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What Are Stem Cells?

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Scientists are searching for a cure to many diseases using stem cell technology, a promising area of research that relies on the unique abilities of stem cells to self-renew and to give rise to different types of cells. These two properties of stem cells distinguish them from all other cells.


Stem Cells Self-Renew


When a stem cell receives an appropriate signal from outside, often called an extracellular cue, it will divide to generate two identical daughter cells, a process referred to as self-renewal. If the self-renewal signal persists, the two daughter cells will divide again to produce four identical stem cells. Those four cells can divide to produce eight identical stem cells, and so on.


The term self-renewal is used to describe the ability of stem cells to generate identical stem cells almost indefinitely. So, even after several generations, each new generation of daughter cells is identical and has exactly the same capacity for self-renewal. Although many other non-stem cells divide quite often -- for instance, skin cells and blood cells -- they all have a finite generation span; after a certain number of divisions they lose the ability to divide.


Stem Cells Differentiate


Stem cells receive many different extracellular cues, and although some prompt them to self-renew, some push them along a different path. After receiving the appropriate signal, a stem cell may divide asymmetrically into two daughter cells that are very slightly different from each other. The daughter cell identical to its parent cell will continue to self-renew, maintaining stem cell levels, but the slightly different daughter cell has now been programmed to follow a different course. Instead of dividing, it starts to change its form by switching on or off different genes. This process is called differentiation. First it differentiates into a progenitor cell. The progenitor cell is then able to differentiate into specialized tissue cells by switching on different gene programs.


For instance, a progenitor cell could give rise to white blood cells, like the B cells and T cells involved in immune response; hormone-secreting cells, like the beta cells in the pancreas that secrete insulin; heart muscle cells; or nerve cells (neurons). Each of these differentiated, specialized cells is very different from its stem cell cousins produced by self-renewal. Once differentiated, these cells have only limited potential for cell division, and some, like nerve cells, cannot divide at all. Also, the differentiation path does not reverse – differentiated cells cannot de-differentiate into stem cells – at least, not in the human body. In future blogs, we will discuss the progress that has been made in the lab towards causing some cell types to de-differentiate.

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