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paraplegia, Alzheimer’s, Huntington’s, Parkinson’s, Lou Gehrig’s, and other neurological diseases. Ira Black, a neuroscientist at the Robert Wood Johnson Medical School, documents this pathbreaking research in his compelling book The Dying of Enoch Wallace: Life, Death, and the Changing Brain. The story began fifty years ago in fascist Italy when Rita Levi-Montalcini discovered a hormone that stimulates nerves to grow in size, length, and number. Since then considerable research on nerve growth factor (NGF) has led to the discovery of numerous chemicals that appear to stimulate nerves to grow. Chan-Nao Liu and William Chambers at the University of Pennsylvania Medical School in Philadelphia, for example, documented new neural growth in the cut spinal cords of cats—not from the damaged nerves but from nerves adjacent to those cut, sprouting like tree branches in search of a match on the other side of the cut cord. Oxford University’s Geoffrey Raisman recorded the regrowth of synaptic connections between neurons in the brain (the synapse is the gap between two neural connections), where the destruction of one synaptic connection triggered nerve fibers from undamaged sites to rush in and reconnect the neurons. After more than a hundred experiments Raisman concluded that “the central nervous system can no longer be considered incapable of reconstruction in the face of damage.”

Although most of this research is done with animals, it never hurts to remind ourselves that we, too, are animals subject to the same laws of biochemistry. Thus, it is encouraging to read about Fernando Nottebohm’s discovery that songbirds generate thousands of new neurons in their brains every day. Birds are not mammals, but at Rockefeller University Elizabeth Gould discovered that rats whose adrenal glands are removed promptly lose massive numbers of neurons in their hippocampus . . . and just as rapidly replace the lost neurons with new ones. Now at Princeton, Gould demonstrated a similar regeneration effect in adult tree shrews, as well as in marmosets (a New World monkey), which, relative to birds and rats, is closely related to humans. Most important, Gould found this neurogenesis (new nerve growth) effect in the monkeys’ neocortex, the brain structure responsible for complex thought, and that the “use it or lose it” principle normally applied to muscle development appears to have applications to nerve generation.

The best hope for neurogenesis in humans, however, is in stem cell research, recently the subject of contentious public debate, with President George W. Bush navigating a delicate course through the ragged shoals of secular and religious pundits arguing for and against what promises to be the medical miracle of the twenty-first century. Stem cells are undifferentiated cells awaiting final instructions on what they will be when they grow up. Under the right conditions, it might be possible to program these juvenile cells to develop into neurons destroyed by Parkinson’s, cells lost to diabetes, or whatever ails you. Italian scientist Angelo Vescovi, for example, found that he could grow brain tissue in a jar from a handful of stem cells taken from a mouse. It remains to be seen if they can successfully be put back into the brain, but Salk Institute geneticist Fred Gage has a promising study in which he autopsied five cancer victims who had received a chemical marker called bromodeoxyuridine (BrdU) that is used to track how many new cells are being created in the body (chemotherapy destroys all duplicating cells in the body, but since cancer cells replicate faster, the hope is that the chemo kills them all before killing the patient). To his astonishment Gage discovered BrdU in primitive neural stem cells in the brain, indicating that these elder people were generating new neurons, perhaps as many as five hundred to a thousand a day. It would appear that you can teach an old dog new tricks.

A subject pool of five is limited, to be sure, and skeptics of animal studies abound—Yale’s Pasko Rakic, for example, thinks that neurogenesis