Course: SAT 170 Introductory Biotechnology

Unit 8 Learning Module

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Stem Cells

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Stem cells are those which are capable of creating more stem cells and also differentiating into different cell types. These cells are either classified as pluripotent or totipotent. As mentioned previously, totipotency is exhibited by cells that are capable of differentiating into all cell types of a particular organism. Pluripotent cells have more limited capabilities and are able to differentiate into some but not all cell types.

In general, embryonic stem (ES) cells which are collected from the morula stage of an embryo are known to be totipotent. Shortly after fertilization where a sperm and egg fuse, the resulting zygote undergoes many rounds of cell division to produce this morula, which subsequently develops into the blastocyst. Within the blastocyst is a collection of cells that will develop into the fetus known as the inner cell mass. This is the source most ES cells. The collection of these cells results in destruction of the embryo and is thus an ethical concern for researchers. These ES cells, however, have the ability to develop into any and all the cells that the developing fetus will need. Once these cells begin to differentiate, they lose some of their plasticity and become pluripotent. These cells are present in the various tissue, organs and organ systems of an individual to provide a mechanism for growth and repair of that tissue, organ, or organ system. As stem cells become more and more differentiated, they begin to lose their potency. Often AS cells are referred to as being multipotent (can develop into many cell types), oligopotent (can develop into a few different cell types) or bipotent and unipotent (can develop into two or one cell type, respectively). This limited differentiation combined with the fact that they are often found in relatively low numbers within the organs and tissues of the adult body which makes them difficult to isolate are major disadvantages to AS cells. The primary advantage to AS cells from a therapeutic perspective is that there is no risk of immune rejection if a patient is treated with their own stem cells. Figure 1: This diagram illustrates the different type of embryonic stem cells which will eventually give rise to the stem cells present in adult tissues (known as adult stem cells).

Another option for obtaining stem cells is to produce Induced pluripotent stem (iPS) cells. These iPS cells are cells that have been collected and induced to go through the process of dedifferentiation by reversing the modifications to the DNA that occur during the differentiation process. Although they are not totipotent, the use of these cells is highly advantageous because they overcome the ethical concerns surrounding ES cells and the limitations of the potential of AS cells.While there are many things that can be accomplished with the use of stem cells, the ability to grow tissues, and potentially organs, for transplants is one of the most important.  We will explore this and several other functions in future units. Figure 2: The reprogramming of differentiated cells (shown in the petri dish in step 1) involves using a virus (shown as a purple circle shaped structure with a green hexagon structure containing certain fragments of DNA which are labeled red) which will deliver genes into the differentiated cells to undo the epigenetic changes that led to that cells differentiation. The cells that were successfully altered with the virus can then be transferred to new media (steps 2 and 3) until the cells have regained their potency (step 4). Figure 3: There are a variety of applications of stem cells, many of which we will discuss in unit 11 because of their relevance to the medical field.

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