![]() a muscle cell), it would not continue to be a zygotic nucleus in this new location. ![]() Conversely, if a nucleus from a bona fide zygote produced by fertilization was transferred to a differentiated cell (e.g. … 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. 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. Just as stem cells are not embryos, embryos are not eggs. These factors are uniquely produced by eggs, and are not manufactured by stem cells or even by embryos. Over fifty maternally derived factors that are necessary for embryonic development have been characterized in mammalian species. The maternal factors (proteins, protein complexes, RNA, and microRNA) provided by the egg are critical components of totipotency. 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. 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.” 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. Molecular differences of this magnitude are comparable to the differences between a stem cell and a skin cell. The differences detected between zygotes and later embryos are even greater. 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.įor example, one recent study identified over 800 genes that are expressed differently in mouse zygotes compared to mature mouse oocytes. Stem cells, (plenipotent, pluripotent, multipotent, or unipotent) exist in cellular states that are quite distinct from that of an embryo. 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. 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.Įmbryos Cannot Be Produced By Reprogramming 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. If transferred to a uterus, it will produce a tumor, not a baby. 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. 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. Totipotency in this strict sense is demonstrated by the ability of an isolated cell to produce a fertile, adult individual. to undergo a coordinated process of development. 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. The difference between these two definitions is not trivial. A zygote is “totipotent” in both senses, yet pluripotent stem cells are “totipotent” only in the second sense.Īs I explain in detail in a recent scientific review article of totipotency: 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. The term “totipotent” is used in the scientific literature in two radically different ways. Neither the concerns nor the exaggerated media claims are supported by the scientific evidence. 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. Recent developments in stem cell technology ( STAP cells and Totipotency factors) have raised concerns that reprogramming techniques could produce human embryos in the laboratory.
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