Source: The New Atlantis » Appendix A - The Science of Embryonic Stem Cell Research

2012

This section is much more positive toward ACT's Technology!

How Embryonic Stem Cells Are Obtained

The extraction by scientists of cells from the developing embryo — a process that destroys the source embryo — is typically carried out as follows: An embryo four to five days old is immersed in a chemical solution that dissolves and destroys its trophoblast cells, which allows for the cells of the ICM, called blastomeres, to be extracted.[28] These cells can then be placed in specialized culture conditions designed to enable them to grow as colonies of stem cells.[29] The chains of cultured embryonic stem cells and their progeny are referred to as embryonic stem cell lines.

Scientists generally employ three tests to assess the pluripotency of stem cells. The stem cells can be injected into an animal with a compromised immune system in order to see if they develop into teratomas, a special type of relatively benign tumor consisting of cells from all three germ layers of the embryonic body.[30] Because the different germ layers represent distinct developmental paths, the ability of a cell to differentiate into cells from each of the three layers indicates its ability to form all the cell types of the body, even if the teratoma does not consist of each and every cell type in the body.[31] A second test of pluripotency is the ability of the stem cell to contribute to the development of a chimera — an organism with some cells that are genetically distinct from the rest of the organism.[32] In this test, the stem cells are injected into an early embryo, where they contribute to the development of the fetus and adult organism, resulting in a chimera in which cells originating from the stem cells are found in all of the tissue types in the adult organism’s body.[33] In the third test of pluripotency, stem cells are injected into an embryo that has been modified so as to make it capable of developing into placental tissues but not the cells of the embryo itself. When the stem cells are added to this special embryo — called a “tetraploid” embryo because the procedure for creating it involves fusing the two cells of the early embryo, resulting in a cell with four sets of chromosomes — the ability of the stem cells to develop into all of the different cell types of the embryo complements the ability of the tetraploid embryo to develop into the tissues of the placenta, thus allowing for normal embryonic development.[34] This procedure, called the “tetraploid complementation assay,” is the most stringent test of pluripotency because it creates an organism that is entirely derived from the stem cells used in the procedure. (It is worth noting that, although scientists use all three of these tests in researching animal stem cells, they do not use the chimera formation test or the tetraploid complementation assay on human stem cells. For ethical and practical reasons, they rely only on the teratoma formation test, in which human embryonic stem cells are injected into immune-compromised mice.[35])

While research on adult stem cells can be traced back decades — indeed, hematopoietic stem cell transplants have been used to treat persons suffering from bone marrow diseases, including cancer, since the 1950s[36] — the key breakthrough for human ES cell research was achieved in 1998 when University of Wisconsin researcher James Thomson announced that he had derived ES cells from human embryos.[37]

Two related issues at this point are of interest because of the ethical questions to which they give rise. The first concerns the origin of the embryos from which ES cells are derived. The most practicable source of ES cells is embryos that have been created in fertility clinics through IVF but are “left over” from attempts to aid infertile couples in conceiving. In IVF, a sperm and an egg cell (oocyte) are joined in a petri dish. The resulting embryo is then allowed to grow for several days before it is either implanted in the woman or, if it is not to be used immediately, frozen and stored.[38] An American IVF clinic will typically produce more embryos than are used in each cycle of treatment, in order to have additional embryos available in case some turn out to be unusable or the implantation is unsuccessful. Therefore, some embryos usually remain after an IVF cycle has been successfully initiated; currently, there are several hundred thousand of such “spare” embryos frozen in IVF clinics in the United States.[39] The parents of these embryos may choose to donate them to be used in research if they do not wish to use them in a future IVF cycle.[40] Many supporters of ES cell research see these embryos as the most promising source of ES cells. However, as enticing as this sitting stockpile may be to interested researchers, most of the stored embryos have not been designated by the parents for research; they may be unsure if they wish to try to conceive again in the future, or may be uncomfortable donating their embryos to research for other reasons.[41] Further, even when the parents do consent, there are various logistical barriers to using these embryos for research, including possible degradations experienced in long storage, the hazards of transportation from clinic to laboratory, and reduced viability to begin with (the fertility clinicians will have selected the strongest-seeming embryos for the first round of implantation).

The same IVF procedure of creating embryos for fertility treatment could also be used to create embryos specifically for research purposes.

Another source of embryonic stem cells involves the process known as somatic cell nuclear transfer (SCNT), a kind of cloning. In this approach, which will be discussed further below, an enucleated oocyte (that is, an egg whose nucleus has been removed) is fused with the nucleus of a somatic cell (a cell containing the full complement of genetic material, unlike a gamete cell such as a sperm or egg, which contains only half). The oocyte “reprograms” the nucleus back to a totipotent (undifferentiated) state. This one-celled organism, which is genetically almost identical to the organism that provided the somatic cell, is now effectively a new embryo, and it begins the process of cellular division and growth. The embryo could be implanted in a womb; this is how Dolly, the cloned sheep, was created.[42] Or the embryo could be used as a source of ES cells.[43]

It is worth noting that the cloned embryo and the ES cells that result from SCNT are usually not completely genetically identical to the original somatic cell and the organism that provided it. The DNA in the new cells’ nuclei would be identical to that in the original cells’ nuclei. But DNA is also present outside the nucleus, in the mitochondria. Except in cases where a female provides both the eggs and the somatic cell for the SCNT procedure, the mitochondrial DNA of the egg used in the SCNT process will be different from that of the donor cell, possibly leading to mitochondrial disorders, which have been observed in cloned mammals.[44]

An alternative version of SCNT that would not require the procurement of egg cells from women is called interspecies SCNT (iSCNT). In this process, the nucleus of a human somatic cell is transplanted into an enucleated animal oocyte in order to produce embryonic stem cells. Because the nucleus of the animal oocyte has been removed, most of the DNA in the resulting embryo will be human, although the small amounts of mitochondrial DNA present in the cytoplasm of the animal oocyte will be present in the resulting embryo. The organisms created via iSCNT have been dubbed “cybrids” — cytoplasmic hybrids — since they have human DNA placed in the cytoplasm of an animal oocyte. While this technique has been successfully used to clone certain mammals of species that were closely related to one another,[45] attempts to perform iSCNT with human nuclei have been so far unsuccessful. Some scientists have expressed doubts about whether iSCNT can work in humans at all, since SCNT relies on the ability of the oocyte to “reprogram” the genome of the nucleus into an embryonic state, but the somatic cell nucleus must be compatible with the oocyte in order for this “reprogramming” to be successful.[46]

Three other procedures also can, in practice or in theory, produce human embryonic stem cells. First, it is possible to reprogram somatic cells to a pluripotent state by fusing them with existing ES cells.[47] Second, blastomeres can be extracted from living embryos without destroying the embryos. This kind of blastomere extraction is already done now in a practice called preimplantation genetic diagnosis (PGD), which is used by IVF clinics to screen embryos before they are implanted. Blastomere extraction apparently does not always significantly interrupt the embryo’s biological functioning, although some embryos are evidently lost as a result of this process, as the rate of successful pregnancies following PGD is lower than with other assisted reproduction technologies, and there is evidence that twins or triplets born following PGD have increased rates of birth defects and infant mortality.[48] Third, dead embryos maintained in culture often contain living cells, which might also provide a source of ES cells in the strict sense.[49]

These latter two procedures highlight the second important issue surrounding embryonic stem cells, namely, the consequences for the embryo of ES cell extraction. When blastomeres are extracted from an IVF embryo or an SCNT embryo by dissolving the trophoblast, the resulting stem cells have been obtained at the cost of the embryo’s life. By contrast, when blastomeres are extracted from living embryos without dissolving the trophoblast, or when blastomeres are extracted from dead embryos, the resulting stem cells will not have been obtained by destroying embryos. Although the long-term medical consequences to a living embryo brought to term after blastomere extraction are not yet known, these techniques suggest the possibility of attaining human embryonic stem cells without the destruction of living human embryos.