On December 3, a 36-year-old man in Serbia became the third known person to receive a testicle transplant.

When a man's testicles are nonexistent, he experiences both reproductive- and hormone-related problems, Dr. Bobby Najari, the director of the Male Fertility Program at NYU Langone Health, told Insider.

But testicle transplants are rare because donating a testicle requires the donor to contribute his sperm to the recipient as well, and there are other ways testosterone can be replaced.

Still, the procedure may have positive implications for transgender people, wounded soldiers, and cancer survivors.

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On Tuesday, December 3, a 36-year-old man became the third known person to receive a testicle transplant.

The man, who underwent the transplant in Belgrade, Serbia, wasn't born with any testicles and got one testicle from his identical twin brother, the New York Times reported.

The two previous testicle transplants were performed nearly 40 years ago, and both also involved identical twins.

Being born without testicles is rare. According to the National Institutes of Health, only 0.15% of full-term male babies are born without testicles. (It's more common — about 3% of full-term male babies — to be born with undescended testicles, but in most of those cases, the testicles drop to the correct position before they're 6 months old.)

When a man's testicles are nonexistant, he experiences both reproductive- and hormone-related problems, Dr. Bobby Najari, the director of the Male Fertility Program at NYU Langone Health who was not involved in the transplant, told Insider.

Although the hormone-related problems can be fixed with hormone replacement therapies, only a testicle transplant can potentially help a man born without testicles gain reproductive abilities.

Testicles have both hormonal and reproductive functions

The testicles serve two purposes. First, they release the hormone testosterone, which is responsible for a man's muscle growth, facial hair growth, and sex drive, according to the Urology Care Foundation. But if a man doesn't have testicles, he won't release testosterone and needs to get the hormone through a replacement method, Najari said.

Replacement methods include injections administered by the man himself or his doctor, oral medications, or topical medications.

The testicles also play a primary role in reproduction, since they contain the sperm needed to fertilize an egg, which develops into an embryo and then a fetus. Without testes, a man can't have genetic children of his own.

Even with a testicle transplant, a man who previously had no testes isn't guaranteed to have kids, Najari said. That's because it would require an extra surgical step during which a surgeon attaches the vas deferens — the male reproductive organ that connects the sperm to the urethra at the tip of the penis — to the recipient's own penis.

If the vas deferens isn't connected to the recipient's penis (it wasn't this new transplant case), then the sperm have no way of leaving the scrotum, entering the penis, and exiting the penis in the form of ejaculate, according to Najari.

If the vas deferens are attached, it adds about an hour to the procedure, which is already about three hours long, Najari said.

The testicles play a primary role in reproduction. nimon/Shutterstock

Testicle transplants are rarely performed

Testicle transplants require surgeons be extremely physically adept because they must fuse the donated testicle's blood vessels with the recipient's blood vessels in order for the donated testicle to function properly, according to Najari.

But the difficulty of the procedure itself isn't why testicle transplants are rare, Najari said. Rather, it's because "you're transplanting the donor's sperm into the recipient, so the recipient's children are going to be the testicular donor's children, from a genetic standpoint."

It's not common for people to want to donate their sperm to a stranger in this capacity, or for a deceased organ donor to consent to this while alive, which explains why the three transplants completed thus far have been with identical twins.

Testicle transplants could be revolutionary for transgender people, wounded soldiers, and cancer survivors

Although testicle transplants are rare, the procedure has the potential to change the lives of people in need of a testicle, Najari said.

Transgender men who aren't born with male reproductive organs, for example, could have more reproductive options if they received a testicle transplant.

Additionally, wounded soldiers and cancer survivors who lost their testes could benefit from the procedure if they don't wish to use other testosterone-providing methods or want to have children.