Their
finite variety
Like the price of a
vintage Morris Minor, stem cells are subject to change. And just as a Minor
is always trying to change into iron oxide (rust), stem cells are also converting
into more specialized cells.
Embryonic stem cells have a number of distinctive properties.
They can live essentially forever -- without forming tumors.
They can take a hint. Their development is directed by subtle chemical cues that vary according to location and conditions in the body.
Pretty
pluripotent
 Totipotent cells can form any human cell -- including the placenta. Pluripotent, or embryonic, stem cells -- can form any body tissue except the placenta. During development, cells derived from these stem cells become progressively more specialized. It's normally a one-way street -- in the body, embryonic stem cells don't stick around long. National Institutes of Health |
That cell, and its immediate descendents, are called "totipotent" stem cells because they form every tissue in the body. Specialization starts almost immediately, as the multiplying cells form a ball called a blastocyst. Inside the blastocyst are "pluripotent" or "embryonic" stem cells that can form just about every cell except the placenta.
It is the descendants of these embryonic stem cells that form the skin in your eyelids and the hair of your eyelashes. They form the muscle cells that raise your eyelid and nerve cells that detect light in your eye.
Embryonic stem cells, however, dislike the status quo. Normally, whether in the body or the lab, they quickly specialize. Within days, their job is done and they disappear.
Cellular
diversity, courtesy of a blood stem cell, seen at the top. The cells'
jobs are written across the bottom. Copyright and
courtesy Hemosol, Inc.
Generic
genius
In contrast, "adult" stem cells are more specialized (despite the name,
you can find them in young people, not just aging acolytes of elderly autos).
As the drawing shows, blood-forming stem cells in bone marrow may be less
versatile than embryonic stem cells, but they can still create red and
white blood cells.
One of the first medical experiments with adult stem cells used cells gathered from aborted fetuses, a controversial source to those who oppose abortion. About a decade ago, Swedish researchers started injecting fetal brain tissue into the brains of people with Parkinson's, a disease that gets unrelentingly worse. Over the ensuing years, many of these people improved, which helped spark the current fetish for stem cells. We previously covered the role of neural stem cells in learning; we'll get to stem cells and Parkinson's shortly.
This
Morris
Minor looks great. But like a stem cell, it's trying to change into
something else.
Transplantation of fetal brain cells also seems to have helped patients with Huntington's disease, a progressive and fatal brain illness that causes uncoordination and psychosis. In a pilot study in Florida, four of seven patients had improvements in movement, although the disease normally gets steadily worse. The implanted cells were alive and connected to other neurons in a patient who died for unrelated reasons, more than one year after the transplant.
While there is no proof that the transplants caused the improvement, it's a reasonable conclusion. And, says Ole Isacson, of Harvard Medical School, "such long survival is good news not just for fetal-cell transplantation but for using stem cells."
What are the pitfalls and potentials of these remarkable cells, in terms of treating the ills that ail us?
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