RAISING BEAST PEOPLE

Scientists are hoping that animals with a small percentage of human brain cells will provide a substitute for human subjects. Scientists call these part-human animals chimeras, after the creature in Greek mythology with the head of a lion, the torso of a goat and a tail sprouting the head of a venomous snake.

Conventional wisdom was shattered in March 1984, when an animal unlike any other ever born, or seen, adorned the cover of journal Nature. Danish embryologist Steen Willadsen had mixed cells from the embryos of a sheep and a goat in a petri dish and created a ‘‘geep’’—a mosaic animal with the head of a goat and the woolly upper torso of a sheep. The geep wasn’t much more than a biological curiosity.

In the past few years, however, explosive advances in stem-cell biology have provided scientists with the ability to create human-animal chimeras. This progress raises a simple question: how far should scientists be allowed to go down the chimera path? Unfortunately, there is no simple answer.

Efforts to create human replacement organs are probably the least controversial use of chimeras. By implanting human stem cells into animal foetuses or embryos, biomedical scientists hope to produce animals that develop a human organ, such as a liver or kidney, in place of the animal version.

Progress along these lines has already been made. In 2002, Alan Flake at Children’s Hospital in Philadelphia and Esmail Zanjani at the University of Nevada incorporated human stem cells into early sheep foetuses in their mothers’ wombs. After birth, a wide variety of lamb tissues including blood, cartilage, muscle and heart displayed human contributions of up to 40 percent, though external body features were always entirely animal-like.

In 2003, Yair Reisner at the Weizmann Institute in Israel implanted nondescript human kidney stem cells into mice, and coaxed the cells to multiply and develop into miniature, but fully functional, human kidneys that actually secreted urine. Results obtained from these lines of research and others suggest that the creation of chimeras may be key to coaxing human stem cells to develop into fully functional replacement kidneys, livers and hearts.

Last December, Gage injected human embryonic stem cells into the brain regions of developing mouse fetuses still inside their mother’s uteruses. The human cells became ‘‘active human neurons that successfully integrate into the adult mouse forebrain’’, where higher brain function is localised, Gage explained.

Stanford professor Irving Weissman believes that mice with brains made entirely of human cells would make a better ‘‘model’’ for human neurological diseases, and he’s proposed creating them.

(LEE M. SILVER )