EMBARGOED FOR RELEASE CONTACT: Gabriel Paal 4:00 p.m. U.S. Eastern Time
202-326-6421 Thursday, 21 January 1999 gpaal(AT)aaas.org
Washington D.C. - Research by a group of Italian and Canadian scientists
indicates that certain adult cells- previously assumed to be permanently
wedded to their specialized roles in the body--can shed their identities
and reinvent themselves as different types of cells. In the study,
reported in the 22 January issue of Science, the authors transplanted
adult stem cells from the brains of adult mice into the bone marrow of
new, mice, where the stem cells changed their behavior and began
generating blood cells. The findings raise the possibility that in the
future adult stem cells could be used to supply a variety of new cells
for important therapeutic uses, for example to generate healthy blood
cells for treating patients with blood disorders.
Researchers recently proposed that using stem cells to grow healthy
tissues was one of several beneficial applications for their new-found
ability to grow human stem cells from early-stage embryos, a finding
announced in Science last November. Embryonic stem cells are primordial
cells that divide indefinitely and give rise to the body's different
cell types as they develop. However, although these cells may have the
potential to provide considerable benefits, they also pose some
controversial ethical implications because they come from human embryos.
Now, a study on mice by Angelo Vescovi, of Neuro-Spheres Limited in
Canada and the National Neurological Institute in Italy, and his
colleagues suggests that stem cells don't have to come from embryos in
order to generate specialized cells. "We went the other way around and
used adult stem cells instead," said Vescovi.
Adult stem cells are more specialized than the primitive stem cells of
an early embryo. These stem cells supply new cells to parts of the body
with a high cellular turnover rate, such as the hematopoeitic (blood
producing) system, the intestines, or the skin. In their experiment,
Vescovi and his colleagues used neural stem cells (NSCs) from the
central nervous system of mice. NSCs generate the major cell types found
in the adult brain, namely neurons and their support cells.
The authors injected the NSCs into a second group of mice whose
hematopoeitic stem cells (these cells reside primarily in the bone
marrow where they produce the different varieties of blood cells) had
been destroyed by a near-lethal dose of radiation. Once in the blood
stream, the NSCs seeded the mice's bone marrow (and the spleen, another
site of blood production). There, they took over the job of the
hematopoeitic stem cells and churned out a fresh supply of blood cells.
Apparently, during their development as NSCs, the stem cells had not
undergone any irrevocable changes that made them unable to specialize as
other types of stem cells as well. "It took us a while to believe our
own data. The tissue of the body has always been seen as unchangeable,"
Vescovi said.
Vescovi and his colleagues speculate that it may be possible in the
future to perform bone marrow transplants using adult stem cells to
create a new supply of healthy blood in patients with cancers of the
hematopoeitic system such as leukemia. To reach that point, however,
researchers will have to confirm that human stem cells behave the same
way the mouse stem cells do. And they will have to accomplish the
daunting task of isolating and growing adult human stem cells.
More directly, the evidence from Vescovi and his colleagues' study
indicates that neural stem cells, or "NSCs," were able to revert to
their unspecialized, embryonic state and then re-specialize as
hematopoeitic stem cells. To further investigate whether this process
was in fact occurring, the researchers also injected a second group of
irradiated mice with foreign hematopoeitic stem cells. Because these
cells had been producing blood cells originally, it took them less time
to produce blood cells after being transplanted than it did the NSCs.
The NSCs may have taken more time because they had to first "unlearn"
their original tasks.
In their paper, Vescovi's group notes that their results are consistent
with a key finding by Ian Wilmut and his colleagues, who announced the
success of their cloning experiments in 1997. In both cases, adult cells
returned to their primordial states and then began new lives: one became
Dolly the sheep, the other a blood producing stem cell.