What Are Stem Cells?Stem cells are unique cells of the body in that they are unspecialized and have the ability to develop into several different types of cells. They are different from specialized cells, such as heart or blood cells, in that they can replicate many times, for long periods of time. This ability is what is known as proliferation. Unlike other cells, stem cells also have the ability to differentiate or develop into specialized cells for specific organs or to develop into tissues.
In some tissues, such as muscle or brain tissue, stem cells can even regenerate to aid in the replacement of damaged cells. Stem cell research attempts to take advantage of the renewal properties of stem cells by utilizing them to generate cells for tissue repair and the treatment of disease.
Where Are Stem Cells Found?Stem cells come from several sources in the body. The names of the cells below indicate the sources from which they are derived.
Embryonic Stem Cells
These stem cells come from embryos in the early stages of development. They have the ability to differentiate into any type of cell in the initial stages of development and become slightly more specialized as they mature.
Fetal Stem Cells
These stem cells come from a fetus. At about nine weeks, a maturing embryo enters into the fetal stage of development. Fetal stem cells are found in fetal tissues, blood and bone marrow. They have the potential to develop into almost any type of cell.
Umbilical Cord Blood Stem Cells
These stem cells are derived from umbilical cord blood.
Umbilical cord stem cells are similar to those found in mature or adult stem cells. They are specialized cells that develop into specific types of cells.
Placental Stem Cells
These stem cells are contained within the placenta. Like cord blood stem cells, these cells are specialized cells that develop into specific types of cells. Placentas, however, contain several times more stem cells than do umbilical cords.
Adult Stem Cells
These stem cells are present in mature body tissues in infants, children, and adults. They may also be found in fetal and umbilical cord blood cells. Adult stem cells are specific to a particular tissue or organ and produce the cells within that particular tissue or organ. These stem cells help to maintain and repair organs and tissues throughout a person's life.
Types of Stem CellsStem cells can be categorized into five types based on their ability to differentiate or their potency. The stem cell types are as follows:
Totipotent Stem Cells
These stem cells have the ability to differentiate into any type of cell in the body. Totipotent stem cells develop during sexual reproduction when male and female gametes fuse during fertilization to form a zygote. The zygote is totipotent because its cells can become any type of cell and they have limitless replicative abilities.
As the zygote continues to divide and mature, its cells develop into more specialized cells called pluripotent stem cells.
Pluripotent Stem Cells
These stem cells have the ability to differentiate into several different types of cells. Specialization in pluripotent stem cells is minimal and therefore they can develop into almost any type of cell. Embryonic stem cells and fetal stem cells are two types of pluripotent cells.
Induced pluripotent stem cells (iPS cells) are genetically altered adult stem cells that are induced or prompted in a laboratory to take on the characteristics of embryonic stem cells. Although iPS cells behave like and express some of the same genes that are expressed normally in embryonic stem cells, they are not exact duplicates of embryonic stem cells.
Multipotent Stem Cells
These stem cells have the ability to differentiate into a limited number of specialized cell types. Multipotent stem cells typically develop into any cell of a particular group or type. For example, bone marrow stem cells can produce any type of blood cell.
However, bone marrow cells don't produce heart cells. Adult stem cells and umbilical cord stem cells are examples of multipotent cells.Mesenchymal stem cells are multipotent cells of bone marrow that have the ability to differentiate into several types of specialized cells related to, but not including, blood cells. These stem cells give rise to cells that form specialized connective tissues, as well as cells that support the formation of blood.
Oligopotent Stem Cells
These stem cells have the ability to differentiate into just a few types of cells. A lymphoid stem cell is an example of an oligopotent stem cell. This type of stem cell can not develop into any type of blood cell as bone marrow stem cells can. They only give rise to blood cells of the lymphatic system, such as T cells.
Unipotent Stem Cells
These stem cells have unlimited reproductive capabilities, but can only differentiate into a single type of cell or tissue. Unipotent stem cells are derived from multipotent stem cells and formed in adult tissue. Skin cells are one of the most prolific examples of unipotent stem cells. These cells must readily undergo cell division to replace damaged cells.
Sources: Stem Cell Basics: Introduction. In Stem Cell Information [World Wide Web site]. Bethesda, MD: National Institutes of Health, U.S. Department of Health and Human Services, 2002. Available at (http://stemcells.nih.gov/info/basics/pages/basics1.aspx)