Embryonic: These master cells naturally give rise to nerve, heart, bone, blood -- all of the body's tissue types. Scientists predict they will improve medicine by replacing defective cells such as those damaged in a spinal-cord injury, and by enhancing understanding of what happens when something malfunctions in the body.

Adult: Stem cells found in bone marrow, umbilical cord blood and some other tissues also have regenerative powers. They have been used in transplants to treat blood disorders and some types of cancer. Researchers are working to coax them toward other uses. It remains to be seen whether they can be as versatile and prolific as embryonic cells.
SHARON SCHMICKLE