Stem cells are the foundation cells for our bodies. The highly specialized cells that compose all the organs and tissues of the body originally came from a pool of stem cells that formed shortly after fertilization.
Throughout our lives, we rely on a pool of stem cells to repair injured tissues and replace cells that are normally lost over the course of time, such as those in our skin, hair, blood, and the lining of our gut.
Stem cells can be broadly defined by two characteristics: their capacity to self-renew (divide in a way that generates more stem cells) and to differentiate (to turn into mature, specialized cells that make up our tissues and organs).
Stem cell research contributes to a fundamental understanding of how organisms develop and grow, and how tissues are maintained throughout adult life. This knowledge is required to understand what goes wrong during disease and injury and ultimately how these conditions might be treated.
Of particular interest is research on human embryonic stem cells, as those cells open a window into unique aspects of human development and biology, and therefore will lead to novel insights into human disease.
There are now more than 4,000 clinical trials involving stem cells in the United States, with Parkinson’s disease, diabetes, heart disease, Alzheimer’s disease, and various blood cancers being treated with experimental stem cell therapies.
What are embryonic stem cells?
Embryonic stem cells are pluripotent stem cells, meaning they can give rise to all specific cell types of the body. They can be grown indefinitely to maintain a pool of stem cells in the laboratory if the correct conditions are met or, if given the right cues, they can be coaxed to differentiate to a variety of mature, specialized cell types. This makes them very valuable for regenerative medicine.
A major focus of research is to find ways to generate cells and tissues from embryonic stem cells that can be used to test new drugs or to replace damaged organs in patients.
Embryonic stem cells are obtained from a very early stage in development, usually the blastocyst stage, which in humans forms about five days after fertilization of an egg.
A blastocyst is a mainly hollow ball, barely visible to the naked eye. Inside is a clump of approximately 150 cells, called the inner cell mass, that develop over time to form the entire body of the developing animal. Embryonic stem cells are formed by removing the cells from the inner cell mass and growing them in culture.
Mouse embryonic stem cells, first isolated in 1981, are the most widely studied. They have taught us a lot about how pluripotent cells grow and specialize, and how early development works. Mouse embryonic stem cells can be manipulated to contain specific genetic changes and then used to generate mice, which contain this change. This has led to the discovery of many genes associated with different human diseases and helped us understand how these diseases develop.
Human embryonic stem cells were isolated relatively recently, in 1998. They are more difficult to work with than their mouse counterparts, and currently, less is known about them. However, scientists are making remarkable progress in learning about human developmental processes, modeling disease, and establishing strategies that could ultimately lead to new medical treatments.
What are induced pluripotent cells?
Induced pluripotent cells (iPS cells) are non-pluripotent cells that are induced to become pluripotent—that is, able to form all cell types of the body. In essence, the process of normal development from embryonic stem cell to differentiated cell is reversed. To form iPS cells, a cell with a specialized function (for example, a skin cell) is “reprogrammed” to an unspecialized state similar to that of an embryonic stem cell. Although iPS cells and embryonic stem cells share many characteristics, they are not identical.
Typically, iPS cells are produced by using viruses to insert copies of three or four genes known to be important in embryonic stem cells into fully mature cells, such as skin cells or immune cells.
Different research groups have used slightly different combinations of genes and different methods to get them into the cell. It is not completely understood how each of these genes functions to confer pluripotency, and ongoing research is addressing this question.
iPS cells hold great promise for creating patient- and disease-specific cell lines for research purposes. A great deal of work remains before these methods can be used to generate stem cells suitable for safe and effective therapies.