Defective embryos could limit controversy surrounding
research, scientist says
Precursor neural cells derived from human embryonic stem cells
grow in a lab. Now, scientists have shown that defective cloned frog embryos
produced useful stem cells.
WASHINGTON
- Defective embryos may be able to yield stem cells, which could offer another source
of the sought-after cells, British researchers said.
THEY SAID
their findings illustrate why bans on cloning and stem cell research could be
premature and could prevent potentially lifesaving discoveries.
“This might
be a quite interesting way around the problem,” John Gurdon of the Wellcome Cancer Research Institute at Britain’s
Cambridge University
said in a telephone interview.
Gurdon and
colleagues found that defective cloned frog embryos produced useful stem cells,
the body’s master cells, which can be manipulated to form any kind of cell in
the body.
It is hoped
such cells can be the source of valuable tissue or even organs for transplant
to treat a range of diseases from Parkinson’s and Alzheimer’s to cancer and
juvenile diabetes.
There are
several sources of stem cells.
Embryonic
stem cells are considered the most flexible. They come from early embryos,
usually obtained from fertility clinics, which make many excess embryos in
trying to help couples conceive test-tube babies.
Embryos can
also, theoretically, be made using cloning technology with a patient’s own
cells. Such therapeutic cloning would not involve growing the embryo to a stage
at which it could be implanted into a woman’s womb.
Stem cells
can also be taken from aborted or miscarried fetuses, and can be found, with
difficulty, in adult tissue and blood.
These
cells are generally farther along the path of differentiation into various cell
types and are not so easy to manipulate, experts say.
No one
opposes the use of adult stem cells and many researchers are working to try to
develop them. But many experts argue it is not known how useful they would be
and would like to be able to work with embryonic stem cells.
MAY BE USEFUL
Many of the
embryos made in in-vitro fertilization attempts to create test-tube babies are
abnormal, and researchers have been afraid to use them as sources of stem
cells. They are thrown away.
But Gurdon and
colleagues, working with frogs, said they found normal stem cells in
abnormal-looking cloned embryos.
“We find we
can take embryos destined to die really quite quickly and retrieve good cells
from them which could be used both for research and for cell replacement,”
Gurdon said.
What are stem cells?What
are embryonic stem cells?What are adult stem cells?How do embryonic and adult stem cells compare?Where do the embryos used in research come from?What are the possible medical uses for stem cells?Are there other potential uses?
Stem cells are master cells that have the ability to
differentiate into other cell types, including those in the brain, heart,
bones, muscles and skin.
Embryonic stem cells are cells contained in embryos that
have the ability to transform themselves into virtually any other type of cell
in the body. It is this quality that enables the tiny embryo to develop into a
fully formed body. About five days after fertilization, the human embryo
becomes a blastocyst — a hollow sphere of about 100
cells. Cells in its outer layer go on to form the placenta and other organs
needed to support fetal development in the uterus. The inner cells go on to
form nearly all of the tissues of the body. These are the embryonic stem cells
used in research.
The name is a misnomer because they are harbored in
mature tissue in the bodies of children as well as adults. Adult stem cells are
more specialized than embryonic ones and give rise to specific cell types. The
mature body uses these cells as “spare parts” to replace other worn out cells.
For example, certain stem cells in the bone marrow spawn red blood cells, white
blood cells and blood platelets. Recent research has suggested adult stem cells
can turn into many more cell types than once believed possible.
Embryonic stem cells boast two important qualities: they
can become almost anything in the body and they can be grown in culture in an
unlimited quantity. The disadvantages are that a patient’s immune system might
reject transplants of embryonic stem cells just as some organ transplants are
rejected, and that runaway growth of embryonic stem cells could produce tumors.
Because adult stem cells would be taken from the very patient who would receive
them later in treatment, there are no rejection issues.
Disadvantages in adult stem cells include: doubts about
whether they can transform themselves as readily as embryonic stem cells;
difficulty in growing them in culture at the quantity needed to facilitate
transplants, and worry that years of exposure to toxins, radiation and DNA
replicating errors could leave them with genetic abnormalities.
Scientists generally harvest embryonic stem cells from
embryos left over in fertility clinics after in vitro fertilization procedures.
When people undergo IVF, there are usually many more embryos created than can
be implanted. Sometimes surplus embryos are discarded; other times, they are
donated to help other infertile couples or for research.
Scientists hope to harness the transformational qualities
of stem cells to provide treatments for a variety of diseases affecting
millions of people worldwide. Because stem cells can turn into many other cell
types with the right prompting, doctors may be able to replace tissues and
organs damaged by disease or injury to restore healthy function.
Therapeutic applications of stem cells potentially could
treat illnesses including: Parkinson's disease; diabetes; Alzheimer’s disease;
stroke; heart attack; multiple sclerosis; blood, bone and bone marrow ailments;
severe burns by providing skin grafts; spinal cord injuries, and cancer
patients who have lost cells and tissue to radiation and chemotherapy. In
addition, stem cells could be harnessed and packaged to deliver gene therapies
to specific targets in the body to treat genetic problems.
Stem cells could allow drugs to be more easily tested
without using a laboratory animal as a proxy, as researchers could grow pure
populations of specific cell types involved in certain diseases. Then these
cells would act as a proving ground for drugs. Ramped-up stem cell technology
would permit the rapid screening of thousands of chemicals.
Writing in
the Proceedings of the National Academy of Sciences, Gurdon said abnormal
cloned frog embryos produced normal stem cells.
“You can see
some of them are so abnormal that they cannot survive,” he said. An embryo at
this point is usually a nice, even sphere of cells, but abnormal ones are lumpy
messes.
Gurdon’s
team made frog embryos genetically engineered with green fluorescent protein —
a jellyfish protein that causes cells to glow under a fluorescent light. “We
took some normal-looking cells from these defective embryos and grafted those
onto normal, non-green hosts,” Gurdon said.
“We then
grew those up into tadpoles and the tadpoles grew for many months until they
were about to turn into frogs.” At that point, the tadpoles still carried cells
with green fluorescent protein, showing the stem cells had survived and propagated.
If the
same thing worked in mammals and humans, Gurdon said, it may offer an
acceptable alternative source for stem cells. “The objection in America
to their use is that these embryos are potential human beings,” Gurdon said.
“My argument is they are not potential humans beings if they are destined to die.”
President
George W. Bush has limited the use of embryonic stem cells by federally funded
researchers in the United States,
although private researchers can do as they please. He supports an outright ban
on all human cloning, even “therapeutic cloning”
designed to create stem cells for medical research.
Therapeutic
cloning and stem cell research is fully legal in Britain.
April 22, 02
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