Scientific References: totipotency, twinning

article comments Irving
copyright May 2, 2003
Reproduced with Permission

Scientific References

Germ Line Cells Are Totipotent:

... Primordial germ cells (PGC), or gonocytes, are generally believed to be both extragonadal and extraembryonic in origin. They are difficult to recognize in very young human embryos. Claims for them have been made as early as in the blastocyst, and they are believed to be segregated at latest by 2 1/2 weeks and possibly much earlier. At 4 weeks they can be identified in the umbilical vesicle (yolk sac) and hindgut. A week or so later, they have migrated to the gonads. When the primordial germ cells have settled in the gonad, they become more spherical, stain less intensely with alkaline phosphatase, undergo mitosis, and are then referred to as oogonia or spermatogonia, depending on whether they are situated in an ovary or a testis.. ... The unifying feature in the formation of primordial germ cells would seem to be the exemption of those cells from the processes of regional, somatic differentiation. . (pp., 23-24)

... Cells differentiate by the switching off of large portions of their genome. Future somatic cells thereby lose their totipotency and are liable to senescence, whereas germ cells regain their totipotency after meiosis and fertilization. (p. 39)

... Stem cells comprise a small subpopulation of multipotent or pluripotent, ultrastructurally unspecialized, slow-cycling cells that possess the ability of self-renewal and can produce cells that are destined to differentiate. (In contrast, primordial germ cells and those of a morula are totipotent; i.e., they can develop into any type of embryonic tissue and can even form an entirely new embryo) ... In addition to embryonic stem cells from preimplantation blastocysts, stem cells can be obtained from an adult. (p. 136) ... Ethical concerns are intensified by the experimental finding in primates (in contrast to the mouse) that embryonic stem cells are totipotent and can develop into a complete embryo with a primitive streak. ... If the source of the stem cells is a human embryo or fetus, however, ethical and legal issues have to be considered. (pp. 136-137)

Germ Line Cells Are Diploid:

Inner Cell Mass (ICM) Cells of Blasotcyst Are Totipotent:

...Cells of early free blastocyst (stage 3) are totipotent: The embryo enters the uterine cavity after about half a week, when probably at least 8-12 cells are present and the endometrium is early in its secretory phase. Each cell (blastomere) is considered to be still totipotent (capable. on isolation, of forming a complete embryo), and separation of these early cells is believed to account for one-third of cases of monozygotic twinning. (p. 37)

Cells of Early Human Embryo Are Totipotent; Twinning (cloning by blastomere separation and blastocyst splitting):

... Mammalian embryogenesis is considered to be a highly regulative process. Regulation is the ability of an embryo or an organ primordium to produce a normal structure if parts have been removed or added. At the cellular level, it means that the fates of cells in a regulative system are not irretrievably fixed and that the cells can still respond to environmental cues. Because the assignment of blastomeres into different cell lineages is one of the principal features of mammalian development, identifying the environmental factors that are involved is important. (pp. 44-49); ... Of the experimental techniques used to demonstrate regulative properties of early embryos, the simplest is to separate the blastomeres of early cleavage-stage embryos and determine whether each one can give rise to an entire embryo. This method has been used to demonstrate that single blastomeres, from two- and sometimes four-cell embryos can form normal embryos, ... (p. 44); ... Fate mapping experiments are important in embryology because they allow one to follow the pathways along which a particular cell can differentiate. Fate mapping experiments, which involve different isozymes of the enzyme glucose phosphate isomerase, have shown that all blastomeres of an eight-cell mouse embryo remain totipotent; that is, they retain the ability to form any cell type in the body. Even at the 16-cell stage of cleavage, some blastomeres are capable of producing progeny that are found in both the inner cell mass and the trophoblastic lineage. (p. 45); ... Another means of demonstrating the regulative properties of early mammalian embryos is to dissociate mouse embryos into separate blastomeres and then to combine the blastomeres of two or three embryos. The combined blastomeres soon aggregate and reorganize to become a single large embryo, which then goes on to become a normal-appearing tetraparental or hexaparental mouse. By various techniques of making chimeric embryos, it is even possible to combine blastomeres to produce interspecies chimeras (e.g., a sheep-goat). (p. 45); ... Blastomere removal and addition experiments have convincingly demonstrated the regulative nature (i.e., the strong tendency for the system to be restored to wholeness) of early mammalian embryos. Such knowledge is important in understanding the reason exposure of early human embryos to unfavorable environmental influences typically results in either death or a normal embryo. (p. 46).

... Some types of twinning represent a natural experiment that demonstrates the highly regulative nature of early human embryos, ... (p. 48); ... Monozygotic twins and some triplets, on the other hand, are the product of one fertilized egg. They arise by the subdivision and splitting of a single embryo. Although monozygotic twins could ... arise by the splitting of a two-cell embryo, it is commonly accepted that most arise by the subdivision of the inner cell mass in a blastocyst. Because the majority of monozygotic twins are perfectly normal, the early human embryo can obviously be subdivided and each component regulated to form a normal embryo. (p. 49)

"Twinning" Most Genetically Accurate Form of Cloning:

References From IVF Textbooks and IVF Clinic Web Sites:

Identical twins occur naturally approximately 3.5 times out of every 1000 human births. And, to date, scientists still don't know why and can't predict that they will, in any given birth, occur. However, in the last half of this century, and indeed, in the past ten to fifteen years, scientific advances have impacted on twins and other multiples and their families in numerous ways.

Now, a new method of actually producing identical twins looms near. Called "blastomere separation" (the separation of a two- to eight-cell blastomere into two identical demi-embryos), it is potentially one method of helping infertile couples have children through in vitro fertilization (IVF).

The following is excerpted from the medical journal Assisted Reproduction Reviews, May 1994. Dr. Joe B. Massey, who heads an in vitro clinic in Atlanta. Dr. Massey reviews the advances in blastomere separation and discusses the potential indications, benefits, limitations, and ethics of using this method to produce monozygotic twin embryos for IVF patients. The Twins Foundation, by presenting Dr. Massey's material for your information neither advocates nor rejects any such procedures.

Embryo Multiplication by Blastomere Separation-One Doctor's Proposal [Massey] In spite of many advances in human vitro fertilization (IVF), there are still many problems. While leading clinics now have success rates of about 30%, many other clinics lag behind. Still, the number of couples undergoing IVF continues to increase despite high costs.

According to Dr. Massey, "Observations on the potential impact of removing less than half of the cells from the human embryo have been well documented in pre-clinical embryo biopsy studies. (For more on this story see Research Update Vol. 9, No. 1, 1994)