An unspecialized cell that has the ability to reproduce itself over an extended period. Unlike other cells in the body, stem cells have no defined functions or structures. They seem to exist for the purpose of becoming whatever kinds of cells the body needs them to become, a process called differentiation. The therapeutic potential of harnessing this capability is that stem cells could be “grown” into cell types and tissues to replace those damaged or destroyed by disease. At present there remain many unanswered questions about stem cells and their possible use to treat diseases such as Parkinson’s. Scientists only partially understand stem cells’ origin and function and are just beginning to learn to manipulate their development and differentiation in the laboratory. There are two kinds of stem cells, embryonic and adult.
Embryonic Stem Cells
Embryonic stem cells are the most primal cell structures of the human body. They exist in the earliest stages of human development, when the embryo (called a blastocyst) is no more than a cluster of about 30 cells during the first three to five days after fertilization and before cell differentiation begins. Embryonic stem cells are totipotent they have the ability to become any kind of cell found in the human body. Genetic encoding determines what kinds of cells they become. In the embryo, differentiation takes place as the genetic blueprint unfolds. In the laboratory researchers can manipulate embryonic stem cells to differentiate into virtually any kind of cell by introducing the genetic material of the desired type of cell.
The same colony of embryonic stem cells continues to replicate undifferentiated in the laboratory for a year or longer, when sustained in an appropriate culture medium. By taking cells from an established colony and placing them into a separate culture medium, scientists can grow millions of embryonic stem cells from each 30-cell blastocyst. When scientists implant embryonic stem cells into other tissues, the stem cells adopt the genetic code of the adjacent cells and differentiate into cells of that type as they replicate. Embryonic stem cells implanted into the PUTAMEN or the substantia nigra, for example, differentiate into dopaminergic neurons.
Such implantation, called embryonic stem cell implant, is investigational as well as a bioethical concern. Embryonic stem cells are obtained from aborted embryos or from embryos created through in vitro fertilization (combining of an ovum, or egg, and a sperm in the laboratory), a treatment for infertility, that are not needed. The moral and legal implications of using human embryos to cultivate embryonic stem cells, and in particular of the potential for researchers to create human embryos for the express purpose of providing such cells, have generated significant controversy within the medical community as well as among the public at large. In 2001, President George W. Bush signed federal legislation restricting embryonic stem cell research to embryonic stem cell cultivations (called stem cell lines) already in existence at about 20 research facilities in the United States. Many other countries also have, or are considering, strict limitations on the sources of embryonic stem cells used in research as well. Conversely, the state of California, as well as some nations like Australia, have passed legislation that control, but specifically permit the use of, embryonic stem cells from in vitro fertilization surpluses and other sources.
Adult Stem Cells
Adult stem cells exist as undifferentiated cells among the differentiated cells that make up numerous body tissues. Scientists have located adult stem cells in the bone marrow, brain, liver, blood vessels, and skin. It is possible that all tissue systems contain stem cells; researchers do not yet know how adult stem cells develop, where they reside, or what activates them to differentiate. They seem to exist in very small numbers and to become activated to differentiate when there is a need to replace tissue, such as when there is an injury. In such circumstances, adult stem cells differentiate at replication, creating new cells of the same type and function as their “host” tissue. But scientists do not understand what mechanisms control this process, or why it does not occur automatically, for example, when a heart attack damages areas of heart tissue. Some adult stem cells are pluripotent: That is, they can differentiate into cells of several kinds of tissues. Scientists are searching for ways to manipulate adult stem cells to differentiate for specific types of cells, in which case they could become treatment options for autoimmune diseases such as diabetes and neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.
Colonies of adult stem cells cultivated in the laboratory have a much shorter life span than cultivated embryonic stem cells, generally living for no longer than six months. Researchers are not sure why this occurs; nor are they sure of the life span of an adult stem cell within the human body. Although one culture can be the foundation for numerous colonies, it is necessary to develop new colonies continually. Also, it is difficult to locate and extract adult stem cells from their native tissues, as there are not many of them and their distribution is diffuse. However, there is little controversy about research and therapy using adult stem cells. Could scientists perfect adult stem cell cultivation and selective differentiation, it would be possible to develop individualized “tissue banks” a person could use as the need arose. The prospect of harnessing the body’s own therapeutic potential as a renewable resource has great appeal.