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:
One cell line could supply endless amounts of cells with carefully defined
characteristics. Like an endless fountain, the cell line itself would
They can make any body cell.
Human embryos remaining after in-vitro fertilization are routinely destroyed
by fertility clinics.
Embryonic stem cells are:
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.
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.
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
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:
to immune attack: If patients receive the products of their own stem
cells, they will not mount an immune response.
Some types, like blood stem cells, are easy to find.
specialized: That reduces the amount of outside direction needed to
create specialized cells.
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:
Not all types of adult stem cells have been found yet.
They can be dangerous to extract (you wouldn't want to poke around in
someone's brain for neural stem cells).
They don't live as long as embryonic cells in culture.
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.
quality: Genetic defects may occur after exposure to sunlight or toxins.
Or the disease being treated may be present in the stem-cell genes.
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.
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,
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?