Stem cells are the body’s “master cells.” They are the building blocks of organs, tissue, blood, and the immune system. The history of stem cell research began in the 1950s when researchers discovered that bone marrow contains at least two kinds of adult stem cells, hematopoietic stem cells and mesenchymal stem cells.
Stem cells have the remarkable potential to develop into many different cell types in the body. In many tissues, they serve as a sort of internal repair system, dividing constantly to replenish lost or damaged cells. Perhaps the most important potential application of human stem cells is in the development of cell-based therapies to treat or cure disease and aid in the repair of tissues damaged by disease or injury. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat multiple conditions, including spinal cord injury, stroke, burns, heart disease, osteoarthritis, autoimmune disease, inherited disorders and neurological disorders.
Two growing fields of stem cell research include regenerative repair and composite (multiple) tissue transplantation (CTA).
All stem cells—regardless of their source—have three general properties:
- They are capable of dividing and renewing themselves for long periods of time Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate themselves—stem cells may replicate many times, or proliferate. A starting population of stem cells that proliferates for many months in the laboratory can yield millions of cells.
- They are unspecialized A stem cell cannot work with its neighbors to pump blood through the body like a heart muscle cell or carry oxygen molecules through the bloodstream like a red blood cell. However, unspecialized stem cells can give rise to specialized cells, including heart muscle cells, blood cells, or nerve cells.
- They can give rise to specialized cell types Under certain conditions, stem cells can be induced to become tissue-specific cells. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. When unspecialized stem cells are induced to become specialized cells (a process called differentiation), they can be used to treat diseases affecting specific organs and tissues. Internal signals from the cell’s genes and external signals that include physical contact with neighboring cells and chemicals secreted by other cells control differentiation.
Until recently, scientists primarily worked with two kinds of stem cells: embryonic stem cells and adult or “somatic” stem cells. In 2006, researchers made another breakthrough in stem cell research when they discovered that specialized adult stem celled could be genetically “reprogrammed” to assume the properties of embryonic stem cells. These new stem cells are called induced pluripotent stem cells (iPSCs).
Embryonic stem cells are derived from embryos that develop from eggs fertilized in an in-vitro fertilization clinic. They are then frozen and stored for implantation at a later time. Embryos that are no longer needed may be donated for research purposes with the informed consent of the donors. They are not derived from eggs fertilized in a woman’s body.
Adult stem cells can be found in all tissues of the body, including brain, bone marrow, blood, blood vessels, fat, skeletal muscle, skin, teeth, heart, gut, liver and hair follicles. The primary job of adult stem cells is to maintain and repair the tissue in which they are found. Stem cells generally remain quiescent (non-dividing) for long periods of time, but are routinely activated by the need to replace cells that are lost through normal wear and tear. They may also be activated by disease or tissue injury. Cord blood is another source of adult stem cells in widespread use to treat disease, particularly in children.
Induced pluripotent stem cells (iPSCs) are adult stem cells that have been genetically reprogrammed to behave like embryonic stem cells. iPSCs are already useful tools for drug development and disease modeling for research discoveries. It is hoped that further study will help researchers learn how to reprogram cells to repair damaged tissues in the human body.