Stem cells have the potential to develop into many other types of cells. Some of them are found in tissues and serve as their internal repair system. Stem cells have the capability to divide without limit. All stem cells are characterised by two characteristics: unspecialized cells capable of renewal and ability to become induced to tissue- or organ-specific cells. Scientists have also been able to show that some stem cells from one tissue have the ability to create cells from another tissue, known as plasticity. However, it is still under great debate and has only been shown in vitro.
With regards to their unspecialized characteristic, stem cells have the ability to proliferate many times unlike other cells. These cells do not contain any tissue-specific structures that force it to have specialized functions; however, they do have the ability to produce specialized cells, a process called differentiation.
There are three types of stem cells which include embryonic stem cells, adult stem cells, and induced pluripotent stem cells.
Embryonic stem cells are derived from four to five day old embryos, or fertilized eggs. This structure is also known as a blastocyst and can be divided into the trophoblast, blastocoel, and inner cell mass. The inner cell mass is where the stem cells are located. To access the stem cells, the scientist would need to destroy the embryo and transfer the inner cell mass into a culture medium. Embryonic stem cells have the ability to differentiate into any type of cell in the body, thus they are pluripotent cells. Additionally, they are very easily grown in culture.
An adult stem cell is an undifferentiated cell that is found in a specific tissue among other differentiated cells. These stem cells can create specialized cells types of the tissue in which they reside. There is a very small number of adult stem cells that are located in the stem cell niche where they remain quiescent for years until they are needed to divide. "These cells have a limited capacity for proliferation, thus making it difficult to generate large quantities of these cells in the laboratory." Adult stem cells are primary used for repair and maintenance mechanisms of the cell. Currently, adult stem cells have been reported in the brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, intestine, teeth, heart, gut, liver, ovarian epithelium, and testes. Adult stem cells are limited to the cell types of their tissue of origin. Since there are very few of them in the stem cell niche, they are relatively rare, harder to isolate and they have a limited ability to proliferate which greatly reduces the ability to effectively produce a large number of cells needed for stem cell replacement therapies.
Finally, induced pluripotent cells (iPS) are "adult cells that have been genetically reprogrammed to an embryonic stem cell-like state by being forced to express genes and factors important for maintaining the defining properties of embryonic stem cells." The cells clinical ability compared to embryonic stem cells is currently unknown and needs more research. With research done from mouse iPS cells, they have shown characteristics of pluripotent stem cells such as their ability to express stem cell markers, form tumors from all three germ layers, and contribute to make different tissues.
Stem cells have many different potential uses. They gave scientists more information about the complexity of development among humans. Scientists now know that the key to identifying how undifferentiated stem cells become differentiated is through the process of turning genes on and off. Additionally, most of the serious medical concerns today such as cancer and birth defects are related to cell division and differentiation. Stem cells would help scientists have a better grasp as to what is happening and what type of treatment may be useful with the help of stem cells. Stem cells are also important to test new drugs. They would allow scientists to test the drugs on the cells rather then a person. Finally, human stem cells could be used for cell-based therapies such as transplants. Now days, there are a limited number of available organs for the numerous people that need organ transplants. Stem cells would allow scientists to grow cells to repair the damaged organs. Since the scientists would be using the individuals own stem cells, then the risk of rejection is eliminated since it is their own cells in the first place.
Here is a basic diagram to show the differences of how embryonic and adult stem cells are created. It also shows the other forms of stem cells but unfortunately, iPS cells are not included.
This website gives a nice interactive introduction to the different types of stem cells.
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