In a notable step for male fertility preservation, a procedure that involves removing and freezing immature testicular tissue and then reimplanting it later has resulted in the first live birth of a healthy monkey.

The technique could theoretically help prepubescent boys who are about to undergo cancer treatments, about 30 percent of whom become infertile from the harsh effects of chemotherapy and radiation. Currently, these boys have no options if they want to have biological children in the future.

Doctors can freeze sperm from adult male patients, but boys do not start producing mature sex cells until they reach puberty. At that point, hormonal changes in the body ramp up testosterone levels, spurring stem cells in the testes to develop into sperm. Chemotherapy and radiation treatments can deplete these stem cells, resulting in low sperm levels and infertility.

In the new study, investigators at the University of Pittsburgh School of Medicine and their colleagues surgically removed and froze testicular tissue from five juvenile rhesus macaques that were too young to produce sperm. The macaques were then neutered to make sure they were sterile. When the monkeys reached puberty, the frozen tissue was thawed and grafted under the scrotum of the same animal from which it was originally taken.

Eight to 12 months later, scientists harvested tissue from the graft sites and extracted sperm from it in the lab. The tissue produced enough sperm to fertilize 138 eggs. Of these, only 16 turned into embryos suitable for implantation. The researchers transferred 11 of these embryos into female macaques, which resulted in one pregnancy and a subsequent live birth. The findings were detailed Thursday in Science.

“We grafted tissue that had zero sperm at the beginning of the experiment, and when we collected the grafts, they were producing millions and millions of sperm,” says Kyle Orwig, senior author of the study and a professor of obstetrics, gynecology and reproductive sciences at the University of Pittsburgh School of Medicine. “We’re confident that if the same thing happens with human tissue, we’ll have more than enough sperm to fertilize an egg and establish a pregnancy.”

As for the low yield of embryos, Orwig says it is possible that the procedure affected the sperm quality but notes other factors, such as egg quality and breeding time, as possible culprits. Typically, macaques breed from October through May. The first two experiments Orwig and his team conducted were at the end and beginning of the breeding season, respectively, and neither attempt resulted in a pregnancy. The researchers’ third attempt, in November 2017, led to the birth of a healthy female baby monkey last April.

“The challenge has been that you’re starting from this very immature stem cell population, and you have to find conditions that will allow them to mature from that very undifferentiated state to completely mature sperm,” says Susan Taymans, a program officer at the National Institute of Child Health and Human Development, who was not involved in the study but whose institution helped fund it.

Scientists had previously used a similar technique in mice to produce healthy pups. Other researchers had attempted to freeze and graft testicular tissue in monkeys, too, but these efforts did not result in a pregnancy. Robert Brannigan, a reproductive urologist and male infertility specialist at Northwestern University, who was not involved in the work, says Orwig and his team might have been successful because they used larger tissue grafts and waited longer to harvest the tissue for sperm after reimplanting it.

A similar procedure for women—ovarian tissue freezing and transplantation—has been available to female cancer patients for several years and has resulted in more than 100 live births. One published case involved a woman who had frozen her ovarian tissue as a prepubescent girl. Tissue freezing is more common for women with a cancer diagnosis because they might not have time to freeze their eggs before undergoing treatment.

In contrast, it is much easier to obtain a semen sample. As a result, testicular tissue freezing is very rarely done in adult men with cancer. Tissue freezing for boys who are starting cancer treatment is currently offered at University of Pittsburgh Medical Center and the Mayo Clinic, as well as a few sites around the world, in anticipation of future fertility technology becoming available. Branigan says the tissue extraction is usually done alongside other planned procedures, such as a biopsy or catheter implantation to deliver chemotherapy. Orwig is optimistic that the transplant procedure will move to clinical trials within the next few years.

Although the technique could allow male childhood cancer survivors to have their own biological children, they would most likely have to use assisted reproductive technology, such as artificial insemination or in vitro fertilization, Orwig says. This is the case because it is challenging to “hook up the plumbing” between the grafted tissue and the rest of male reproductive system, he says. “When we graft tissue, there’s no way for sperm to come out into the ejaculate.” So the tissue will have to come out of the body and be dissected to release the sperm.

One concern with taking tissue from cancer patients is the possibility of removing tumor cells along with the testicular tissue that then gets transplanted back into patients later on. For boys with some types of cancer, such as testicular cancer, leukemia or lymphoma, this method might come with a higher risk. Taymans says that doctors would need to follow patients over time to make sure their cancer does not reemerge. In rare cases, a patient’s cancer might damage the testes to the point where he is unable to produce sperm.

Another question is when the tissue grafts should be reimplanted and harvested for sperm. Orwig says it might be better to do it when the boy is a teenager rather than waiting until he is ready to have a family. “That way, he’ll know even when he’s a teenager that he has sperm waiting for him, and he’ll be able to have a biological child in the future.”