Stem cells is a hierarchy of special cells of living organisms, each of which can subsequently change (differentiate) in a special way (that is, getting specialization and further develop as a normal cell). (Kiessling 2003)

Stem cells differ from other types of cells in the body. All stem cells, regardless of their source, have three general properties:
– they are able to divide and self-renewal for a very long time;
– they are unspecialized;
– they can lead to the creation of specialized cell types. (Kiessling 2003)

Stem cells can divide and regenerate over time. Unlike muscle cells, blood cells and nerve cells that normally do not proliferate, stem cells can do this many times. Even a small stem cells, which multiply within a few months in the laboratory can bring millions of cells in the end.

Scientists are trying to understand two fundamental properties of stem cells, which belong to their long-term self-renewal: Why embryonic stem cells can grow for a year or more in the laboratory without differentiating, but most of other stem cells can not? What are the factors in living organisms that normally regulate stem cell self-renewal, as in human organism they are not updated continuously as in the laboratory? Answers to these questions may allow to understand how cell proliferation is regulated during normal embryonic development or in the process of abnormal cell division that leads to cancer. Such information will also allow scientists to grow embryonic stem cells more effectively in the laboratory.

The specific factors and conditions that allow stem cells to remain unspecialized are of great interest to scientists. It took two decades to learn how to grow human embryonic stem cells in the laboratory, following the development of stem cells. Stem cells are unspecialized. One of the fundamental properties of stem cells is that they have no tissue-specific structure, which limits the performance of specialized functions only. For example, stem cells can not work with its neighbors to pump blood throughout the body (such as heart muscle cells), they also can not carry molecules of oxygen through the blood (such as red blood cells). However, unspecialized stem cells can lead to specialized cells, including heart muscle cells, blood cells or nerve cells.

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