At this point in development, each cell is pluripotent, meaning that it has the power to become many different parts of the body. Once the embryo becomes differentiated from the placenta, however, the stem cells start to become specialized into those that become heart, liver, spine, nervous system, etc. and lose their potentiality to become anything else. Germ cells, unlike other cells in the developing fetus, are not committed to developing into particular organs. They remain pluripotent, in order to carry their genetic information forward in the next generation. Stem cells taken from the germ cells of a fetus may be equivalent to stem cells taken from a blastocyst, but this remains to be proven.
Stem cells are immortal and can be grown indefinitely in the laboratory.
Researchers hope that they will be able to direct stem cells to form organs for transplantation. At this point, no one knows how to do this, so transplantable organs may be a long way off. Experiments with mouse stem cells show that if injected into an organ of another mouse, for example, the heart, the stem cells specialize into cells for that organ, but this is not the same as growing an entire heart for transplantation and may not even be medically useful. Even if it becomes possible to grow entire organs, problems of ejection by the body s immune system may remain. Just about anything with the label embryo or fetus arouses many people s concerns about the dignity of human life or human potential.