A 36-year-old man born without testicles received one transplanted from his identical twin brother in a six-hour operation performed in Belgrade, Serbia, by an international team of surgeons, according to a New York Times report.

The surgery was intended to give the recipient more stable levels of the male hormone testosterone than injections could provide, to make his genitals more natural and more comfortable, and to enable him to father children, said Dr. Dicken Ko, a transplant surgeon and urology professor at Tufts University School of Medicine in Boston, who flew to Belgrade to help with the procedure.

The operation was only the third known transplant of this type. The first two were performed 40 years ago in St. Louis, also for identical twins, each pair with a brother lacking testicles.

The absence of testicles is an exceedingly rare condition, but doctors say that the surgery may have broader applications for transgender people, accident victims, wounded soldiers and cancer patients. But the procedure raises questions about the ethics of transplants that are not lifesaving, and about the possibility of recipients’ someday fathering children with sperm from donors who may not even be related to them.

The surgery was performed at the University Children’s Clinic in Tirsova, a section of Belgrade. The Serbian brothers are doing well, doctors said. Τhe recipient already has normal testosterone levels.

He’s good, he looks good, his brother looks good,” Dr. Ko said in a telephone interview on Friday. The donor, who already has children, should remain as fertile as he was before, despite giving up a testicle.

Because the patients are identical twins with the same genetic makeup, there is no concern that the recipient’s body will reject the transplant, so he does not have to take the immune-suppressing drugs that most transplant patients need.

Surgeons operated on the brothers simultaneously, in adjoining rooms. The procedure was challenging because it required sewing together two arteries and two veins that were less than 2 millimeters wide.

Once you remove the testicle from the donor, the clock starts ticking very fast,” said Dr. Branko Bojovic, an expert in microsurgery at Harvard Medical School and part of the team in Belgrade.

Within two to four hours, you have to have it re-perfused and working again,” Dr. Bojovic said. Without a blood supply, a testicle is viable for only four to six hours.

It can take 30 to 60 minutes to make each of the four blood-vessel connections. But the team managed to complete them all in less than two hours, he said.

The team did not connect a structure called the vas deferens, which carries sperm out of the testicles. The surgeons could not find the tissue in the recipient needed for the connection, which means that for now, he cannot father children in the usual way.

Another operation to make the connection may be possible. Otherwise, if the recipient wants children, he might undergo a procedure to extract sperm from the testicle for in vitro fertilization. Or his twin brother’s sperm could be used.

Dr. Ko and Dr. Bojovic were both part of the surgical team that performed the first penis transplant in the United States, in 2016, on a man whose penis had been removed because of cancer.

Dr. Miroslav Djordjevic, who led the team in Belgrade, specializes in urologic reconstruction and sex reassignment surgery at Mount Sinai Hospital in New York and at the University of Belgrade. He said the brothers approached him after learning that he had performed a successful uterus transplant between twins sisters, which enabled the recipient to give birth.

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