THE SPERM THAT NEVER DIES

By MICHAEL D. LEMONICK

Imagine a mouse impregnating a rat, a bull impregnating a horse, a deer impregnating an elephant. Imagine a man fathering a child--or 100 children--a century after his death. Sound preposterous? Think again. A team of scientists at the University of Pennsylvania and the University of Texas Southwestern Medical Center has taken the first tiny steps toward scenarios at least as bizarre, and perhaps even more so. Writing in the current issue of Nature Medicine, the researchers report that they have frozen spermatogonial stem cells--the cells that make sperm--thawed them and coaxed them back to life. And in a companion paper in the journal Nature, the same scientists say they have taken stem cells from rats and implanted them in mice, which have then produced fully functional rat sperm.

The first experiment is, by itself, something of a reproductive coup. Stem cells are to sperm what a widget factory is to widgets. Being able to keep the cells on ice is thus a lot more useful than the already common practice of freezing sperm itself; sperm lasts only so long, but a sperm factory can, in theory, go on forever. That could be enormously comforting to men about to undergo, say, radiation treatment or chemotherapy, which can destroy stem cells and render patients permanently infertile. These men could sidestep the problem by having their stem cells removed, frozen and reimplanted after treatment.

Similarly, a commercially valuable animal--a champion bull or a prize hog, for example--could keep producing sperm indefinitely, even after death, using lesser specimens as surrogate spermmakers. Stem cells also give rise to new stem cells, which can then be harvested and frozen in turn. As a result, says Pennsylvania veterinary physiologist Ralph Brinster, a co-author of both studies, "we can make any individual male biologically immortal."

The second experiment is equally groundbreaking. If rat stems can produce rat sperm while living in a mouse's testicles, it's plausible at least that any mammalian species could play host to the stem cells of any other--thus helping biologists, for example, preserve the genetic material from endangered species.

Stranger still--and even more disturbing--is the idea that humans might try to produce designer children by sampling the sperm of superstars. What makes using frozen stem cells different from frozen sperm, says Art Caplan, director of the Center for Bioethics at the University of Pennsylvania, is that it creates the possibility of an unlimited supply. "That," he says, "opens the opportunity for mass use."

What would women pay to be impregnated, by remote control or even after the donor's death, by the world's smartest physicist or most talented violinist or most accomplished adventurer? That isn't so preposterous as it may sound. A few years back, William Shockley, Nobel-prizewinning co-inventor of the transistor, attracted ridicule by making a deposit in a sperm bank that accepted donations only from men with high IQs. But with biological immortality as a lure, more of the world's most accomplished men--or, failing that, a bunch of rock stars and politicians--might be only too happy to sign up.

--By Michael D. Lemonick. Reported by Alice Park/New York

With reporting by Alice Park/New York