What's a Stem Cell?
The simplest description of a stem cell is a generic cell that has the potential to become many types of specialized cells through a process of differentiation.
The archetypical stem cell is a fertilized egg the single root cell that forms all the rest of the cells in an organism. For the first couple divisions after a fertilized egg cell divides to form a group of cells, each of the progeny cells retain this characteristic of totipotency to form a whole organism. Identical twins, triplets, quintuplets, etc. start from a few cells formed from a single fertilized egg cell that separate and develop independently.
Totipotency and Pluripotency
After the first few divisions, however, the embryo cells formed from the original fertilized egg cell lose totipotency. Each embryo cell, however, still does have the potential differentiate into many different types of cells—they are pluripotent. These are embryonic stem cells (ES cells).
The more developed the embryo is, the less pluripotent most of the cells become. However, all through development and growth, some cells in different parts of the body retain a degree of pluripotency. Regina Bailey, About's Biology Guide, lays out the variations of stem cell potency nicely.
Adult Stem Cells
Throughout life, the body has to produce new cells to replace lost one and repair itself. For example, new blood cells need to be made constantly, cells in the digestive system slough off and need to be replaced continually, cuts and bone fractures need to heal, etc.
The body's replacement cells are grown from populations of adult stem cells, also often called progenitor cells. New blood cells, for example, are grown in bone marrow. Adult stem cells typically only have potency to develop into a limited number of cell types, much fewer than ES cells.
Induced Pluripotent Stem Cells
In 2006, a group from Kyoto University in Japan found that they could make mouse skin cells pluripotent by introducing just four genes. Once they made the cells, they were able to get them to differentiate into other types of cells like muscle, cartilage, and nerve cells. Basically, the four genes "de-differentiated" cells to create what appeared to be undifferentiated stem cells. To prove they behaved like stem cells, the researchers then cultured them in ways so that they differentiated into a completely different type of cells.
It surprised many that fully differentiated normal adult cells could be transformed into stem cells with just four genes. A few months later, the same lab found that they could actually make iPS cells with just three of genes and another lab also replicated the results in different systems. Following this discovery, researchers began working in earnest on induced pluripotent cells (iPS cells). The ability to purposely induce differentiation using genetic engineering provided an ideal tool to study the biological mechanisms behind this process.
Potential for Stem Cell Therapy
The potential applications for stem cell therapy are broad. With the ability to turn into virtually any of the body's cells, stem cells have the potential to treat a wide range of injuries and degenerative diseases, including Alzheimer's, muscular dystrophy, spine injuries, blindness, diabetes, and heart damage, just to name a few.
While embryonic stem cells may turn out to be the best choice for some therapies, research work with other stem cell system, such as iPS cells and adult stem cells, might provide more practical utility for clinical use in the long run. Also, once we have better knowledge of the cell differentiation process, there is also the possibility of reprogramming fully differentiated cells right in the body to replace damaged tissue or organs.
Stem Cell Research is Just Starting
Although there's been a lot of discussion about developing therapies with stem cells over the last several years, the science is still really just in its infancy. It is only in the last 10-20 years that we have had the genomic research tools to really start uncovering the mechanisms behind the process of differentiation. While it is clear this technology will have a major impact on health care in the years to come, at this point, there is no way to predict which lines of research will lead to the most promising treatments. Right now, it is important to push for more progress on all fronts.