What are the unique properties of all stem cells?

Stem cells differ from other kinds of cells in the body. All stem cells regardless of their source have

three general properties: they are capable  of dividing  and renewing  themselves for long periods; they are unspecialized;  and they can give rise to specialized  cell types.

Scientists are trying  to understand  two fundamental  properties of stem cells that relate to their  long-term self -renewal:

1.  why can  embryonic stem cells proliferate for a year or more in the laboratory without differentiating, but most  adult stem cells cannot; and

2.  what are the factors in living organisms  that normally regulate stem cell  proliferation and

self- renewal ?

Discovering  the answers to these questions may make it possible to understand  how cell proliferation is regulated  during normal embryonic development  or during the abnormal  cell division that leads to cancer. Importantly, such information  would enable scientists to grow embryonic and adult stem cells more efficiently in the laboratory.

Stem cells are unspecialized.  One of the fundamental  properties of a stem cell is that it does not have any tissue - specific structures  that allow it to perform specialized  functions. A stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell); it cannot carry  molecules of oxygen  through the bloodstream (like a red blood cell); and it cannot fire electrochemical  signals to other cells that allow the body to move or speak (like a nerve cell). However,  unspecialized  stem cells can give rise to specialized  cells, including  heart muscle cells, blood cells, or nerve cells.

Stem cells are capable of dividing and renewing themselves for long periods. Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate themselves—stem cells may replicate many times. When cells replicate themselves many times over it is called proliferation.  A starting  population of stem cells that proliferates  for many months in the laboratory can yield millions of cells. If the resulting cells continue to be unspecialized,  like the parent stem cells, the cells are said to be capable  of long- term self- renewal.

The specific factors and conditions that allow stem cells to remain unspecialized  are of great interest to scientists. It has taken scientists many years of trial and error to learn to grow stem cells in the laboratory without them spontaneously  differentiating into specific cell types. For example, it took 20 years to learn how to grow  human embryonic stem cells in the laboratory following the development of conditions for growing mouse stem cells. Therefore, an important area of research is understanding the signals in a mature organism that cause a stem cell population to proliferate and remain unspecialized until the cells are needed for repair of a specific tissue. Such information is

critical for scientists to be able to grow large numbers of unspecialized stem cells in the laboratory for further experimentation.

Stem cells can give rise to specialized cells. When  unspecialized stem cells give rise to specialized cells, the process is called  differentiation. Scientists are just beginning to understand the signals inside and outside cells that trigger stem cell differentiation. The internal signals are controlled by a cell's  genes, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell. The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the  microenvironment.

Therefore, many questions about stem cell differentiation remain. For example, are the internal and external signals for cell differentiation similar for all kinds of stem cells? Can specific sets of signals be identified that promote differentiation into specific cell types? Addressing these questions is critical

because the answers may lead scientists to find new ways  of controlling  stem cell differentiation in the laboratory, thereby growing cells or tissues that can be used for specific purposes including  cell -based therapies.

Adult stem cells typically  generate the cell types of the tissue in which they reside. A blood- forming adult stem cell in the bone marrow,  for example, normally gives rise to the many types of blood cells such as red blood cells, white blood cells and platelets. Until recently, it had been thought  that a blood- forming cell in the bone marrow which is called a  hematopoietic stem cell  could not give rise to the cells of a very different tissue, such as nerve cells in the brain.  However,  a number of

experiments  over the last several years have raised the possibility that stem cells from one tissue may be able to give rise to cell types of a completely different tissue, a phenomenon known as  plasticity . Examples of such plasticity  include blood cells becoming  neurons , liver cells that can be made to produce insulin, and hematopoietic  stem cells that can develop into heart muscle. Therefore, exploring the possibility of using adult stem cells for cell- based therapies has become a very active area of investigation by researchers.