FERTILITY RESEARCHERS have been wary of modifying sperm since the outcry five years ago over fluorescent tagging of X and Y sperm, allowing parents to choose the sex of their child.

But the spotlight is back on the testicles after the successful treatment of an infertile man using animal 'foster testes'.

Dr Charles Briner, of Johns Hopkins medical school, Baltimore, removed the 32-year-old man's spermatogonia (the sperm-making cells) and transplanted them to a ram's testicles, where they were stimulated to start producing normal sperm.

'When the cells were grafted back, they continued to work perfectly,' Briner said.

JB