The feat, reported in this week's Nature, should apply to all mammals, the researchers say. As a result, it could one day help young men reproduce even after they've been rendered infertile by cancer or other causes. It also has implications for the preservation of endangered species and prized livestock, the study points out.
Doctors have already performed testicle transplants in male tumor patients using both whole testes and material from the reproductive organs that's frozen and returned after cancer therapy. However, that procedure has been dogged by fears that it may reintroduce cancerous cells.
The new technique could turn lab animals into virtual human testes that would offer an effectively unlimited supply of sperm. At the same time, it avoids the possibility of sparking recurrent tumors, says Dr. Sherman J. Silber, director of the Infertility Clinic of St. Louis.
Silber, who performed the first testicle transplant in 1977, calls the new study "seminal," and says it may also apply to young women facing the loss of their ovaries to chemotherapy.
Led by Dr. Ina Dobrinski, a veterinarian at the University of Pennsylvania, the researchers grafted small chunks of testicles from newborn goats, pigs and mice underneath the skin of castrated, hairless lab mice. The rodents also had no immune function so they couldn't reject the new cells.
More than 60 percent of the transplants survived to produce sperm, the scientists say. By 10 weeks after the procedure, the grafted testes had swelled from a millimeter or less to as broad as 8 millimeters.
Harvesting the fruits of their labor, Dobrinski's group then fertilized mouse eggs with direct injections of sperm from all three animals. Although the cross-species embryos would not develop because of chromosomal imbalances, Dobrinski says the fact that they could spark cell division proved the sperm were functional.
Since the resulting sperm were testicular -- that is, they hadn't been through the full ejaculation process -- they are only 10 percent as motile as sperm in seminal fluid, Dobrinski says. To overcome this indolence, doctors could simply take single sperm and place them directly into a waiting egg cell. That procedure, called intracytoplasmic sperm injection, or ICSI, is commonly done in fertility clinics. "I think that's definitely the way to go," she says.
The next step will be to take the sperm from grafted testes and fertilize eggs from the same species. Dobrinski says her group is now trying to do just that.
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