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II. What are the unique properties of all stem cells?

Stem cells have unique abilities to self-renew and to recreate functional tissues.

Stem cells have the ability to self-renew.

Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate— stem cells may replicate many times. When a stem cell divides, the resulting two daughter cells may be: 1) both stem cells, 2) a stem cell and a one more differentiated cell, or 3) both more differentiated cells.  What controls the balance between these types of divisions to maintain stem cells at an appropriate level within a given tissue is not yet well known.

Discovering the mechanism behind self-renewal may make it possible to understand how cell fate (stem vs. non-stem) is regulated during normal embryonic development and post-natally, or misregulated as during aging, or even in the development of cancer. Such information may also enable scientists to grow stem cells more efficiently in the laboratory. The specific factors and conditions that allow pluripotent stem cells to remain undifferentiated are of great interest to scientists. It has taken many years of trial and error to learn to derive and maintain pluripotent stem cells in the laboratory without the cells spontaneously differentiating into specific cell types.

Stem cells have the ability to recreate functional tissues.

Pluripotent stem cells are undifferentiated; they do not have any tissue-specific characteristics (such as morphology or gene expression pattern) that allow them to perform specialized functions. Yet they can give rise to all of the differentiated cells in the body, such as heart muscle cells, blood cells, and nerve cells. On the other hand, adult stem cells differentiate to yield the specialized cell types of the tissue or organ in which they reside, and may have defining morphological features and patterns of gene expression reflective of that tissue.

Different types of stems cells have varying degrees of potency; that is, the number of different cell types that they can form. While differentiating, the cell usually goes through several stages, becoming more specialized at each step. Scientists are beginning to understand the signals that trigger each step of the differentiation process. Signals for cell differentiation include factors secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment.