1.Politics and potential

2.Meet the stem cells

3.Troublesome treatment

4.Treating Parkinson's?

Bloody important cells. These undistinguished-looking critters are the source of all blood cells.
Courtesy Dr. John Krause's Hematopathology Homepage, Tulane University School of Medicine.

Embryonic stem cells
Embryonic, or "pluripotent," stem cells exist in the human embryo for a few weeks after conception. In 1998, James Thomson of the University of Wisconsin-Madison proved that they can be sustained indefinitely in the lab, creating hundreds of generations of identical stem cells. This is a crucial step toward one medical goal of stem cells research: creating well-known lines of cells. That would give transplant surgeons a standard bottle labeled "Cell Type 3-A, for use in stroke patients."

What is the medical potential of the two varieties of stem cells?

Advantages. Embryonic stem cells are:
Immortal: One cell line could supply endless amounts of cells with carefully defined characteristics. Like an endless fountain, the cell line itself would remain intact.

Flexible: They can make any body cell.

Available: Human embryos remaining after in-vitro fertilization are routinely destroyed by fertility clinics.

Disadvantages. Embryonic stem cells are:
Hard to control: They may pass through several intermediate stages before becoming the cell type needed to treat a particular disease; this process is controlled by complex chemical cues.

Ethically controversial: Many who believe life begins at conception say that the informed consent by patient donors does not remove the ethical stigma of doing research on human embryos.

Rejected by the immune system: The immune profile of the specialized cells would differ from that of the recipient. The problem might be overcome by creating cell lines with generalized compatibility, perhaps through genetic engineering.

Adult stem cells
Adult stem cells are partly specialized cells that descended from pluripotent, or embryonic, stem cells. Less "eager" to specialize than embryonic stem cells, they may linger in the adult body for decades, although they may become more scarce with age.

Advantages. Adult stem cells are:
Immune to immune attack: If patients receive the products of their own stem cells, they will not mount an immune response.

Available: Some types, like blood stem cells, are easy to find.

Partly specialized: That reduces the amount of outside direction needed to create specialized cells.

Flexible: Adult stem cells may form other tissue types. Last fall, scientists reported that skin and blood stem cells both produced cells that look like neurons -- in the lab. Ira Black, of the Robert Wood Johnson Medical School, who lead the blood work, told Science News, "It's absolutely astonishing. There are stem cells in a variety of places in the body that have the capability of giving rise to neurons" (see "New Sources..." in the bibliography).

Disadvantages. Adult stem cells are:
Scarce: Not all types of adult stem cells have been found yet.

Unavailable. They can be dangerous to extract (you wouldn't want to poke around in someone's brain for neural stem cells).

Vanishing: They don't live as long as embryonic cells in culture.

Rare: Adult stem cells are never very common, and grow more scarce as we age, when the cells might be needed most. Like the following problem, this is relevant for self-transplants of a patient's stem cells.

Questionable quality: Genetic defects may occur after exposure to sunlight or toxins. Or the disease being treated may be present in the stem-cell genes.

A University of Wisconsin-Madison team first cultured human embryonic stem cells. The colony at right includes a core of undifferentiated cells inside some that have changed into less-versatile, more "committed" cells.
© 1998 Science. Courtesy University Communications.

More uses for stem cells
Beyond replacing parts -- the human equivalent, say, of a balky starter motor -- stem cell research has other theoretical advantages. For one thing, stable populations of human cells would be a boon to the pharmaceutical industry, which could test new drugs on real, live and fairly normal human cells. If the meds worked, they could be put through animal and finally human tests. Stem cells could increase the accuracy of early drug discovery tests while reducing costs and the need to use animals such as, well, guinea pigs...

Second, knowing more about the change and specialization of cells could help in two diseases where such processes go awry -- birth defects and cancer. In birth defects, some cells fail to become their intended tissue type, while in cancer, cells revert to a less-specialized form and lose the usual inhibition on endless multiplication.

By studying the sequence of genes that turn on and off during specialization, we could learn to control and treat these diseases.

But the big payoff could come in the clinic. Could stem cells cure Parkinson's? When?