Dear Straight Dope: In an age when organ transplants are becoming more commonplace, it's difficult to find information on which organs can and can't be replaced. My friends and I would like to know if testicles can be transplanted. If so, would the resulting sperm cells carry the genetic code of the donor or recipient? Brian Tinsman, Seattle

SDStaff paperbackwriter replies:

Noting that we’ve received letters essentially identical to this one from readers Walsh and Kurt Buchmann as well, let me just say: Guys, I sense that there may be some underlying issues here, but I’ll stick to the questions you’ve asked and leave Freud out of this. I’m sure we all prefer it that way.

There are three kinds of transplant procedures. The first is allogenic transplantation, the transplanting of tissue or organs from one person’s body to another’s — what you might think of as a “normal” transplant. Tissues and organs that have been successfully transplanted this way include adrenal glands, thymus glands, bone, bone marrow, cartilage, corneas, fat, hair, hearts, kidneys, lungs, small intestines, and skin. There are also xenotransplants, where tissue from another species, such as a pig’s heart valve, is implanted into a human. Lastly there are autologous transplants, also called autotransplants, where tissue or an organ is removed from its location and transplanted into a new location within the same person’s body.

The only type of testicle transplants done in humans are autotransplants, performed to treat certain types of a condition called cryptorchidism, or undescended testicles. During the development of a male embryo, the testicles grow in the area that will eventually be the abdominal cavity near the kidneys. Normally, they move downwards and forwards, and then drop into the scrotum. An undescended testicle is one that has failed to complete this trip. In some cases, the testicle stays high in the abdomen, and autotransplantation is the surgical procedure to correct this. An untreated undescended testicle can lead to increased chances of infertility or testicular cancer later in life.

Researchers are currently investigating another type of testicular autotransplant. Prepubescent boys who undergo chemotherapy or radiation therapy for cancer often lose the ability to create sperm. One way of preventing this may be to remove the testicles and freeze them before cancer treatment, then reimplant them after the danger has passed. Couldn’t they just extract sperm from the boys and freeze it for later? No, because the spermatogenic cells in the testicles remain dormant until puberty.

OK, why no allogenic testicle transplants for adult men? Mainly because there’s no reason for it. Hormone treatments can address low testosterone production and many causes behind lack of sperm production. And men who aren’t producing enough sperm but don’t respond to hormone treatment wouldn’t benefit from a testicle transplant anyway — assuming the benefit they’re looking for is the ability to pass their genes along to their children. Spermatogenic cells are derived from primordial germ cells that enter the testicles during the fifth week of pregnancy. This means sperm will always carry genetic information that was implanted during embryonic development, which in turn means that the donor of a transplanted testicle, and not the recipient, would be the genetic father of any child that resulted from its sperm. This being the case, simply using a sperm donor or adopting makes a whole lot more sense than having someone else’s testicles transplanted into your body.

Update:

After this article was originally published, reader j-in-japan asked about a case he remembered hearing about on the Phil Donahue Show in the late 70’s about a pair of identical twins that had such a procedure. One twin was born without testicles, the other with two normal ones, so the fully-equipped brother donated one to his twin. The recipient twin was subsequently able to impregnate his wife. Because the men were identical twins, the child was genetically equivalent to his own. This unique case was reported in the journal Fertility and Sterility in 1978. Since that time, there have been no further reports of such a procedure.

SDStaff paperbackwriter, Straight Dope Science Advisory Board

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