In one Petri dish are scores, perhaps hundreds, of thrombocytes: human platelets, the cells that circulate in our bloodstream and help us stop bleeding when we're cut. Normally, platelets are produced when they bud off from megakaryocytes, their parent cells, in our bone marrow. The newly formed platelets circulate around our bodies for about a week, and then-if they haven't been used in clotting-they are destroyed in the spleen and liver, to be replaced by freshly created cells. But the platelets in this Petri dish have never been inside a bone or traveled through a vein or an artery; they will never encounter a spleen or a liver; they will never be a part of a human body or pumped by a heart. Only a few weeks ago, these cells were undifferentiated human embryonic stem cells, floating in this same Petri dish like clouds in a tiny sea of gel. Now, having been bathed by a researcher in the right combination of materials, they have become platelets.

In a neighboring Petri dish there are still clouds floating: human embryonic stem cells from the same line as those that have already been transformed into platelets. These cells are being cultivated, divided, and multiplied. They will supply the researcher with an essentially limitless number of genetically identical cells on which to test and re-test techniques for inducing thrombocytic differentiation-for making specialized human blood cells without blood, bone marrow, or a human body.

In these two Petri dishes we see the twofold magic of stem cells: they have the ability to replicate themselves repeatedly, and they can transform into a diverse range of specialized cells. So-called "embryonic" stem cells are taken from what is in fact the pre-embryonic blastocyst stage of development (i.e., a fertilized egg that has divided into a small cluster of cells). They have the capacity to develop into every kind of cell. So-called "adult" stem cells are found at numerous sites around the body at every post-embryonic stage of development. They have the capacity to differentiate into a range of specialized cell types found in their organs of origin; this permits them selectively to repair and replenish specialized tissue.

Both adult and embryonic stem cells have tremendous potential for exploitation in the development of therapies for disease. The fact that they can self-replicate indefinitely means that they are of great utility in testing and comparing cellular responses to different drugs and biological materials. Moreover, if scientists can master the mechanisms by which stem cells can be made to differentiate into specialized cell types, stem cells may become a source of replacement cells for people with cellular diseases like diabetes, Parkinson's and Alzheimer's.