Stem Cell.

Ci-dessous un extrait traitant le sujet : Stem Cell.. Ce document contient 2181 mots. Pour le télécharger en entier, envoyez-nous un de vos documents grâce à notre système d’échange gratuit de ressources numériques ou achetez-le pour la modique somme d’un euro symbolique. Cette aide totalement rédigée en format pdf sera utile aux lycéens ou étudiants ayant un devoir à réaliser ou une leçon à approfondir en : Echange

---

Stem Cell.
I

INTRODUCTION

Stem Cell, type of cell that can make any kind of cell required to build an organism. When a stem cell divides, one new cell that results can remain a stem cell while the
other new cell becomes an ordinary cell with a particular function in the organism. Sometimes a stem cell that divides produces two identical stem cells. With either
process, stem cells can renew themselves indefinitely. By contrast, ordinary cells can only make copies of themselves when they divide and can only divide a limited
number of times. Sometimes a stem cell may divide into two ordinary cells, producing no more stem cells. The ability of stem cells to produce new cells of specific types
is of special interest to medical science.
Medical researchers and other scientists study stem cells to understand basic processes in cell development and disease. The special properties of stem cells make them
potentially powerful medical tools that could repair or replace diseased or injured tissues or organs in humans. Research is usually done with stem cells from mice or
from humans. The study or use of stem cells derived from human embryos has led to controversy, however.

II

TYPES OF STEM CELLS

Two basic types of stem cells occur in humans and animals: embyronic stem cells and adult stem cells. Embryonic stem cells have the potential to develop into any
organ or type of tissue in the body. This property is called pluripotency. Embryonic stems cells exist in fully developing embryos for a limited period of time (about three
weeks). However, embryonic stem cells produced in laboratory conditions continue to divide and can be sustained almost indefinitely in nutrient cultures.
Adult stem cells, also called somatic stem cells, are found in humans and animals after birth and remain active in the body throughout a lifetime. Adult stem cells can
only turn into certain specialized types of cells. Different types of adult stem cells act as a repair system and are able to replace such cells as blood cells, bone cells, or
certain nerve cells in the body. Researchers continue to discover new types of adult stem cells in different parts of the body and have discovered that blood stem cells
originate in the placenta. In living humans and animals, development of adult stem cells appears to be influenced by their niche or microenvironment in the body, which
restricts what they become. Some progress has been made in reprogramming adult stem cells in the laboratory to act more like embryonic stem cells and produce more
types of cells. Cultures of adult stem cells have been difficult to sustain in laboratories, however.
Other types of stem cells are related to embryonic or adult stem cells but have different properties. Fetal stem cells are similar to embryonic stem cells and occur in a
developing fetus. Umbilical or neonatal stem cells are found in the umbilical cord at birth and are similar to certain types of adult stem cells but less mature.
Cancer stem cells are found in some tumors and in some blood cancers (see leukemia), and appear to be a factor in making such cancers grow. Similar to normal adult
stem cells, cancer stem cells can divide to produce a new cancer stem cell and a particular type of cancer cell.

III

THE SOURCE OF STEM CELLS

To explore potential medical uses of stem cells, scientists need to produce stem cell lines. These lines are colonies of stem cells that grow and replicate themselves in
culture--that is, in a special nutrient substance in a laboratory dish. A stem cell line provides a scientist with a virtually endless supply of material to explore and test.
Stem cells, however, seem to lose some of their ability to make a wide range of cells as they age. In other words, an older stem cell may not be as versatile as a
younger one. The exception seems to be adult stem cells derived from bone marrow, which retain their ability to transform into any cell type. As a result, adult stem
cells from bone marrow and the earliest stem cells--those found in an embryo--may provide the most powerful tools for use in medical treatment.
Scientists are investigating two primary approaches to using stem cells in medicine. The first approach involves developing stem cells that could be transplanted to
combat a specific disease. For instance, someone with a liver disease might receive a dose of liver stem cells. Some day, a regular medical regimen might even include
an occasional dose of stem cells, much like a booster shot, to keep the body tuned up to fight off diseases and tissue damage. The second approach involves learning to
use the body's own supply of stem cells. Perhaps these stores of cells could be turned on or off as needed to help the body defend itself against health problems.

IV

DEBATING THE USE OF STEM CELLS

Despite the promise of stem cells, research using human stem cells has stirred considerable controversy in the United States and in some other parts of the world. The
controversy mainly surrounds the use of stem cells that come from human embryos, in particular embryos left over from infertility treatments. During a treatment
known as in vitro fertilization, eggs that have been surgically removed from a female ovary are placed in a laboratory dish with male sperm. In some cases more than
one egg becomes fertilized, creating extra embryos. Embryonic stem cells come from embryos at a very early stage, about the time the embryo would have attached to
the wall of a uterus.
The use of human embryonic stem cells in medical research raises a fundamental question: Do these cells come from human tissue or from humans? Some people
oppose the use of anything, including stem cells, from an embryo that is viable (able to grow). For people who take this philosophical position, stem cell research
involves the destruction of a human life. An opposing viewpoint states that these embryos would never develop into humans, because they would be either discarded or
kept frozen in laboratories for future research. Consequently, some people argue that this material should be used in any way that could possibly improve human life.
Some people who oppose the use of embryonic stem cells point out that scientific investigators could rely on other sources. A variety of body tissues--including bone
marrow and blood from an umbilical cord--can provide adult stem cells. However, adult stem cells are not equal to embryonic cells in their versatility and thus in their
potential for treating human disease.

V

MEDICAL RESEARCH

In 1981 scientists first grew cultures of stem cells from mice embryos. Although that achievement marked the beginning of extensive research, growing human stem
cells in a laboratory remained an elusive goal until 1998. That year two research teams independently announced that they had isolated and grown human stem cells.
The teams were led by biologists John Gearhart at Johns Hopkins University and James Thomson at the University of Wisconsin at Madison.
During the late 1990s scientists discovered many characteristics of stem cells. Perhaps most interesting, various investigators showed that even mature stem cells from
one tissue--the blood, for example--can create cells of another tissue type, such as neurons (nerve cells) for the brain. In some of the most exciting results, researcher
Fred Gage at the Salk Institute for Biological Studies showed that the brains of adult humans can create new neurons. Before Gage's discovery, neurobiologists assumed
that our brain did not create any new cells after birth. Presumably, the capacity for ongoing creation of neurons comes from stem cells. In addition, Gage and his
colleagues found that a mentally stimulating environment or even exercise can enhance the creation of neurons in the brain.

The medical profession used adult stem cells to treat diseases long before anyone isolated one. In 1968 scientists performed the first successful bone marrow
transplant, a procedure in which a patient receives an infusion of healthy bone marrow cells. The purpose of such transplants is to restore the blood-making capabilities
of the patient's diseased bone marrow after extremely strong chemotherapy has destroyed that bone marrow. From the beginning investigators suspected that stem
cells in the infused bone marrow make this technique work. Bone marrow transplants are now a standard therapy for certain cancers, such as leukemia and lymphoma,
and for other diseases of the blood and bone. Other stem cell therapies in current use involve blood stem cells isolated from drawn blood and taken from umbilical
cords. See also Medical Transplantation.
By the beginning of the 21st century researchers had not yet developed any medical treatment that relied on isolated stem cells grown in culture. Because of their
ability to repair tissue damage, stem cells could serve as starting points in therapy for a wide variety of medical conditions, including Alzheimer's disease, which
damages brain cells, especially those related to memory, and Parkinson disease, which damages nerves that control the muscles. Other diseases, such as diabetes,
heart disease, and leukemia, might also be treated with stem cells to replace or repair damaged, diseased, or lost cells and tissues.

VI

THE GOVERNMENT AND STEM CELL RESEARCH

After the isolation of stem cells, the controversy that developed over their use soon involved the United States government, which funds much of the country's medical
research. In November 1998 President Bill Clinton asked the National Bioethics Advisory Commission, an advisory committee appointed by the president, to investigate
the medical and ethical issues behind stem cell research. That commission encouraged federal funding for research on embryonic stem cells, as long as these cells came
from the tissue of dead fetuses or from embryos that remained after infertility treatments. Yet differing opinions prevented a firm government commitment to funding
research in this field.
In August 2001 President George W. Bush announced a new government position: Federal funding would be available for stem cell research, but only for research on
existing stem cell lines. In other words, stem cells already in culture could be used in federally funded projects, but scientists could not isolate additional sources of stem
cells. Researchers can choose to adhere to these guidelines or rely on private funding.
The head of Bush's Council on Bioethics has said embryonic cloning is morally wrong because embryos are destroyed in the process of extracting stem cells, women
could be exploited as egg donors, and the techniques could lead to cloning human beings. In 2006 Bush vetoed a bill to allow increased federal funding for research on
embryos that had been discarded by fertility clinics. The bill had already been passed by Congress. Current U.S. government policy equates the destruction of an
embryo with the destruction of a human life and prohibits federal funding for such research. In addition, a number of states forbid embryonic stem cell research, and
some ban cloning for any purpose.
Researchers are seeking new techniques for harvesting stem cells without harming the embryo, thus avoiding the ethical issues that have blocked stem cell research. In
2006 Massachusetts researchers at the Worcester, Massachusetts, laboratory of Advanced Cell Technology, Inc., announced a technique for extracting a single cell
(blastomere) from a two-day-old embryo that consists of eight cells. The extracted blastomere cell is then grown in the laboratory to start a stem cell line. The sampled
embryo itself is unharmed. In early 2007 researchers in North Carolina reported success in harvesting stem cells from amniotic fluid, the liquid that surrounds a fetus in
the womb. The cells can be captured during amniocentesis tests given to pregnant women. More research is needed to confirm the effectiveness of both techniques.
Other researchers have used cloning techniques to insert the nucleus from a skin cell from one individual into a human egg that has had its own nucleus with DNA
removed. The egg began to develop into a blastocyst with DNA matched to the new individual's immune system. The egg did not develop to the stage of producing
embryonic stem cells, but this step may be reached in the future. However, using a human egg carries similar ethical issues to using embryos, in the views of some
people.
An important advance in the study of stem cells has been the genetic reprogramming of ordinary differentiated cells to act like embryonic stem cells. In work first
announced in 2007, viruses were used to insert genetic instructions into human skin cells that made the cells pluripotent (able to make all other types of cells), similar
to embryonic stem cells. However, using viruses carries the risk of tumors or cancer, and other, safer methods of reprogramming cells will need to be developed. It is
not yet known if such induced pluripotent stem cells are truly equivalent to embryonic stem cells. Using a patient's own cells to create the reprogrammed equivalents of
stem cells would avoid problems with immune system rejection, however. Because such techniques do not require human embryos or human eggs, research and
therapies with reprogrammed cells could also avoid possible ethical objections and legal restraints.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.