The teams took different approaches. The Wisconsin group, led by James Thomson, took cells from human embryos created by in vitro fertilization. Specifically, embryos were cultured to the blastocyst stage then cell masses were removed and grown separately. Using mouse feeder cells, the researchers were able to maintain embryonic stem cells without any specialization developing or cells dying. Some were even successfully stored cryogenically.
Indications are that all the cells can be maintained indefinitely.
Meanwhile, John Gearhart at Johns Hopkins extracted embryonic germ cells from the 8 week old fetuses of aborted pregnancies. Although not as versatile as stem cells, the germ cells still have the ability to develop into a variety of different tissues. The next step in the process is to discover and understand the mechanisms which trigger cell differentiation into specialized tissue. It is at this stage that the medical applications of the discovery truly multiply. Of course, such research raises some complex ethical issues.