The Balls Hold The Key.

I found this story at BBC.com:

Testicles 'are stem cell source'

The cells in a man's testicles may be able to do a lot more than just make sperm - they could provide any cell type in his body.

German and UK research suggests that sperm cells can be coaxed into stem cells with similar properties to those found in the embryo.

The study, in Nature, raises hopes eventually of a supply of "repair tissue" for other parts of the body.

However, a UK expert said it was too early to draw firm conclusions.