Surgeons have transplanted a  single testicle from a 36-year-old Serbian man to his identical twin brother, who was born with none.

The six-hour operation was only the third of its kind, performed by an international team of surgeons, including an expert from Tufts University School of Medicine in Boston.

With the newly-transplanted testicle, the recipient will be able to have children of his own and, because the twins are identical, the DNA he passes to his offspring will be a nearly exact match for his own. 

The historic procedure could offer hope that transgender patients and men with catastrophic groin injuries could have children too, but the ethics become murky outside of the (exceedingly unique) case of identical twin brothers who are donor and recipient. 

Babies born missing both testicles are extremely rare. There are few statistics to come by that estimate just how rare, but one 1974 study suggests that about one in every 5,000 male babies are born missing one testicle. 

Just one in every 20,000 are estimated to be born with neither testicle, according to the University of Southern California research paper. 

The causes of absent testicles are unclear. Some scientists have speculated that it might be a genetic condition, though identical twin brothers in Serbia cast some doubt over that theory. 

Without either testicle, a man's body cannot produce testosterone. 

In the absence of testosterone, men may not go through puberty, they may grow too little hair, or too much in certain areas, develop breast tissue and, lack sex drive and energy and are more prone to muscle loss, and depression. 

And, of course, without the structures that produce and contain sperm, men without testicles are infertile. 

Hormone injections can help to manage the other symptoms that come without testosterone, but for the patient who underwent the operation in Serbia, the goal was to be able to have children and feel more comfortable, as well as having normal testosterone levels. 

The transplantation of a testicle is a fairly long, delicate procedure, but has to be completed with maximum efficiency. 

'Once you remove the testicle from the donor, the clock starts ticking very fast,' said Dr Branko Bojovic, a Harvard Medical School microsurgeon who assisted in the procedure in Belgrade told the New York Times.  

If it's not re-connected to a blood supply within four to six hours, the testicular tissue will start to die. 

There are four crucial blood vessels that need to be carefully sewn together for the testicle to receive life-giving, oxygen-rich blood, and doing so can take up to 60 minutes per vessel, leaving precious little margin for error. 

The team managed to make all four connections successfully, however, in about two hours, the New York Times reported. 

Ideally, they also would have connected a tubule called the vas deferens that would carry sperm from the new testicle to the recipients penis. 

But the surgeons reportedly couldn't find enough of the gland tissue in the recipients body to complete this part of the surgery. 

While his testicle will carry sperm that can be harvested and used to fertilize embryos, for now, he won't be able to reproduce sexually. 

Eventually, he may be able to undergo a second procedure to reconstruct the duct. 

Earlier this year, a complete penis and scrotum transplant was completed at Massachusetts General Hospital, for a veteran who has identified himself only as 'Ray.' 

Ray lost his entire penis, scrotum, most of his legs and a part of his abdomen after getting blown up by a roadside bomb in Afghanistan. 

Four other penis transplants had been performed, but non had included the scrotum. 

But for Ray's transplant, which involved body parts from a deceased donor, doctors removed the testicles from inside the scrotum, because of the ethical quandary that might be presented if he fathered children with the semen - but not the consent - of his donor. 

Identical twin brothers have almost identical DNA, and the donor brother was living, making it possible for all parties to consent to the two sharing sperm that shares nearly all genetic material any way. 

The first testicular transplant, performed in the US in the 70s also involved twin brothers, aged 30.  

Doctors who work with transgender patients are hopeful that a way to make sexual reproduction possible for post-bottom-surgery trans men. 

For now, it's still complicated, and doctors have reconstructed penises for trans men from their own tissue, but have not transplanted donor penises, let alone testicles.  

Still, the surgery that took place in Serbia is an historic one and, so far, it seems to be a successful one.  

By Friday, the recipient's previously low levels of testosterone had perked right up to the normal range, according to the New York Times. 

'He’s good, he looks good, his brother looks good,' Dr Dicken Ko, Boston urologist who flew to Serbia to take part in the operation told the Times. 

The two brothers were expected to be able to go home over the weekend. 

Compiled by Olalekan Adeleye

MailOnline