With just a bit of hindbrain and a flap of ear where the head should be, the mouse pup is a freak of science--but it is also the first proof that a single gene plays an essential role in creating a head. William Shawlot and Richard Behringer of the University of Texas M. D. Anderson Cancer Center in Houston created 125 headless mice by knocking out a gene called Lim1 in the developing embryos. The gene, they reported last March, turns out to be an organizer gene: it switches other genes on and off, and in so doing tells cells at the front end of the embryo to become a head.
Only four of the headless embryos survived until birth, and with no nostrils, no mouth to breathe through, says Behringer, they died immediately. The experiment wasn’t just an exercise in scientific sadism, though. Lim1 belongs to a set of genes, called the homeobox genes, that are essential to embryonic development--and that are present in all animals. Lim1, for instance, has already been found in frogs. So by studying headless mice, the researchers are finding out what goes into making a human head too. The frog gene and the mouse gene are almost identical, says Behringer. I would be very surprised if there wasn’t a human gene.
