Cultures of the cells in the laboratory could be nudged down different developmental pathways to become heart or bone marrow or pancreatic cells. Guided then by the body's own internal regulatory signals, the cells would develop into new, young heart tissue, supplementing or replacing the heart cells already there. The same approach should in principle work with any tissue of the body. Human embryonic stem cells would thus serve as a universal spare parts system. Because the cells grow and divide indefinitely in the laboratory, very few blastocysts would be needed.
Many technical problems remain to be resolved.
The art of directing embryonic stem cells down specific pathways is in its infancy. But heart muscle cells have been grown from mouse embryonic stem cells and successfully integrated with the heart tissue of a living mouse. Dr. Thomson in 1995 isolated the embryonic stem cells of a monkey, and Geron intends to do pilot experiments in these cells. Another problem lies in making grafted cells compatible with the patient's immune system. Dr. Thomas B. Okarma, Geron's vice president for research, said Geron would explore several ways of doing this.