The morning began with a first gamy whiff of what lay in store. Shortly after 9 a.m., Bradley Martin, his assistant Jin-Quang Kuang and a researcher named Ellen Flynn marched along a dimly lit, institutional-tiled corridor at the Johns Hopkins Hospital in Baltimore. After pausing to take a deep breath, they pushed through a green door and entered a small room where several robust Yorkshire pigs greeted them with braying squeals and frothing curiosity. Flynn wheeled a heart-imaging echocardiogram machine into the narrow aisle between the cages, and then Martin, a flimsy yellow surgical gown covering his blue jeans and sports shirt, stepped gingerly into one of the cages and gently wrapped an arm around the huge porker, a gesture that wavered between a hug and a headlock. "All those years of graduate school," Martin grunted over his shoulder, "are finally paying off."

Spending your morning wrestling a 180-kilogram pig into position and holding it steady, while a colleague rubs a jelly-coated probe over the animal's chest in search of a good echocardiogram signal, against deafening squeals of porcine protest and the in-your-face odor of big animals kept in close quarters-that's not exactly how most people imagine the world of cell biology. But then Martin is not interested in ordinary cells-or ordinary biology. His foray into the animal room represents what could be one of the last steps in readying a futuristic form of coronary medicine for testing in humans. If all goes well, those human studies could begin as early as the end of this year.

Martin, a sandy-haired, good-humored senior researcher at Baltimore-based Osiris Therapeutics, has been paying weekly visits to this room for six months. It is a cardiac ward of sorts: all of the pigs in the room have suffered heart attacks. Some of them, however, have subsequently received a highly unusual form of treatment, an injection of stem cells-specifically, an adult form of these versatile progenitor cells isolated from bone marrow. It is Martin's hope that these special cells, known to biologists as adult mesenchymal stem cells, have grown and transformed themselves within the pigs' hearts to form new, healthy tissue right at the site of injury.

Indeed, it is the uncanny ability to zero in on areas of physiological damage and then to organize the process of healing and repair that makes these and other kinds of stem cells so laden with medical possibility. Most of the cells in the body are specialized to perform specific functions in specific tissues, but stem cells-found both in embryos and in various locations in the adult body-can form a number of different tissues and so could in theory be used to treat a vast array of diseases. Rebuilding hearts after heart attacks, regenerating livers ravaged by cirrhosis or viral disease, reconstructing damaged joints, seeding the brain with fresh neurons to reverse the effects of Parkinson's disease and Lou Gehrig's disease-those are just some of the fantastic medical promissory notes that doctors predict these remarkably potent cells will ultimately redeem.