What is Regenerative Medicine? (Excerpted from NIH site)
Research on stem cells is advancing knowledge about how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. This promising area of science is also leading scientists to investigate the possibility of cell-based therapies to treat disease, which is often referred to as regenerative or reparative medicine.
What are stem cells and why are they important?
Stem cells have two important characteristics that distinguish them from other types of cells. First, they are unspecialized cells that renew themselves for long periods through cell division. The second is that under certain physiologic or experimental conditions, they can be induced to become cells with special functions such as the beating cells of the heart muscle or the insulin-producing cells of the pancreas.

Scientists primarily work with two kinds of stem cells from animals and humans: embryonic stem cells and adult stem cells, which have different functions and characteristics. Scientists discovered ways to obtain or derive stem cells from early mouse embryos more than 20 years ago. Many years of detailed study of the biology of mouse stem cells led to the discovery, in 1998, of how to isolate stem cells from human embryos and grow the cells in the laboratory. These are called human embryonic stem cells. The embryos used in these studies were created for infertility purposes through in vitro fertilization procedures and when they were no longer needed for that purpose, they were donated for research with the informed consent of the donor.

Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst, stem cells in developing tissues give rise to the multiple specialized cell types that make up the heart, lung, skin, and other tissues. In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury, or disease.
What are the unique properties of all stem cells?
Stem cells differ from other kinds of cells in the body. All stem cells—regardless of their source—have three general properties: they are capable of dividing and renewing themselves for long periods; they are unspecialized; and they can give rise to specialized cell types.

  • Stem cells are unspecialized. One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialized functions. A stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell); it cannot carry molecules of oxygen through the bloodstream (like a red blood cell); and it cannot fire electrochemical signals to other cells that allow the body to move or speak (like a nerve cell). However, unspecialized stem cells can give rise to specialized cells, including heart muscle cells, blood cells, or nerve cells.
  • Stem cells are capable of dividing and renewing themselves for long periods. Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate themselves—stem cells may replicate many times. When cells replicate themselves many times over it is called proliferation. A starting population of stem cells that proliferates for many months in the laboratory can yield millions of cells. If the resulting cells continue to be unspecialized, like the parent stem cells, the cells are said to be capable of long-term self-renewal.
  • Stem cells can give rise to specialized cells. When unspecialized stem cells give rise to specialized cells, the process is called differentiation. Scientists are just beginning to understand the signals inside and outside cells that trigger stem cell differentiation. The internal signals are controlled by a cell's genes, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell. The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment.
Where are adult stem cells found and what do they normally do?
Adult stem cells have been identified in many organs and tissues. One important point to understand about adult stem cells is that there are a very small number of stem cells in each tissue. Stem cells are thought to reside in a specific area of each tissue where they may remain quiescent (non-dividing) for many years until they are activated by disease or tissue injury. The adult tissues reported to contain stem cells include brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.
How do adult stem cells regenerate?
Adult stem cells typically generate the cell types of the tissue in which they reside. A blood-forming adult stem cell in the bone marrow, for example, normally gives rise to the many types of blood cells such as red blood cells, white blood cells and platelets.

Until recently, it had been thought that a blood-forming cell in the bone marrow—which is called a hematopoietic stem cell—could not give rise to the cells of a very different tissue, such as nerve cells in the brain. However, a number of experiments over the last several years have raised the possibility that stem cells from one tissue may be able to give rise to cell types of a completely different tissue, a phenomenon known as plasticity.

Examples of such plasticity include blood cells becoming neurons, liver cells that can be made to produce insulin, and hematopoietic stem cells that can develop into heart muscle. Therefore, exploring the possibility of using adult stem cells for cell-based therapies has become a very active area of investigation by researchers.
What is known about adult stem cell differentiation?

Hematopoietic and stromal stem cell differentiation.

Scientists have reported that adult stem cells occur in many tissues and that they enter normal differentiation pathways to form the specialized cell types of the tissue in which they reside. Adult stem cells may also exhibit the ability to form specialized cell types of other tissues, which is known as transdifferentiation or plasticity.

Normal differentiation pathways of adult stem cells. In a living animal, adult stem cells can divide for a long period and can give rise to mature cell types that have characteristic shapes and specialized structures and functions of a particular tissue. The following are examples of differentiation pathways of adult stem cells (See above figure).

  • Hematopoietic stem cells give rise to all the types of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, macrophages, and platelets.
  • Bone marrow stromal cells (mesenchymal stem cells) give rise to a variety of cell types: bone cells (osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and other kinds of connective tissue cells such as those in tendons.
  • Neural stem cells in the brain give rise to its three major cell types: nerve cells (neurons) and two categories of non-neuronal cells—astrocytes and oligodendrocytes.
  • Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells.
  • Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles. The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis.

Adult stem cell plasticity and transdifferentiation. A number of experiments have suggested that certain adult stem cell types are pluripotent. This ability to differentiate into multiple cell types is called plasticity or transdifferentiation. The following list offers examples of adult stem cell plasticity that have been reported during the past few years.

  • Hematopoietic stem cells may differentiate into: three major types of brain cells (neurons, oligodendrocytes, and astrocytes); skeletal muscle cells; cardiac muscle cells; and liver cells.
  • Bone marrow stromal cells may differentiate into: cardiac muscle cells and skeletal muscle cells.
  • Brain stem cells may differentiate into: blood cells and skeletal muscle cells.
What is the difference between adult stem cells and embryonic stem cells?
Embryonic stem cells are the cells found in a developing human embryo. These cells can differentiate into any cell type that makes up all of the organs and tissues in the body.

Adult stem cells exist in limited amounts in most adult tissues. The cells create new cells for the types of tissue in which they reside. For example, stem cells found in bone marrow generate red blood cells, white blood cells and platelets. In recent studies, however, research has shown that even these somewhat specialized adult stem cells can be used develop cells of very different tissue, such as nerve cells in the brain.
What conditions can be treated with adult stem cells?
A variety of conditions can be treated through adult stem cell products. These include diseases, degenerative conditions, wounds, scars, and other conditions. Specific diseases and condition that can benefit from adult stem cell technology include Alzheimer's, Parkinson's, ischemic head trauma, spinal cord injuries, and other age related macular degenerative conditions. Other diseases and conditions that adult stem cell products can be used for include retinitis pigmentosa, diabetic retinopathy, burns, scars, skin ulcers and vesicoureteral reflux, and stroke.
Glossary of terms associated with adult stem cells and regenerative medicine.
  • Adult (or somatic) stem cell — An undifferentiated cell found in a differentiated tissue that can renew itself and differentiate (with certain limitations) to give rise to all the specialized cell types of the tissue from which it originated.
  • Astrocyte — A type of supporting (glial) cell found in the nervous system.
  • Bone marrow stromal cells — A mixed population of stem cells found in bone marrow that does not give rise to blood cells but instead generates bone, cartilage, fat, and fibrous connective tissue.
  • Cell division — Method by which a single cell divides to create two cells. There are two main types of cell division: mitosis and meiosis.
  • Cell-based therapies — Treatment in which stem cells are induced to differentiate into the specific cell type required to repair damaged or destroyed cells or tissues.
  • Differentiation — The process whereby an undifferentiated embryonic cell acquires the features of a specialized cell such as a heart, liver, or muscle cell.
  • Directed differentiation — Manipulating stem cell culture conditions to induce differentiation into a particular cell type.
  • Hematopoietic stem cell — A stem cell that gives rise to all red and white blood cells and platelets.
  • Long-term self-renewal — The ability of stem cells to renew themselves by dividing into the same non-specialized cell type over long periods (many months to years) depending on the specific type of stem cell.
  • Multipotent — Ability of a single stem cell to develop into more than one cell type of the body.
  • Neural stem cell — A stem cell found in adult neural tissue that can give rise to neurons and glial (supporting) cells. Examples of glial cells include astrocytes and oligodendrocytes.
  • Neurons — Nerve cells, the structural and functional unit of the nervous system. A neuron consists of a cell body and its processes—an axon and one or more dendrites. Neurons function by starting and conducting impulses. Neurons transmit impulses to other neurons or cells by releasing neurotransmitters at synapses.
  • Plasticity — The ability of stem cells from one adult tissue to generate the differentiated cell types of another tissue.
  • Pluripotent — Ability of a single stem cell to give rise to all of the various cell types that make up the body. Pluripotent cells cannot make so-called "extra-embryonic" tissues such as the amnion, chorion, and other components of the placenta.
  • Regenerative medicine — A treatment in which stem cells are induced to differentiate into the specific cell type required to repair damaged or destroyed cell populations or tissues.
  • Somatic cell — Any body cell other than gametes (egg or sperm).
  • Somatic stem cells — Non-embryonic stem cells that are not derived from gametes (egg or sperm cells).
  • Stem cells — Cells with the ability to divide for indefinite periods in culture and to give rise to specialized cells.
  • Stromal cells — Non-blood cells derived from blood organs, such as bone marrow or fetal liver, which are capable of supporting growth of blood cells in vitro. Stromal cells that make the matrix within the bone marrow are also derived from mesenchymal stem cells.
  • Transdifferentiation — The process by which stem cells from one tissue differentiate into cells of another tissue.
  • Undifferentiated — A cell that has not yet generated structures or manufactured proteins characteristic of a specialized cell type