livestock by allowing sperm production from immature males," she pointed out. "Accessibility of the tissue in the mouse host makes it possible to manipulate spermatogenesis and steroidogenesis in a controlled manner that is not feasible in the donor animal, and certainly not in humans. This in turn will allow analysis of the effects of toxicants and potential male contraceptives on testis function in the target species."

Turning from animals to humans, Dobrinski made the point, "There are several immediate, highly relevant applications for testis-tissue grafting. First, it provides a new option for male germline preservation. In contrast to the conventional sperm cryopreservation method, this procedure supplies a potentially inexhaustible source of male gametes, even from immature gonads. And unlike autologous [self-source] transplantation of isolated germ cells to restore fertility in an individual after cancer therapy, xenologous grafting and use of the resultant sperm for assisted fertilization will eliminate the potential risk of transmitting pathogenic tumor cells.

"Cancer chemotherapy," Dobrinski pointed out, "relates to the fact that especially in children who have to undergo such treatment for cancer, it often impairs their future fertility; they frequently become infertile. If a man has to undergo chemotherapy, he usually saves sperm ahead of time, so if he becomes infertile he has the sperm stored. But in a child you can't do that. You could instead preserve a piece of testis tissue, which might be used later down the road to produce sperm."

From Beast To Man Via Nude, Neonatal Mice

As for potential clinical applications, she allowed, "I guess in any situation where you are dealing with an individual who is not sexually mature, but you cannot get sperm, conceptually you might take a piece of tissue and expose it to freezing. Then later on using it for sperm production would be a potential way to preserve this particular individual's fertility.

"Goats and pigs are farm animals," Dobrinski noted, "and we are trying to extend that use to other domestic species. We have not done any exotic or threatened species. Now we're trying to use cattle, horses and also monkeys - subhuman primates. If transplantation is successful in the monkey," she concluded, "this will open up the field for medical applications. We're thinking about human clinical trials down the road with the appropriate approvals of ethics and safety. It would certainly be interesting to see whether this would work or not.

"The university," she noted, "has filed a preliminary patent application," with herself as principal inventor.