Confusions About Totipotency: Stem Cells Are Not Embryos

 
 

The ability to both produce all cell types and to organize them into a coherent body plan is the defining feature of a human organism. All stem cells lack essential elements supplied by the egg cell and cannot develop into a fetus.

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Recent developments in stem cell technology (STAP cells and Totipotency factors) have raised concerns that reprogramming techniques could produce human embryos in the laboratory. They have also raised a chorus in the scientific news media claiming that scientists have produced “totipotent stem cells” that could be used for human cloning. Neither the concerns nor the exaggerated media claims are supported by the scientific evidence.

What Is Totipotency?

The term “totipotent” is used in the scientific literature in two radically different ways. The strict sense of totipotency refers to a one-cell embryo or zygote that is “capable of developing into a complete organism.” The second, weaker sense of totipotency refers to the ability of a cell to differentiate into any of the cells or tissues of the body, including cells of the placenta. A zygote is “totipotent” in both senses, yet pluripotent stem cells are “totipotent” only in the second sense.

As I explain in detail in a recent scientific review article of totipotency:

The difference between these two definitions is not trivial. Producing a mature organism requires the ability to both generate all the cells of the body and to organize them in a specific temporal and spatial sequence; i.e. to undergo a coordinated process of development. Totipotency in this strict sense is demonstrated by the ability of an isolated cell to produce a fertile, adult individual. Consequently, a cell that is totipotent is also a one-cell embryo; i.e., a cell that is capable of generating a globally coordinated developmental sequence.

In contrast, a cell that can produce but not organize all of the cells of the body (including cells found in the placenta) is not an embryo. If transferred to a uterus, it will produce a tumor, not a baby. Such cells are sometimes referred to as “totipotent” in the second, weak or cellular sense, a misuse of the term that causes great confusion for many non-scientific readers. I have proposed the term “plenipotent” (from the Latin plenus, or “full”) to distinguish such cells from pluripotent stem cells (i.e., stem cells that do not produce placental tissue) and to avoid confusing mere tumors with actual, totipotent embryos.

Embryos Cannot Be Produced By Reprogramming

The key point is this: although stem cells can vary in their ability to produce some or all of the cell types found in the body, in all cases, stem cells are not embryos. Stem cells, (plenipotent, pluripotent, multipotent, or unipotent) exist in cellular states that are quite distinct from that of an embryo. Recent work has shown that in normal development, oocytes, zygotes, and later embryos (i.e., the stages from which stem cells are derived) are all very different from each other.

For example, one recent study identified over 800 genes that are expressed differently in mouse zygotes compared to mature mouse oocytes. The differences detected between zygotes and later embryos are even greater. Molecular differences of this magnitude are comparable to the differences between a stem cell and a skin cell. Similarly, extensive analysis of the molecular state of stem cells and embryos has conclusively demonstrated that over 200 genes are expressed differently between the two. Indeed, the authors of one study flatly stated: “We conclude that the protein expression patterns of markers that define stemness in ESC do not identify the totipotent cells in human preimplantation embryos.”

Stem cells are not only distinct from embryos on a molecular level, stem cells (regardless of how they are produced) cannot be embryos, because they lack required elements of totipotency that are normally provided to the embryo by the egg. The maternal factors (proteins, protein complexes, RNA, and microRNA) provided by the egg are critical components of totipotency. Over fifty maternally derived factors that are necessary for embryonic development have been characterized in mammalian species. These factors are uniquely produced by eggs, and are not manufactured by stem cells or even by embryos. Just as stem cells are not embryos, embryos are not eggs. Once a cell has entered into a state similar to a zygote, it does not produce the egg-specific components that are absolutely required for an embryo to exist.

Thus, as noted in my recent review,

… even if the nucleus of a somatic cell is fully reprogrammed to be identical in every respect to that of a zygote (a state that is distinct from a pluripotent stem cell and one that would require different reprogramming factors), it would still not be totipotent, because it lacks the non-genetic factors … that are critical components of totipotency. Conversely, if a nucleus from a bona fide zygote produced by fertilization was transferred to a differentiated cell (e.g. a muscle cell), it would not continue to be a zygotic nucleus in this new location. It would be (or would rapidly become) a muscle cell nucleus that functions in the manner specified by the muscle cell cytoplasm.

Consequently, mere nuclear reprogramming, regardless of the method by which it is accomplished, categorically cannot produce an embryo because it does not provide the many maternal factors required for a cell to be totipotent. There are many types of plenipotent stem cells, but there are no totipotent stem cells. Stem cells are not embryos.

All "Pluripotent" Stem Cells Produce Placental Tissue

In addition to the fact that stem cells are distinct from embryos on a molecular level and that they manifestly lack the egg-derived cytoplasmic factors that are critical components of totipotency, stem cells have repeatedly been shown to behave like tumors, not like organisms, both in the laboratory and when transferred to a host animal. This is true despite the fact that most stem cells exhibit a wide range of potencies that are (at times) mischaracterized by both the popular media and by scientists.

For example, it is often asserted that mouse and human embryonic stem cells (ESCs) do not produce placental tissue. Yet experiments performed in the late 1980s unequivocally demonstrate that mouse ESCs (mESCs) contribute to all of the tissues of a developing mouse, including “extraembryonic” tissues such as the placenta. These contributions are only seen in 25 percent of the animals examined and involve a relatively small number of cells compared to the large contributions mESCs make to tissues of the postnatal body, but they clearly demonstrate mESCs have a broader range of potency than is typically acknowledged. In a form of scientific “shorthand,” mESCs are sometimes said to “not produce” placental tissue, but like many things in science, this is merely a matter of relative degree: mouse ESCs simply contribute less to the placenta than they do to postnatal tissues. Multiple recent studies have confirmed that subpopulations of mESCs (see Morgani, Canham, and Macfarlan) or—under specific culture conditions—the great majority of these cells (Hayashi, Mfopou) are able to produce placental tissue and are therefore plenipotent.

Similarly, the paper reporting the initial isolation of human ESCs (hESCs) clearly concluded these cells are also capable of producing placental tissue. For ethical reasons, the potency of hESCs has not been tested by injection into human embryos, yet their capabilities in the laboratory suggest they would have a somewhat broader potency than mESCs if tested in this way. Thus, both hESCs and mESCs are able to produce placental tissue, yet neither of these cell types is totipotent. It has been demonstrated by literally thousands of papers over the last fifteen years that clusters of stem cells (both human and mouse) do not make babies; they make tumors.

Just as for stem cells isolated from embryos, stem cells produced by reprogramming (induced pluripotent stem cells; iPSCs) are also able to make placental tissue and are therefore best described as plenipotent. Both human and mouse iPSCs produce placental tissue under specific conditions in culture. Moreover, mouse iPSCs make significant contributions to placenta when injected into developing embryos, an observation that led the authors to speculate these cells had “totipotency features.” Yet the use of the term “totipotent” in this context does not indicate these cells are embryos.  Reprogrammed iPS cells are clearly plenipotent, but they are not embryos because they are not capable of undergoing development on their own.

Finally, a recent report by Obokata demonstrating a new technique for generating pluripotent stem cells (“stimulus triggered acquisition of pluripotency” or STAP cells) has raised concerns because STAP cells also demonstrate a wide range of potency and are able to contribute to the placenta in 60 percent of animals examined. While mouse STAP cells injected into developing mice appear to contribute more readily to placenta than do mESC (and are therefore likely to be more similar to hESCs and iPSCs), all four cell types have similar potency in most respects. And, as over three decades of research on mESCs has clearly demonstrated, clusters of stem cells do not undergo development and do not produce babies, either in the laboratory or when transferred to a host animal. Regardless of the level of potency stem cells exhibit or how efficiently they contribute to the placenta when injected into an existing embryo, stem cells do not possess the factors required for totipotency that are uniquely provided by the oocyte, and therefore, stem cells are not embryos.

Conclusion

Totipotent zygotes are distinct from pluripotent or even plenipotent stem cells because they undergo development. The ability to both produce all cell types and to organize them into a coherent body plan is the defining feature of a totipotent human organism. And totipotency critically requires multiple factors derived from oocytes. Human embryos can be produced by fertilization or by cloning, yet both of these procedures start with and require the cytoplasmic factors contributed by a human egg. All other scientific techniques that result in embryos containing mixtures of stem cells start with an embryo produced by either fertilization or by cloning.

Although future developments may discover new ways of producing stem cells with expanded potency, unless the technique explicitly utilizes human eggs or painstakingly supplies the multiple egg-derived factors that are currently known to be required for development, such cells will not be totipotent and will not be human embryos.

Maureen L. Condic is Associate Professor of Neurobiology and Anatomy at the University of Utah School of Medicine.

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