What are stem
cells, and why are they important?
Stem cells have
the remarkable potential to develop into manydifferent cell types in the body
during early life and growth. In addition, in many tissues they serve as a sort
of internal repair system, dividing essentially without limit to replenish
other cells as long as the person or animal is still alive. When a stem cell
divides, each new cell has the potential either to remain a stem cell or become
another type of cell with a more specialized function, such as a muscle cell, a
red blood cell, or a brain cell. Stem cells are distinguished from other cell
types by two important characteristics. First, they are unspecialized cells
capable of renewing themselves through cell division, sometimes after long
periods of inactivity. Second, under certain physiologic or experimental
conditions, they can be induced to become tissue- or organ-specific cells with
special functions. In some organs, such as the gut and bone marrow, stem cells
regularly divide to repair and replace worn out or damaged tissues. In other
organs, however, such as the pancreas and the heart, stem cells only divide
under special conditions. Until recently, scientists primarily worked with two
kinds of stem cells from animals and humans: embryonic stem cells and
non-embryonic “somatic” or “adult” stem cells. The functions and
characteristics of these cells will be explained in this document. Scientists
discovered ways to derive embryonic stem cells from early mouse embryos nearly
30 years ago, in 1981. The detailed study of the biology of mouse stem cells led
to the discovery, in 1998, of a method to derive stem cells from human embryos
and grow the cells in the laboratory. These cells are called human embryonic
stem cells. The embryos used in these studies were
created
for reproductive purposes through in vitro
fertilization procedures. When they were no longer needed for that purpose,
they were donated for research with the informed consent of the donor. In 2006,
researchers made another breakthrough by identifying conditions that would
allow some specialized adult cells to be “reprogrammed” genetically to assume a
stem cell-like state. This new type of stem cell, called induced pluripotent
stem cells (IPSCs), will be discussed in a later section of this document.
Stem
cells are important for living organisms for many reasons. In the 3- to
5-day-old embryo, called a blastocyst, the inner cells give rise to the entire
body of the organism, including all of the many specialized cell types and
organs such as the heart, lungs, skin, sperm, eggs and other tissues. In some
adult tissues, such as bone marrow, muscle, and brain, discrete populations of
adult stem cells generate replacements for cells that are lost through normal
wear and tear, injury, or disease.
Given
their unique regenerative abilities, stem cells offer new potentials for
treating diseases such as diabetes and heart disease. However, much work
remains to be done in the laboratory and the clinic to understand how to use
these cells for cell-based therapies to treat disease, which is also referred
to as regenerative or reparative medicine.
Laboratory
studies of stem cells enable scientists to learn about the cells’ essential
properties and what makes them different from specialized cell types.
Scientists are already using stem cells in the laboratory to screen new drugs
and to develop model systems to study normal growth and identify the causes of
birth defects.
Research
on stem cellscontinues to advance knowledge about how an organism develops from
a single cell and how healthy cells replace damaged cells in adult organisms.
Stem cell research is one of the most fascinating areas of contemporary
biology, but, as with manyexpanding fields of scientific inquiry, research on
stem cells raises scientific questions as rapidly as it generates new
discoveries.
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