Types and Characteristics
(1) Prenatal Stem Cells: Embryonic & Fetal Stem Cells
In the NIH report previously cited, the authors offer the following definition:
"An embryonic stem cell is derived from a group of cells called the inner cell mass, which is part of the early (4 to 5 day old) embryo called the blastocyst. Once removed from the blastocyst, the cells of the inner cell mass can be cultured into embryonic stem cells." (From "Stem Cells: Scientific Progress and Future Research Directions," available at http://stemcells.nih.gov).
Beyond the 8th week of development, the embryo has matured into a fetus and the embryonic stem cells have matured into fetal stem cells, which exist until birth (after which time they are known as "adult" stem cells).
Embryonic and fetal stem cells are known to be pluripotent, which makes them attractive candidates for use in medical therapies. However, from their pluripotency they are also capable of forming teratomas (tumors). Additional risks involved in embryonic stem cell treatment shall be addressed in the following sections. (The reader is referred to "Advantages and Disadvantages: A Comparison").
In order to create an embryonic stem cell line, the destruction of the embryo is required. For this reason, the topic of embryonic stem cell therapy is often a sensitive one, with ethically and politically charged implications.
(2) Postnatal Stem Cells: Placental and Umbilical Cord Stem Cells
Placental stem cells are isolated from placentas, and umbilical cord stem cells are isolated from umbilical cords, at the time of a healthy birth. In both cases, the material from which the stem cells are derived would have otherwise been discarded. Unlike with embryonic stem cells, the destruction of an embryo is not involved in the gathering of placental and umbilical cord stem cells, yet the placental and umbilical cord stem cells exhibit the same pluripotency as do embryonic stem cells.
Animal and human studies have demonstrated that stem cells isolated from placentas and umbilical cords may differentiate into a variety of specialized tissue, such as nerve, liver, heart, kidney, bone marrow, and cartilage cells, among many other types. Furthermore, placental and umbilical cord stem cells are free of antigens, HLA or other cellular markers that could potentially be rejected by the body.
Although it was originally thought that only embryonic and fetal stem cells exhibit pluripotency and are free of antigens or HLA, it is now known that placental and umbilical cord stem cells also exhibit these same desirable characteristics. The additional fact that placentas and umbilical cords would normally be discarded after a healthy birth makes such stem cells readily, easily and inexpensively available, worldwide. This offers a further advantage since, unlike with embryonic stem cells, placental and umbilical stem cells are ethically and politically noncontroversial.
Umbilical cord stem cells have been shown to be effective in the treatment of stroke, spinal chord damage, heart damage, diabetes, lymphomas, multiple myelomas, leukemias, breast cancer, neural blastoma, renal cell carcinoma, ovarian cancer, and in the successful repair of a wide range of tissue injury and damage. Additionally, umbilical cord stem cells were used in the successful treatment of a woman who had been paralysed for 19 years. (Please see the section on "Successful Treatments").
(3) Adult Stem Cells
By definition, "adult" stem cells are present from the moment of birth, and thereafter throughout the remainder of an individual's lifetime. They are naturally present within the body and are constantly being utilized in the repair and healing of every wound and injury, as well as in the continual regeneration of every tissue. They are found at various locations throughout the entire body, such as, for example, in brain, bone marrow, blood, fat, skin, gut, skeletal, and other cells.
"Adult" stem cells are also sometimes called "somatic" stem cells, or "master" cells. The precise origin of adult stem cells in mature tissues is unknown, although bone marrow is suspected. Indeed, recent studies have found that even liver regeneration comes from bone marrow, which produces new (adult) stem cells capable of "homing in" on the liver. This phenomenon will be described in more detail later, as new discoveries in the versatility and "plasticity" of adult stem cells are continually being made.
In 2004, NIH allocated $ 190 million of its budget to adult stem cell research, and $ 25 million to embryonic stem cell research. The role of adult stem cells in medical science is well established.
"Adult" stem cell therapy is not new. Adult stem cells capable of forming blood, and derived from bone marrow, have been used in transplants for over 30 years. Prior to this, laboratory research on adult stem cells first began over 40 years ago. The first studies to be conducted on adult stem cells occurred in the 1960s, with bone marrow.
Although adult stem cells have been identified in many organs and tissues, there is a very small number of these adult stem cells which exist in any given location. Adult stem cells are thought to reside in a specific area of each tissue where they may remain quiescent (nondividing) for many years, until they are activated by disease or tissue injury.
Until recently, adult stem cells were believed to be either multipotent or monopotent, and this feature was seen as a major "drawback" to the use of adult stem cells in medical therapies. New discoveries, however, have demonstrated a highly versatile pluripotency to adult stem cells. These new data shall be addressed in more detail in the following sections.