Stem cells may be found in all animals from early stages of development as an embryo until the end of life. There are several types of stem cells depending on their source and properties.
Embryonic stem cells
Embryonic stem cells can be derived from the blastocyst stage of an embryo. These are also called pluripotent cells as they have the capacity to produce all of the body’s cell types.
The blastocyst is a mostly hollow sphere of cells smaller that a pin head. In its interior is the inner cell mass with around 30 cells. These cells are cultured in the laboratory to yield millions of cells.
The embryos used for obtaining these stem cells are obtained from the excess embryos that couples willingly donate after informed consent and after they have successfully had their offspring. Embryos that have been fertilized within a woman’s body are not used.
Embryonic stem cells are more flexible and can be made into any type of cell that is desired. They are generally easier to collect, purify and maintain in the laboratory than adult stem cells. These cells, however, need to be differentiated into specialized cells before they can be transplanted or else they may lead to tumors called teratomas.
Adult stem cells
Adult stem cells are found in certain tissues in fully developed humans. This could be present in babies, children, adolescents or adults. These stem cells are limited to producing only certain types of specialized cells.
The primary roles of adult stem cells in the body are to maintain and repair the tissues in which they are found. These stem cells are also called somatic stem cells instead of adult stem cells as they may be found in persons of all ages (not just adults).
Research with adult stem cells began in the 1960’s when scientists found that the bone marrow contains at least two kinds of stem cells – the blood cell forming hematopoietic stem cells and the bone marrow stromal cells that are a cell population that generates bone, cartilage, fat, and fibrous connective tissue.
Adult stem cells are also found in brain, peripheral blood, blood vessels, skeletal muscle, skin and liver. There is a very small number of adult stem cells at these sites.
Amniotic Stem Cells
These are stem cells found in the amniotic fluid. These stem cells are very active and can proliferate without feeders. In addition, unlike embryonic stem cells they do not cause tumors. These can be made into fat cells, bone cells, muscle cells, blood vessel walls, liver and nerve cells.
Induced Pluripotent Stem Cells
These are not adult stem cells but are created from adult skin cells after genetically programming them to become pluripotent stem cells.