Types of Stem Cells
Several terms have been employed to describe different existing stem cell types. These will often depend on the cell source or on its stage of development. It is likely that you may have heard of one of the following terms:
1. Adult stem cells or tissue-specific stem cells
Many adult tissues possess stem cells that can replace dead cells, or repair damaged tissue. Skin, muscle, gut and bone marrow all have their own stem cells. In bone marrow, millions of new blood cells are produced daily, derived from blood producing stem cells.
Adult stem cells are tissue-specific, which means that they are found in certain body tissues where they generate mature cells for that tissue or organ. It is not clear whether all organs, the heart for example, have stem cells. The term “adult stem cell” is widely used and may refer to fetal stem cells or cord blood cells.
Some stem cell therapies are well accepted by the medical community and employ tissue-specific stem cells. This is the case of bone marrow stem cell transplants or cord blood used either to treat blood disorders, or for the blood system to recover after anticancer therapies; it also includes stem cells used to treat skin after severe burns or corneal limbal cell transplant for the eye. In each of these cases, stem cells repair the tissue they originate from.
Another type of stem cell is the mesenchymal cell. It is found in a series of different tissue types including bone marrow, among others, and can generate bone, cartilage and fat tissue. These are cells which may also contribute to tissue regeneration. Several animal studies are currently being conducted to establish whether they can be used for the treatment of conditions such as arthritis or non-consolidation of bone fractures. Furthermore, these or other cells may also modulate the immune system in response to injury.
2. Fetal Stem Cells
As their name suggests, fetal stem cells are obtained from the fetus. After approximately 10 weeks gestation, the developing baby is called a fetus. Most fetal tissues contain stem cells which drive rapid organ growth and development. Just as for adult stem cells, fetal stem cells are in general specific for each type of tissue, and generate the same mature cell type as the one in the tissue or organ they reside in.
3. Umbilical cord stem cells
At birth, blood in the umbilical cord contains a great number of blood-forming stem cells. Clinical applications of cord blood cells are similar to those of bone marrow stem cells, they are currently used to treat blood disorders or for the blood system to recover after anticancer therapies. Just like adult bone marrow stem cells, cord stem cells are tissue specific.
4. Embryonic Stem Cells
Embryonic stem cells derive from early stage embryos and can, in theory, give rise to any cell type in the body. However, to successfully transform these cells into specific cell types in the lab is a complex task. Embryonic stem cells may also undergo malignant transformation into cancer cells after transplant. In order to employ them in cell transplantation, these cells should probably target a more mature cell type, both to achieve effective treatment as well as to decrease potential cancer risk. Although it is a fact that these cells are helping us to better understand diseases and look very promising for future therapies, there are currently no embryonic stem cell treatments accepted for use by the medical community.
5. Reprogrammed cells or iPSCs (induced pluripotent stem cells)
In 2006, scientists discovered how to “reprogram” in the lab, cells with specific functions (skin cells for example), in such a way that they behaved like embryonic stem cells. These cells, known as induced pluripotent stem cells (iPSC) are created by inducing specialized cells to express genes which normally originate in embryonic stem cells and control cell function. Embryonic stem cells and iPSCs share many common traits, among them, the ability to develop into cells of any organ or tissue. Nevertheless, they are not identical, and on occasion display slightly different behavior. iPSCs constitute a powerful tool to generate specific patient or disease-related cell lines for research purposes. However, techniques used to create them must be carefully perfected before they can be used to develop iPSCs, that are adequate for safe and efficient treatments.
Reprogramming procedures resulting from the introduction of 4 genes into adult cells were first described in 2006 by Dr. Shinya Yamanak from Japan (in rodent cells) and in 2007 (in human cells), research for which he was awarded the Nobel Prize for Medicine in 2012.