During your most recent biology class, you may have heard the term 'stem cells,' although the words themselves may sound foreign to untrained ears. Stem cells play an integral role in the past, present, and future of medicine and biochemical research.
To understand their role within our bodies and beyond, we must start with an important question: What are stem cells?
By the most standard definition, stem cells are unspecialised cells within the body. If the human body is compared to that of an industrial system, each individual cell within the system contributes by completing a particular role. For example, manufacturing a product, or dealing with the 'business' aspect of the industrial process. Stem cells, however, differ. As aforementioned, stem cells are unspecialised, in the context of the industrial process they are at the stage where they can choose what 'role' they want to have in the system -- or the human body but differentiating into a particular cell.
The image above demonstrated how these undifferentiated cells become specialised into an assortment of cells.
The scope of this cell allows for tissue rejuvenation, essentially permitting self-renewal of the body. There are varying forms of stem cells, such as totipotent, pluripotent, multipotent, unipotent, and more. Each of these types has an implication on the ability of the stream cell to divide and differentiate into the assortment of cells as described above.
The table inserted below demonstrates the difference between the stem cell potencies:
Essentially, unipotent cells aren't able to differentiate as much as pluripotent cells, as developmental potency is reduced at each step of cell differentiation, making it harder for the cell to retain its ability to completely divide and differentiate.
The stem cell with the greatest ability to divide and differentiate is the totipotent cells. These totipotent cells are often formed due to the fusion of an egg and a sperm cell, allowing them to differentiate not only into an embryo and placenta but also an assortment of cells within the human body - allowing them to form a viable organism. The derivation of stem cells can be in places such as the embryo and the human bone marrow. Once extracted, these stem cells can develop from the layers of the placenta, a few days later (typically 4) the inner cell mass becomes increasingly pluripotent due to the specialisation of the totipotent cells, allowing pluripotent stem cells, which have the ability to differentiate into all cells within the human body, to form.
Pluripotent stem cells differ from totipotent stem cells in their inability to form either the embryo or the placenta, as they only retain the ability to form cells within the human body. Once pluripotent stem cells have been derived, their ability, like totipotent cells, to remain undifferentiated decreased. This gives rise to other forms of stem cells such as unipotent or oligopotent stem cells (these are extremely limited in their ability to differentiate).
The ability of stem cells to remain undifferentiated over a period of time is due to the unwinding of their DNA by a cluster of proteins during cell division. When they must undergo cell division, this is completed through asymmetric cell division. Asymmetric cell division occurs when the Stem Cells divide and give rise to two daughter cells with different cellular fates - this is the future identity the cell will take in the human body. It is important that the stem cells retain this ability to remain undifferentiated for a period of time, as this allows them to turn into specific cells.
To learn more about stem cells (the basics) read: Stem cells, an Insider's Guide by Paul Knoepfler.
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Zakrzewski, Wojciech, et al. “Stem Cells: Past, Present, and Future.” Stem Cell Research & Therapy, BioMed Central, 26 Feb. 2019, https://stemcellres.biomedcentral.com/articles/10.1186/s13287-019-1165-5.