Human stem cells hold great promise in health care. Stem cells have the ability to divide without limit and give rise to many specialized cells. They are best described in the context of normal human development, which begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. This fertilized egg is totipotent, meaning that these cells give rise to all the cells in the body. In the first hours after fertilization, this cell divides into identical totipotent cells. This means that either one of these cells, if placed into a woman's uterus, has the potential to develop into a fetus. Approximately four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells and inside the hollow sphere, there is a cluster of cells called the inner cell mass.

Stem cells revolutionizing medicine?

The outer layer of cells will go on to form the placenta and other supporting tissues needed for fetal development. The inner cell mass cells will go on to form virtually all of the tissues of the human body. The inner cell mass cells can form virtually every type of cell found in the human body. These inner cell mass cells are pluri-potent, they can give rise to many types of cells but not all types of cells are necessary for fetal development. Because their potential is not total, they are not totipotent and they are not embryos. In fact, if an inner cell mass cell were placed into a woman's uterus, it would not develop into a fetus.

These pluri-potent stem cells undergo further specialization into stem cells that give rise to cells that have a particular function. Examples of this include blood stem cells, which give rise to red blood cells, white blood cells and platelets; and skin stem cells that give rise to the various types of skin cells. These more specialized stem cells are called multipotent. Multipotent stem cells are also found in children and adults.

How Are Pluri-potent Stem Cells Derived?

At present, human pluri-potent cell lines have been developed from two sources, directly from the inner cell mass of human embryos or from the fetal tissue obtained from terminated pregnancies.

Potential Uses Of Pluri-potent Stem Cells

There are several important reasons why human pluri-potent stem cells are so important. Perhaps the most far-reaching potential application of human pluri-potent stem cells is the generation of tissues and organs. Many diseases and disorders result from destruction of tissues of the body. Today, donated organs and tissues are often used to replace diseased organs. Pluri-potent stem cells offer the possibility of replacement cells and tissue to treat diseases, including parkinson's and alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis.

Multi-potent Stem Cells

Multi-potent stem cells can be found in some, but not all types of adult tissue. In fact, stem cells are needed to replenish the supply cells in our body that normally wear out, blood cells, for example, need to be replaced every 60 days.

While adult stem cells hold real promise, there are also some limitations. First of all, stem cells from adults have not been isolated for all tissues of the body. For example, adult cardiac stem cells or adult pancreatic islet stem cells have not been identified in humans. Secondly, adult stem cells are often present in only minute quantities and their numbers may decrease with age.

Any attempt to use stem cells from a patient's own body for treatment would require that stem cells would first have to be isolated from the patient and then grown in culture. For some diseases, there may not be enough time to grow enough cells. In diseases caused by a genetic defect, the genetic error would likely be present in the patient's stem cells. There is evidence that stem cells from adults may not have the same capacity to grow as younger cells do. This could limit the usefulness of adult stem cells.

The development of stem cell lines, both pluri-potent and multi-potent, that could potentially produce tissues of the human body is an important scientific breakthrough. Further human stem cells may help doctors improve their understanding of the complex events that occur during normal human development and of what goes wrong to cause diseases and conditions such as birth defects and cancer. Stem cells have the potential to revolutionize the practice of medicine and improve the quality and length of life.

Definitions

DNA - Abbreviation for deoxy-ribonucleic acid which makes up genes.

Gene - A functional unit of heredity which is a segment of DNA located in a specific site on a chromosome. A gene directs the formation of an enzyme or other protein.

Somatic cell - Cell of the body other than egg or sperm.

Somatic cell nuclear transfer - The transfer of a cell nucleus from a somatic cell into an egg from which the nucleus has been removed.

Stem cells - Cells that have the ability to divide for indefinite periods in culture and to give rise to specialized cells.

Pluri-potent -Capable of giving rise to most tissues of an organism.

Totipotent - Having unlimited capability. Totipotent cells have the capacity to specialize into extraembryonic membranes and tissues, the embryo, and all postembryonic tissues and organs.


Scientific Information from National Institutes of Health (NIH)