Immortality and unlimited potential. That’s a stem cell in a nutshell. It is an unspecialized ancestor cell, capable of living practically forever or morphing into any cell type from any tissue. Talk about power in a small package!
There are two kinds of stem cells: embryonic and adult. Embryonic stem cells come from fertility clinics. In the fertility laboratories, donor eggs are fertilized in vitro (in a test tube) and watched for a few days. After 4 or 5 days, successfully fertilized eggs have become blastocysts, which are tiny balls of cells. Some of the blastocysts are used for implantation in a woman’s uterus. The rest are frozen, destroyed or donated for stem cell research, with the signed consent of the donors. Stem cells are extracted from the center of the blastocyst and grown in cell cultures in a research lab. As long as they aren’t crowded or signaled to change, the stem cells can multiply unchanged for years. Or, given the right signals, certain genes within these cells will “turn on,” causing the stem cell to differentiate into a very specific cell type from a certain tissue. Embryonic stem cells are pleuripotent (pleuri- as in pleural, implying many, or multiple potent as in potential, meaning these cells have the potential to become any other cell in the system.
In recent years, scientists have discovered that adult humans actually retain a few stem cells. Tucked away in the lining of our intestines and the recesses of our brains, tiny swat teams of these cells huddle, breaking cover only when their home tissue is diseased or injured. These cells, also called “somatic stem cells,” help repair damage, and differ from embryonic stem cells in that they seem to be limited in their potential. In other words, unspecialized stem cells from a certain tissue can become specialized cells of that tissue, or possibly of another tissue type or two, but they are not pleuripotent like embryonic stem cells. So far, adult stem cells have been identified in brain, bone marrow, blood vessels, peripheral blood, skeletal muscle, skin and liver.
Adult stem cells have already been used in medical treatments. Have you heard of a bone marrow transplant? That is a stem cell transplant. Bone marrow stem cells become blood cells. If someone’s bone marrow is wiped out, say by radiation for cancer, they can’t make their own blood cells anymore. Give them a bone marrow transplant, and the donor stem cells take over the job, saving the recipient from catastrophe.
What else are stem cells good for? To date, the answer to this is largely theoretical, since the research is in progress. There are three general areas of research and application: regenerative therapies (like transplantation), drug testing, and development research.
The fact that stem cells can differentiate into different tissues holds promise for tissue transplants. Many diseases cause destruction or degeneration of whole organs or types of tissues. Currently, donated organs and tissues are used to replace these damaged tissues, but the demand far outweighs the supply. Stem cells could be used as a renewable source of transplantable tissues. The possibility of using adult stem cells for this purpose is especially exciting, because if one’s own stem cells could be cultured and placed back in the body, the risk of tissue rejection might be less. Some of the specific diseases being considered for these cell-based therapies are Diabetes, burns, heart disease, spinal cord injury, arthritis, Parkinson’s and Alzheimer’s diseases.
Drug testing for safety and efficacy is an obvious necessity, before new drugs are made available for medical use. At present, some cancer drugs are being tested on cells, on cancer cell lines, which are grown in a lab like stem cells. The scientific hope in this area is that stem cells can be induced to specialize into certain cell types, on which tissue-specific drugs could then be tested.
Finally, studying stem cells and the way they differentiate could give scientists a lot of information about the complex events that occur during human development, normal and abnormal. What tells cells to divide and differentiate? What goes wrong in this process to cause birth defects? What signal makes cells turn cancerous? Stem cell research is being used to understand these very complicated processes, in hopes that one day birth defects and cancer, among other conditions, might be prevented.
Exciting as these medical applications might sound, they are a long way from reality. Many fundamental questions remain. What causes a cell to differentiate? Are the signals internal to the cell or external? How do certain genes get turned on by these signals? How do stem cells remain unspecialized and self renewing for years and years? How can scientists direct a stem cell to become a specific tissue cell? How can a somatic stem cell from one tissue be used to make a different tissue? These are some of the basic questions that are being asked in research labs around the world. Only when they are answered can the medical promise of stem cells be fulfilled.