The first human heart transplant was performed in Cape Town, South Africa, in 1967. The 54-year-old patient survived for just 18 days following the surgery. In the five years that followed that first operation, several a hundred transplants were performed, but each with very little success: Only 15 percent of patients survived even a year after the procedure, and few of those survived longer than that.

The greatest hurdle to successful heart transplantation in those early days was the patient's own immune response, which caused the body to reject the foreign organ. Today, doctors have greatly improved the rate of transplant success, using drugs that suppress the patient's immune response. Despite advances in transplantation medicine, however, the availability of human hearts suitable for transplanting remains quite low. This constant shortage has prompted medical researchers to look for options other than donated organs, including cross-species transplants, artificial hearts, and tissue engineering and cloning technologies.

The procedure of transplanting animal organs into humans, or xenotransplantation, became a reality in 1964 (before the first human-to-human heart transplant) with the first chimpanzee-to-human heart transplant. While this pioneering procedure was ultimately unsuccessful (the chimpanzee heart was not large enough to sustain the circulatory needs of the human patient) it promised a steady supply of organs -- if the right species could be found. Today, most xenotransplantation research focuses on pigs, since the size and output of an average-sized pig heart is very close to that of a human heart. Unfortunately, rejection is even more likely in cross-species transplants. And drugs that can keep a patient from rejecting a pig heart render that patient's body almost completely defenseless in the event of infection.

Artificial hearts have proven to be a good way for patients and doctors to buy time while waiting for suitable donor organs. There are more than two dozen different types of artificial hearts available, but none of these devices provides a permanent solution. One pump currently being tested on animals, however, has some potential to replace live organ transplants, according to researchers. This thumb-sized, battery-powered device is implanted directly inside the left ventricle, where it pumps up to 16 pints of blood per minute. Human trials with this device should begin by 2007.

Another technology that may someday prove to be a transplant surgeon's dream: an endless supply of organs and tissue matched to their recipient. It sounds like science fiction but, in theory at least, may one day be possible. The technique involves stem cells, those cells in embryos (and, to a lesser extent, adults) that have the potential to become any one of the 200 types of cells that make up the human body, including heart cells. Researchers are working now, with some success, on directing stem cells to develop into cardiac tissue. As yet, no one has successfully grown a human heart, but that day may not be as far off as it once seemed.