In the first lesson, we reviewed what makes a cell and the different parts that all eukaryotic cells have. In this lesson, we are going to define what a stem cell is and explain the 3 defining characteristics of stem cells.

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Fundamentally, stem cells are eukaryotic cells that are undifferentiated, meaning that they have not developed into a specific cell type yet. Again, there are many different types of cells that make up an organism, from skin cells to muscle cells to neurons; stem cells don’t have a definite type yet. However, because stem cells are undifferentiated, they have the potential to become a specific cell type when ready. Thus, when a stem cell divides, each new cell has the potential to stay a stem cell— or to become a specific cell type. These characteristics of being in an undifferentiated state and being able to divide into other cell types are unique to stem cells; other types of cells located in the body (also known as somatic cells) do not have these capabilities.

 

The three defining characteristics of a stem cell are plasticity, differentiation, and self-renewal. The first defining characteristic is plasticity. Plasticity means that the stem cell is flexible and can develop into another cell type; this allows different types of stem cells to have different levels of potency, or different capacities to develop into new cells. Multipotent stem cells are stem cells that can differentiate into a few select types of cells. For example, a stem cell that is only able to differentiate into somatic cells associated with the central nervous system is multipotent. Pluripotent stem cells are stem cells that can differentiate into all types of cells found in the adult body, or all the types of somatic cells. Totipotent stem cells are stem cells that are pluripotent, but they also have the capability to differentiate into cell types found during the early stages of development of an organism.

 

The second and third defining characteristics of a stem cell are differentiation and self-renewal. Differentiation is the unique process through which a stem cell can specialize and become another cell type. Self-renewal is the capability to divide in a way that the cell renews itself and does not develop into another cell type. When it is time for a stem cell to divide, there are two ways in which the cell can divide: symmetrically or asymmetrically. Symmetric division is when the two resulting cells are the same type of cell. For example, a stem cell can self-renew in this manner and create two new stem cells. Asymmetric division is when the two resulting cells are different types of cells. For example, a stem cell can differentiate and self-renew by asymmetric division, resulting in a progenitor cell and a stem cell. A progenitor cell is a type of cell that has lost the comprehensive potency of the original stem cell and can only divide to eventually become a certain type of somatic cell.

 

 

 

 

 

 

 

 

 

 

 

 

 

There are three major types of stem cells: embryonic, adult, and induced pluripotent stem cells. Embryonic stem cells are pluripotent and immortal, meaning that they can develop into all types of somatic cells and have the capability to self-renew indefinitely. Embryonic stem cells come from the inner cell mass, which is a mass of cells found in the blastocyst stage of a developing embryo about 4-5 days into development. The other type of stem cell that is found in nature are adult stem cells. Adult stem cells are multipotent and mortal, meaning that they can only develop into a specific set of somatic cells and have a limited amount of times they are able to self-renew. There are different types of adult stem cells, meaning that, cumulatively, all the different types of somatic cells are derived from a specific adult stem cell. Their main function in nature is to regenerate somatic cells when somatic cells die. For example, when someone gets a papercut, skin adult stem cells are activated to replace the skin cells that died as a result of the cut. Finally, the third type of stem cell is the induced pluripotent stem cell (iPS cell). iPS cells are not found in nature; instead, they are made by scientists in a laboratory setting. iPS cells are pluripotent and immortal like embryonic stem cells, so they can self-renew indefinitely and develop into all types of somatic cells as well. We will go further in depth about each type of stem cell in the following lessons.

 

So why do we even care about stem cells? Due to their unique qualities, they have a lot of potential applications in the field of medicine and can be used to save lives. Researchers have figured out that stem cells can help regenerate tissue. For example, stem cells have the potential to be used to help repair heart muscle after a heart attack and can help regenerate neurons lost in those who have developed Alzheimer’s disease. Additionally, stem cells can be used to potentially grow whole organs outside the body and to therefore eliminate the need for organ donors. Stem cells could also be used to model diseases outside of the body so that scientists can learn more about the disease and perform experiments without performing a procedure on the patient. Thus, we care about stem cells because there are many potential applications for them that will help make people's’ lives better.