Pluripotency

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In cell biology, pluripotency (from the Latin plurimus, meaning very many, and potens, meaning having power) refers to a Stem Cell that has the potential to differentiate into any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous system).[1] [2] Thus, pluripotency is a description of the power or potency of a stem cell.

It is worth noting that the definition of pluripotent is somewhat vague. At the strictest version of the definition, a cell must contribute to every embryonic cell type. However, pluripotency can also refer to the abilty to form cells from each germ layer. [1] [3] However, in order to be considered pluripotent via every interpretation of the defintion, a cell must simply be able to differentiate into any embryonic cell type (as the embryo is comprised of the three germ layers). [3] This reflects how pluripotency is a continuum or spectrum, ranging from the completely pluripotent cell that can form every cell of the embryo proper to the incompletely or partially pluripotent cell that can form cells of all three germ layers but that may not exhibit all the characteristics of completely pluripotent cells.

Types of Pluripotent Cells

The most well-known type of pluripotent stem cell is Embryonic Stem Cell (ESCs). They can be found naturally in the Inner Cell Mass during development; in fact, they comprise the entire ICM of the blastula! [4] These cells differentiate to produce the entire resulting organism. In vivo, ESCs are only around for a temporary period of time: they eventually all differentiate into something else. [4]

Another type of pluripotent stem cell is [[Induced Pluripotent Stem Cell]s (iPSCs) They are formed through reprogramming; this traditionally occurs the exogenous addition of pluripotency-associated transcription factors. [5] It is important to note ESCs and iPSCs are not equivalent. They both are considered pluripotent, with studies even showing tetraploid complementation with iPSCs. [6][7][8] Despite these similarities, without successive reprogramming or passaging, there remains key differences in epigenetics and gene expression between the two cell populations. [5]

Other pluripotent stem cells, such as Muse Cells, have been reported. [9] However, most of the existence and pluripotent nature of these alternative cell populations is not widely accepted. This is often due to the lack of reproducibility and previous misconduct relating to STAP cells. [10]

Testing for Pluripotency

In order to claim a cell population is pluripotent, several experimental procedures are typically used. In general, a research group must prove the cell can contribute to all three germ layers or, more strictly, be able to produce every embryonic cell. [3] The following experiments are all frequently used to indicate whether a stem cell is pluripotent or not. [11] It is worth noting that the experiments to test pluripotency are often completed in this order. [12] [9] [13] [14] [15]. Furthermore, where applicable, embryonic stem cells are typically used as a base-line, as they are the most well-known and extensively studied pluripotent stem cell. [12] [15]

  • Gene Expression: The first test for an indication of pluripotency often examines the gene expression pattern of the cell population in question. There are several genes known to be associated with pluripotency, such as Nanog, Oct-4, Sox2, and Klf4. [12] [4] The presence of proteins and the expression of genes associated with pluripotency are often both approximated through methodologies like Western Blotting, Immunocytochemistry, and rt-PCR. [12] [9] The expression pattern in ESCs is often used as a benchmark, as it is known that these cells are pluripotent. It is important to note, however, that gene expression patterns do not prove pluripotency. It is a useful "first pass" to indicate pluripotency!
  • Cell Differentiation: An obvious requirement before a cell can be deemed pluripotent is that it is able to produce cell types of all three germ layers. A simple way to do this is to allow the cell population to differentiate rather than keeping it in stem cell culture conditions. Several exogenous factors, such as drugs, may be added to improve the efficiency for differentiating into a particular lineage. [16] For this experiment to indicate pluripotency, daughter cells should differentiate into two or more cell types from each germ layer.
  • Teratoma Formation: Teratomas and pluripotency are often considered to go hand-in-hand. [17] In fact, tumorgensis and teratoma formation are considered a "hallmark of pluripotency" [18] Teratomas are a useful test for pluripotency as these tumors are often uncontrolled or unorganized growths; as such, they often contain cells from all three germ layers. [18] In order to see if a cell population will form a teratoma, sample cells are injected into a mouse (often into the testes). [12]
  • Chimera Contribution: An even more comprehensive evidence of pluripotency is that of chimera formation. This is because, in order to contribute to a chimera, a cell must be able to differentiate into all three germ layers in a organized manner. During this experiment, the cell population in question is fluorescently tagged; this allows them to be differentiated from traditional cells. [11] A sample of these cells are then injected into a mouse embryo. If the embryo is viable and able to develop, the contribution of the fluorescently marked cells is observed. Pluripotent cells should contribute extensively throughout the embryo. [11]
  • Germ Line Contribution: The "gold standard" for pluripotency is that a cell is able to contribute to a chimera and become a part-of the germ line. In order to determine this, chimeric mouse are out-crossed. The resulting F1 progeny are examined to see if they contain any of the fluorescent marked cells. This is considered the "gold standard", as cells are set aside early during development for the formation of germ cells. [19] As such, in order to form every cell in the embryo, a pluripotent stem cell must be able to contribute to the germ line.
  • Tetraploid Complementation: The best test for pluripotency is Tetraploid Complementation. [4] The procedure is somewhat similar to chimera formation, as the cell population in question is injected into an embryo. However, the embryo is now only comprised of tetraploid cells; this means the egg initially had 2x the genetic material. [4][20] An embryo with twice the genetic material per cell will not be viable long-term; however, it will be able to form extra-embryonic and the trophectoderm cells. [20]. This means that when diploid cells are injected into the tetraploid embryo, the embryo itself will only be formed by the diploid injected cells. This is the best test for pluripotency, as it demonstrates that the cell in question can truly form every embryonic cell.

It is important to note that different experiments are utilized depending on the strictness of the definition of pluripotency. For instance, tetraploid complementation is necessary to truly indicate that a cell can form every embryonic cell (the strictest definition of pluripotency). However, teratoma formation may be sufficient for less-strict definitions of pluripotency.

See Also

Stem Cell

Cell potency

Embryonic Stem Cell

Induced Pluripotent Stem Cell

Citations

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