Stem cells

Contents 1 What are human stem cells? 2 What classes of stem cells are there? 3 Where do embryonic stem cells come from? 4 Why do scientists want to use stem cell lines? 5 Why are doctors and scientists so excited about human embryonic stem cells? 6 What will be the best type of stem cell to use for therapy? 7 What are the potential uses of human stem cells and the obstacles that must be overcome before these potential uses will be realized?

What are human stem cells?

Stem cells are cells that have the remarkable potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells for as long as the person or animal is still alive. When a stem cell divides, each �daughter� cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

What classes of stem cells are there?

There are three classes of stem cells: totipotent, multipotent, and pluripotent. A fertilized egg is considered totipotent, meaning that its potential is total; it gives rise to all the different types of cells in the body.

� Stem cells that can give rise to a small number of different cell types are generally called multipotent.

� Pluripotent stem cells can give rise to any type of cell in the body except those needed to develop a fetus.

Where do embryonic stem cells come from?

Embryonic stem cells, as their name suggests, are derived from embryos. Specifically, embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro�in an in vitro fertilization clinic�and then donated for research purposes with informed consent of the donors. They are not derived from eggs fertilized in a woman's body. The embryos from which human embryonic stem cells are derived are typically four or five days old. Cells from these embryos can be used to create stem cell �lines� � cell cultures that can be grown indefinitely in the laboratory. Stem cell lines have also been developed from fetal tissue older than 8 weeks of development.

Why do scientists want to use stem cell lines?

Once a stem cell line is established from a cell in the body, it is essentially immortal, no matter how it was derived. That is, the researcher using the line will not have to go through the rigorous procedure necessary to isolate stem cells again. Once established, a cell line can be grown in the laboratory indefinitely and cells may be frozen for storage or distribution to other researchers.

Stem cell lines grown in the lab provide scientists with the opportunity to �engineer� them for use in transplantation or treatment of diseases. For example, before scientists can use any type of tissue, organ, or cell for transplantation, they must overcome attempts by a patient�s immune system to reject the transplant. In the future, scientists may be able to modify human stem cell lines in the laboratory by using gene therapy or other techniques to overcome this immune rejection. Scientists might also be able to replace damaged genes or add new genes to stem cells in order to give them characteristics that can ultimately treat diseases.

Why are doctors and scientists so excited about human embryonic stem cells?

Stem cells have potential in many different [[Stem Cells � Uses and Treatment Possibilities | areas of health and medical research]]. To start with, studying stem cells will help us to understand how they transform into the dazzling array of specialized cells that make us what we are. Some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur somewhere in this process. A better understanding of normal cell development will allow us to understand and perhaps correct the errors that cause these medical conditions.

Another potential application of stem cells is making cells and tissues for medical therapies. Today, donated organs and tissues are often used to replace those that are diseased or destroyed. Unfortunately, the number of people needing a transplant far exceeds the number of organs available for transplantation. Pluripotent stem cells offer the possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases, conditions, and disabilities including Parkinson�s and Alzheimer�s diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis.

What will be the best type of stem cell to use for therapy?

Pluripotent stem cells, while having great therapeutic potential, face formidable technical challenges. First, scientists must learn how to control their development into all the different types of cells in the body. Second, the cells now available for research are likely to be rejected by a patient�s immune system. Another serious consideration is that the idea of using stem cells from human embryos or human fetal tissue troubles many people on ethical grounds.

Until recently, there was little evidence that multipotent adult stem cells could change course and provide the flexibility that researchers need in order to address all the medical diseases and disorders they would like to. New findings in animals, however, suggest that even after a stem cell has begun to specialize, it may be more flexible than previously thought.

There are currently several limitations to using adult stem cells. Although many different kinds of multipotent stem cells have been identified, adult stem cells that could give rise to all cell and tissue types have not yet been found. Adult stem cells are often present in only minute quantities and can therefore be difficult to isolate and purify. There is also evidence that they may not have the same capacity to multiply as embryonic stem cells do. Finally, adult stem cells may contain more DNA abnormalities�caused by sunlight, toxins, and errors in making more DNA copies during the course of a lifetime. These potential weaknesses might limit the usefulness of adult stem cells.

What are the potential uses of human stem cells and the obstacles that must be overcome before these potential uses will be realized?

There are many ways in which human stem cells can be used in basic research and in clinical research. However, there are many technical hurdles between the promise of stem cells and the realization of these uses, which will only be overcome by continued intensive stem cell research. The major potential uses of stem cells are:

• Studies of human embryonic stem cells may yield information about the complex events that occur during human development, especially how a particular gene is turned on or off. A better understanding of the genetic and molecular controls may yield information about how diseases arise and suggest new strategies for therapy. However, scientists do not yet fully understand the signals that turn specific genes on and off to influence the differentiation of the stem cell.

• Human stem cells could also be used to test new drugs. New medications could be tested for safety on differentiated cells generated from human pluripotent cell lines. However, to screen drugs effectively, the conditions must be identical when comparing different drugs.

• Human stem cells can generate cells and tissues that could be used for cell-based therapies. Today, donated organs and tissues are often used to replace ailing or destroyed tissue, but the need for transplantable tissues and organs far outweighs the available supply. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including Parkinson's and Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.

To realize the promise of novel cell-based therapies for such pervasive and debilitating diseases, scientists must be able to easily and reproducibly manipulate stem cells so that they possess the necessary characteristics for successful differentiation, transplantation and engraftment. Also, to avoid the problem of immune rejection, scientists are experimenting with different research strategies to generate tissues that will not be rejected.