This primer explains what stem cells are. Here you can learn more about what adult stem cells and pluripotent stem cell are, how they are important to advancing healthcare.

Adult stem cells: Stem cells in an individual after birth.

Apheresis of bone marrow: Inserting a needle into bone and extracting bone marrow. A painful and sometimes dangerous procedure. Apheresis as a term includes extracting peripheral blood.

Autologous: In blood transfusion and transplantation, a situation in which the donor and recipient are the same person. Patients scheduled for non-emergency surgery may be autologous donors by donating blood for themselves that will be stored until the surgery. An autologous graft is providing a graft, for example of skin, to yourself.

Bioreactor: A cylindrical chamber that is rotated on its axis in a manner to grow biological cells, which includes cell growth media.

CD15: poly-N-acetyllactosamine. An anti-body. CD15 recognizes a human myelomonocytic antigen. The structure recognized by CD15 antibodies is lacto-N-fucopentose III. The CD15 antigen is present on greater than 95% of mature peripheral blood eosinophils and neutrophils and is present at low density on circulating monocytes. In lymphoid tissue, CD15 reacts with Reed-Sternberg cells of Hodgkin’s disease and with granulocytes; however, CD15 reacts with few tissue macrophages and does not react with dendritic cells.

CD33: Sialoadhesin; sialic acid-dependent cytoadhesion molecule. CD33 antigen, detected by WM53 monoclonal antibodies, is expressed on human peripheral blood monocytes and weakly on granulocytes. Expression is also found on myeloid progenitor cells, such as granulocyte and macrophage precursor cells in bone marrow. No reactivity has been found with normal lymphocytes, erythrocytes and platelets, nor with pluripotent stem cells.

CD34+: A surface antigen or protein residing on the surface of a blood cell. CD34+ protein is present on the surface of hematopoietic stem and progenitor cells all stages of development. The CD34+ cells are the only cell type in an apheresis or bone marrow collection that are usually responsible for blood cell recovery after transplant. Since it is present on the surface of hematopoietic stem and progenitor cells, it can be used to test for them.

CD38-: A cytoplasmic protein used much like CD34+ for determining expansion of hematopoietic stem and progenitor cells.

CD45: A hematopoietic cell useful like CD34+ and CD38- to test expansion of stem cells.

Cell-to-cell geometry: The geometry of cells in the human blood. Includes the spacing, distance between, and physical relationship of the cells relative to one another.

Cell-to-cell support: The support one cell provides for an adjacent cell in the liquid blood.

CFU-granulocyte-erythroid-macrophage-megakaryocyte (CFU-GEMM): The proliferative state of human pluripotent hemopoietic progenitors.

Colony-forming units granulocyte-macrophage (CFU-GM): A haematopoietic progenitor cell in the granulocytic series that can grow into a myeloblast in the presence of appropriate stimulators in vitro. Called also colony forming u.-culture.

Cord cells: Blood cells drained from an umbilical cord or placenta immediately after birth. Rich in unexpanded stem cells. Available only a few hours after birth.

Cytokines: A small protein released by cells that has a specific effect on the interactions between cells, on communications between cells or on the behavior of cells. The cytokines includes the interleukins, lymphokines and cell signal molecules, such as tumor necrosis factor and the interferons, which trigger inflammation and respond to infections.

Dimethyl sulfoxide (DMSO): A solvent that can be absorbed directly through the skin.

DNA: abbreviation for deoxyribonucleic acid, which makes up genes.

Dulbecco’s medium: A proprietary product commonly used in culturing cells. Embryonic stem cells: Stem cells taken from an unborn fetus.

G-CSF: A laboratory-made agent similar to a normally existing substance in the body that stimulates the production of blood cells. The colony-stimulating factors (CSFs) include granulocyte colony-stimulating factors (G-CSF) and granulocyte-macrophage colony-stimulating factors (GM-CSF).

GM-CSF: See G-CSF

Gene: A functional unit of heredity which is a segment of DNA located in a specific site on a chromosome. A gene directs the formation of an enzyme or other protein.

Hematopoietic colony-forming cells: Progenitor cells from which all blood cells derive.

Heparinized: Prepared to delay clotting.

IgG: Abbreviation for immunoglobulin G, a major class of immunoglobulins found in the blood, including many of the most common antibodies circulating in the blood. Also known as gamma globulin.

IL6: Interleukin 6. Aberrant production of IL6 by neoplastic cells has been implicated as a strong contributory factor to the growth of multiple myeloma and other B-cell dyscrasias, T-cell lymphoma, renal and ovarian cell carcinomas, and Kaposi sarcoma. An IL6 promoter polymorphism is associated with a lifetime risk of development of Kaposi sarcoma in men infected with human immunodeficiency virus.

Ischemic: Ischemic stroke refers to strokes caused by thrombosis or embolism and accounts for 80% of all strokes.

Leukapheresis: Removal of the blood to collect specific blood cells; the remaining blood is returned to the body.

Macrophages: A type of white blood cell that surrounds and kills microorganisms, removes dead cells, and stimulates the action of other immune system cells. A type of large leukocyte that travels in the blood but can leave the bloodstream and enter tissue; like other leukocytes, it protects the body by digesting debris and foreign cells.

Mesenchymal: Refers to the cells that develop into connective tissue, blood vessels, and lymphatic tissue.

Mononuclear cells: Cells having only one nucleus.

Multipotent: Stem cells that can give rise to several other cell types, but those types are limited in number. An example of a multipotent stem cell is a hematopoietic cell — a blood stem cell that can develop into several types of blood cells, but cannot develop into brain cells or other types of cells. At the end of the long series of cell divisions that form the embryo are cells that are terminally differentiated, or that are considered to be permanently committed to a specific function

Myocardial infarction: Destruction of heart tissue resulting from obstruction of the blood supply to the heart muscle. In layman’s terms, a heart attack.

Peripheral blood cells: Blood cells circulating through the body.

Pluripotent: The potential of a cell to develop into more than one type of mature cell, depending on environment; capable of giving rise to most tissues of an organism.

Regeneration: The reproduction or renewal of tissues, cells, etc., which have been used up or destroyed.

Regenerative Medicine: For our purposes, the use of stem cells or tissue to regenerate other cells or tissue in the body.

Somatic cell: Cell of the body other than egg or sperm.

Somatic cell nuclear transfer: The transfer of a cell nucleus from a somatic cell into an egg from which the nucleus has been removed.

Stem cells: Cells that have the ability to divide for indefinite periods in culture and to give rise to specialized cells.

T-cells: A small lymphocyte developed in the thymus; it orchestrates the immune system’s response to infected or malignant cells.

Three-dimensional geometry: The geometry of cells in the blood in a three-dimensional state (their natural state) as opposed to two-dimensional in a Petri dish.

Totipotent: Having unlimited capability to produce all the cells and tissues in the body.

Stem cells have the ability to divide for indefinite periods in culture and to give rise to specialized cells. They are best described in the context of normal human development. Human development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. This fertilized egg is totipotent, meaning that its potential is total. In the first hours after fertilization, this cell divides into identical totipotent cells.

This means that these cells, if placed into a woman's uterus, have the potential to develop into a fetus. In fact, identical twins develop when two totipotent cells separate and develop into two individual, genetically identical human beings. Approximately four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells and inside the hollow sphere, there is a cluster of cells called the inner cell mass.

The outer layer of cells will go on to form the placenta and other supporting tissues needed for fetal development in the uterus. The inner cell mass cells will go on to form virtually all of the tissues of the human body. Although the inner cell mass cells can form virtually every type of cell found in the human body, they cannot form an organism because they are unable to give rise to the placenta and supporting tissues necessary for development in the human uterus. These inner cell mass cells are pluripotent - they can give rise to many types of cells but not all types of cells necessary for fetal development. Because their potential is not total, they are not totipotent and they are not embryos. In fact, if an inner cell mass cell were placed into a woman's uterus, it would not develop into a fetus.

The pluripotent stem cells undergo further specialization into stem cells that are committed to give rise to cells that have a particular function. Examples of this include blood stem cells which give rise to red blood cells, white blood cells and platelets; and skin stem cells that give rise to the various types of skin cells. These more specialized stem cells are called multipotent.

While stem cells are extraordinarily important in early human development, multipotent stem cells are also found in children and adults. For example, consider one of the best understood stem cells, the blood stem cell. Blood stem cells reside in the bone marrow of every child and adult, and in fact, they can be found in very small numbers circulating in the blood stream. Blood stem cells perform the critical role of continually replenishing our supply of blood cells - red blood cells, white blood cells, and platelets - throughout life. A person cannot survive without blood stem cells.

At present, human pluripotent cell lines have been developed from two sources with methods previously developed in work with animal models. In the work done by Dr. Thomson, pluripotent stem cells were isolated directly from the inner cell mass of human embryos at the blastocyst stage. Dr. Thomson received embryos from IVF (In Vitro Fertilization) clinics-these embryos were in excess of the clinical need for infertility treatment. The embryos were made for purposes of reproduction, not research. Informed consent was obtained from the donor couples. Dr. Thomson isolated the inner cell mass (see Figure III) and cultured these cells producing a pluripotent stem cell line. In contrast, Dr. Gearhart isolated pluripotent stem cells from fetal tissue obtained from terminated pregnancies. Informed consent was obtained from the donors after they had independently made the decision to terminate their pregnancy. Dr. Gearhart took cells from the region of the fetus that was destined to develop into the testes or the ovaries. Although the cells developed in Dr. Gearhart's lab and Dr. Thomson's lab were derived from different sources, they appear to be very similar.

The use of somatic cell nuclear transfer (SCNT) may be another way that pluripotent stem cells could be isolated. In studies with animals using SCNT, researchers take a normal animal egg cell and remove the nucleus (cell structure containing the chromosomes). The material left behind in the egg cell contains nutrients and other energy-producing materials that are essential for embryo development. Then, using carefully worked out laboratory conditions, a somatic cell - any cell other than an egg or a sperm cell - is placed next to the egg from which the nucleus had been removed, and the two are fused. The resulting fused cell, and its immediate descendants, are believed to have the full potential to develop into an entire animal, and hence are totipotent. As described in Figure I, these totipotent cells will soon form a blastocyst. Cells from the inner cell mass of this blastocyst could, in theory, be used to develop pluripotent stem cell lines. Indeed, any method by which a human blastocyst is formed could potentially serve as a source of human pluripotent stem cells.

There are several important reasons why the isolation of human pluripotent stem cells is important to science and to advances in health care. At the most fundamental level, pluripotent stem cells could help us to understand the complex events that occur during human development. A primary goal of this work would be the identification of the factors involved in the cellular decision-making process that results in cell specialization. We know that turning genes on and off is central to this process, but we do not know much about these "decision-making" genes or what turns them on or off. Some of our most serious medical conditions, such as cancer and birth defects, are due to abnormal cell specialization and cell division. A better understanding of normal cell processes will allow us to further delineate the fundamental errors that cause these often deadly illnesses.

Human pluripotent stem cell research could also dramatically change the way we develop drugs and test them for safety. For example, new medications could be initially tested using human cell lines. Cell lines are currently used in this way (for example cancer cells). Pluripotent stem cells would allow testing in more cell types. This would not replace testing in whole animals and testing in human beings, but it would streamline the process of drug development. Only the drugs that are both safe and appear to have a beneficial effect in cell line testing would graduate to further testing in laboratory animals and human subjects.

Perhaps the most far-reaching potential application of human pluripotent stem cells is the generation of cells and tissue that could be used for so-called "cell therapies." Many diseases and disorders result from disruption of cellular function or destruction of tissues of the body. Today, donated organs and tissues are often used to replace ailing or destroyed tissue. Unfortunately, the number of people suffering from these disorders far outstrips the number of organs available for transplantation. Pluripotent stem cells, stimulated to develop into specialized cells, offer the possibility of a renewable source of replacement cells and tissue to treat a myriad of diseases, conditions, and disabilities including Parkinson's and Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis. There is almost no realm of medicine that might not be touched by this innovation. Some details of two of these examples follow.

Transplant of healthy heart muscle cells could provide new hope for patients with chronic heart disease whose hearts can no longer pump adequately. The hope is to develop heart muscle cells from human pluripotent stem cells and transplant them into the failing heart muscle in order to augment the function of the failing heart. Preliminary work in mice and other animals has demonstrated that healthy heart muscle cells transplanted into the heart successfully repopulate the heart tissue and work together with the host cells. These experiments show that this type of transplantation is feasible.

In the many individuals who suffer from Type I diabetes, the production of insulin by specialized pancreatic cells, called islet cells, is disrupted. There is evidence that transplantation of either the entire pancreas or isolated islet cells could mitigate the need for insulin injections. Islet cell lines derived from human pluripotent stem cells could be used for diabetes research and, ultimately, for transplantation.

While this research shows extraordinary promise, there is much to be done before we can realize these innovations. Technological challenges remain before these discoveries can be incorporated into clinical practice. These challenges, though significant, are not insurmountable.

First, we must do the basic research to understand the cellular events that lead to cell specialization in the human, so that we can direct these pluripotent stem cells to become the type(s) of tissue needed for transplantation.

Second, before we can use these cells for transplantation, we must overcome the well-known problem of immune rejection. Because human pluripotent stem cells derived from embryos or fetal tissue would be genetically different from the recipient, future research would need to focus on modifying human pluripotent stem cells to minimize tissue incompatibility or to create tissue banks with the most common tissue-type profiles.

The use of somatic cell nuclear transfer (SCNT) would be another way to overcome the problem of tissue incompatibility for some patients. For example, consider a person with progressive heart failure. Using SCNT, the nucleus of virtually any somatic cell from that patient could be fused with a donor egg cell from which the nucleus had been removed. With proper stimulation the cell would develop into a blastocyst: cells from the inner cell mass could be taken to create a culture of pluripotent cells. These cells could then be stimulated to develop into heart muscle cells. Because the vast majority of genetic information is contained in the nucleus, these cells would be essentially identical genetically to the person with the failing heart. When these heart muscle cells were transplanted back into the patient, there would likely be no rejection and no need to expose the patient to immune-suppressing drugs, which can have toxic effects.

As noted earlier, multipotent stem cells can be found in some types of adult tissue. In fact, stem cells are needed to replenish the supply cells in our body that normally wear out. An example, which was mentioned previously, is the blood stem cell.

Multipotent stem cells have not been found for all types of adult tissue, but discoveries in this area of research are increasing. For example, until recently, it was thought that stem cells were not present in the adult nervous system, but, in recent years, neuronal stem cells have been isolated from the rat and mouse nervous systems. The experience in humans is more limited. In humans, neuronal stem cells have been isolated from fetal tissue and a kind of cell that may be a neuronal stem cell has been isolated from adult brain tissue that was surgically removed for the treatment of epilepsy.

Until recently, there was little evidence in mammals that multipotent cells such as blood stem cells could change course and produce skin cells, liver cells or any cell other than a blood stem cell or a specific type of blood cell; however, research in animals is leading scientists to question this view.

In animals, it has been shown that some adult stem cells previously thought to be committed to the development of one line of specialized cells are able to develop into other types of specialized cells. For example, recent experiments in mice suggest that when neural stem cells were placed into the bone marrow, they appeared to produce a variety of blood cell types. In addition, studies with rats have indicated that stem cells found in the bone marrow were able to produce liver cells. These exciting findings suggest that even after a stem cell has begun to specialize, the stem cell may, under certain conditions, be more flexible than first thought. At this time, demonstration of the flexibility of adult stem cells has been only observed in animals and limited to a few tissue types.

Given the enormous promise of stem cells to the development of new therapies for the most devastating diseases, it is important to simultaneously pursue all lines of research. Science and scientists need to search for the very best sources of these cells. When they are identified, regardless of their sources, researchers will use them to pursue the development of new cell therapies.
The development of stem cell lines, both pluripotent and multipotent, that may produce many tissues of the human body is an important scientific breakthrough. It is not too unrealistic to say that this research has the potential to revolutionize the practice of medicine and improve the quality and length of life.