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STAR

Houston, Texas

Representation of several types of stem cells
STaR Center
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Stem Cell Basics

In our center we study adult, embryonic and cancer stem cells. The summaries below explain the different types of stem cells.

Adult Stem Cells

These stem cells come from different organs of fully developed organisms. Sometimes referred to as "somatic" stem cells (from the body, or soma), these cells can be obtained from many different tissues, and contribute to regeneration of those tissues from which they are derived.

For example, the bone marrow regenerates the blood (or "hematopoietic") system constantly. The bone marrow stem cells ("hematopoietic stem cells) are the key elements that are transferred during a bone marrow transplant that recreate an entire blood system in the transplant recipient. The donor can afford to donate his or her bone marrow, because the remaining stem cells will self-renew, replenishing their own stem cell supply so that the donor continues to have a plentiful blood supply.

Such somatic stem cells are found in the skin, digestive tract, and many other tissues. In most tissues, these stem cells in general do not divide very often (they are quiescent), but can be called into duty rapidly to regenerate that tissue. While they are essential for maintaining many adult tissues, and very effective at doing so, they are limited in their capabilities -- for the most part, they can ONLY regenerate those tissues from which they are derived: bone marrow stem cells make bone marrow and blood, skin stem cells make skin, gut stem cells replace the lining of the intestines, muscle stem cells make muscle, and so on.

Despite their limitations, they have great therapeutic potential. Thus, many researchers are studying these, trying to identify them better and understand how they are regulated. Much work of this type is done in our center.

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Embryonic Stem Cells

Stem cells can also be made by propagating part of a very early embryo. At this stage, the embryo is only a few days old, and is a primitive ball, with only an outer sheet of protective cells, and an inner clump of cells.

When these inner cells are put into a culture dish under special conditions, they expand and become a stem cell "line." We then call them embryonic stem, or "ES," cells. These ES cells can be grown and expanded indefinitely, and are nearly unrestricted in their potential. They have the capacity to make every cell type in the body.

The last 20 years of research using stem cells from mice has transformed the biomedical field, Researchers found that these ES cells can be expanded and also used to generate specific cell types. Researchers have used them to introduce a modification, and study how that modification affects the function of a down-stream cell, such as a liver cell, or a blood cell.

These kinds of experiments have led to very many important discoveries as well as novel therapies. Human ES cells were first derived in 1998 from embryos that were planned to be discarded after in vitro fertilization procedures. These human ES cells, like their mouse counterparts, can be expanded indefinitely and will generate virtually any cell type of the body.

For this reason, they are likely to transform basic research into human biology, as well as lead to new therapies based on generation of specific cell types. Current federal guidelines allow some human ES cell lines to be used with federal funding. Research on embryonic stem cells is conducted in our center.

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Cancer Stem Cells

Recently, an old theory about cancer has been revisited. Cancer refers to the collection of diseases in which a cell of the body looses its normally very tight restrictions on growth, and divides out of control.

Because the cancer cells divide much faster than almost all cells of the body, most cancer therapy simply is directed against any rapidly dividing cell. Unfortunately, many cancers relapse after some time, even when they appear to be eliminated.

There are several reasons for this, but the idea that a small subset of cells, possibly a cancer stem cell, causes this relapse, has been revived. These cancer stem cells may have many similarities to normal somatic stem cells. Normally, the cancer stem cells are probably not dividing, or are dividing very slowly. This enables them to escape the chemotherapy agents. Some time after the bulk of the cancer has been destroyed, the cancer stem cells are somehow awakened -- potentially leading to relapse.

Cancer stem cells are poorly understood at this time. They are very difficult to identify, which has hampered the development of therapies that could be targeted towards them. Research to identify and understand cancer stem cells is being conducted in our center.

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Additional Information

To learn more about stem cells, visit the International Society for Stem Cell Research and the National Institutes of Health , both are superb information sources.

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