I. TheCarnegie Stages of Early Human Embryonic Development
The Carnegie Stages of Early Human Embryonic Development are found at the National Museum of Health and Medicine, Human Developmental Anatomy Center [2500 Linden Lane, Silver Spring, MD 20910; USA; [email protected]].
The known facts of the science of human embryology are not “new”. The first to study the human embryo systematically was Wilhelm His, Sr., who established the basis of reconstruction, i.e., the assembling of three-dimensional form from microscopic sections. His, who has been called the “Vesalium of human embryology,” published his three-volume masterpiece Anatomie menschlicher Embryonen in 1880-85 [His, Vogel, Leipzig]. In it the human embryo was studied as a whole for the first time internationally. A detailed Handbook of Human Embryology by Keibel and Mall appeared in 1910-12 . Franklin P. Mall, who studied under His, established the Carnegie Embryological Collection in Baltimore and was the first person to stage human embryos (in1914)
Mall’s collection soon became the most important repository of human embryos in the world and has ever since served as a “Bureau of Standards” for the science of human embryology. Mall’s successor, George L. Streeter , laid down the basis of the currently used staging system for human embryos (1942-48), which was instituted in 1942, completed by O’Rahilly (1973) and revised by O’Rahilly and Muller (1987). [See history of Carnegie Collection, at: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.collections.burdi; see also, Ronan O’Rahilly and Fabiola Muller, Human Embryology & Teratology  (New York: Wiley-Liss, 2001); also, O’Rahilly and Muller, ibid., (3rd ed., 1994), p. 3].
New URLs Online For The Carnegie Stages
The standard source for generations for the documentation for both human sexual and the human asexual reproductive technique of “twinning” has been the Carnegie Stages, and especially when they developed their internet online website, volumes of articles and books have been written referencing those Stages and the URLs where they could be found. Those online URLs have now changed, precluding others reading these articles and books from double-checking them for accuracy. In order to continue to be able to verify such documentation, the following NEW URLs for theCarnegie Stagesis provided:
- — See the National Museum of Health and Medicine(NMHM): http://www.medicalmuseum.mil/ [2500 Linden Lane, Silver Spring, MD 20910; [email protected]].
- — See the current website of the NMHM’s Human Development Anatomy Center: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.index. This is also the home of the Carnegie Stages of Early Human Embryonic Development. See Carnegie Stage One (phases a, b, and c) at: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage01.pdf.
- — See Chart of all 23 Stagesof the early developing human embryo, at: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.index. Click into the “textbook” at the bottom left side of the screen to access more extensive details of each stage and the extensive scientific references.
Go to the URLs listed below (examples of first 6 Stages only). Any particular Stage will first be shown as a brief summary of the scientific facts on that website URL. But if you look toward the bottom left of the webpage, you will see a “photo” of a human embryology textbook by O’Rahilly and Muller; it is actually a hyperlink. Click into that hyperlink, and you will be taken to the full original pages of the Carnegie Stage giving extensive details and documentation.
A. Examples of new online URLs for the Carnegie Stages
- — Stage 1: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.s01 — Stage 1 TEXT: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage01.pdf
- — Stage 2: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.s02 — Stage 2 TEXT: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage02.pdf
- — Stage 3: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.s03 — Stage 3 TEXT: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage03.pdf
- — Stage 4: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.s04 — Stage 4 TEXT: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage04.pdf
- — Stage 5: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.s05 — Stage 5 TEXT: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage05.pdf
- — Stage 6: http://www.medicalmuseum.mil/index.cfm?p=collections.hdac.anatomy.s06 — Stage 6 TEXT: http://www.medicalmuseum.mil/assets/documents/collections/hdac/stage06.pdf
B. Direct Quotes on HumanSexual Reproduction (Fertilization) in theCarnegie Stages
Stage 1 of the Carnegie Stages is divided into three sub-categories: (a), (b), and (c).As it always has and still documents, the new human embryo sexuallyreproduced begins to exist at the beginningof the process of fertilization — i.e., when the sperm makes contact with the oocyte (Stage 1 (a). The next sub-category of the embryo’s development is (b), when the male and female pronuclei come together (the “pronuclear embryo “). The last sub-category of the embryo’s development is (c), when the “zygote ” is formed. Thus, to claim that the new human embryo sexually reproduced begins to exist with the formation of the zygote is scientifically false, and ignores the already existing embryo who has already developed as documented in sub-categories (a) and (b). It so happens that a great deal of genetic engineering and related research takes place when the already existing embryo is at sub-categories (a) and (b) — beforethe formation of the zygote. Here are short excerpts of the accurate scientific facts about Stage 1 of the developing human embryo docutmented and quoted here verbatim from the Carnegie Stages. I encourage the readers to go online and read it themselves.Stage 1 (a), (b), (c) includes the new unicellular human organism, the new human embryo, the new human being, who is sexually reproduced, and who begins to exist from the beginning of the process of fertilization. After that critical event, the new sexually reproduced human embryo simply continues to grow bigger and more complex continuously through the later embryonic, fetal, infant, childhood through adult stages of human development(emphases added fir
those unfamiliar with the science):
- Stage 1 is the unicellular embryo that contains unique genetic material and is an individually specific cell that has the potential to develop into all of the subsequent stages of a human being. It is the beginning of embryonic life and ontogenetic development that starts when an oocyte, arrested in metaphase of meiosis II, is penetrated by a sperm. This is the first event of fertilization. The embryo has a postovulatory age of approximately one day, is between 0.1 to 0.15 mm in diameter and weighs approximately 0.004 mg.
- “… Fertilization is a series of events that begins when a sperm makes contact with an oocyte and ends with the intermingling of paternal (male) and maternal (female) chromosomes on the spindle at metaphase of the first mitotic division of the single cell. The events of fertilization require just over 24 hrs. to complete and normally takes place in the ampulla of the uterine tube [i.e., the fallopian tube, not the uterus].
- “… Stage 1 is divided into three substages; a, b and c. Stage 1a is referred to as the primordial embryo since all the genetic material necessary for the new individual, plus some redundant chromosomes, is now within a single plasma lemma (cell membrane). [Note: all of the components define the “embryo”, not just the genes; and these components must work in sync with each other, thus themselves pre-determining the final coding of the genome – which itself can change from internal and external causes during early development. The human genome is defined as including all the DNA in a cell — both nuclear and mitochondrial — not just the nuclear.] From the perspective of the female gamete it has also been named the penetrated oocyte. The fertilizing sperm has passed through the zona (capsula) pellucida and its plasmalemma has fused with that of the oocyte.
- “… Penetration activates the embryo into resuming its arrested meiosis II  and after anaphase it enters telophase with the expulsion of the redundant chromosomes as a second polar body. This marks the beginning of Stage 1b in which the single-cell is referred to as the pronuclear embryo . From the perspective of the female gamete it has also been named the ootid because its female component is haploid like a spermatid. However, in the pronuclear embryo there are two separate haploid components: one maternal, or female, pronucleus and one paternal, or male, pronucleus.
- “… The pronuclei move toward each other and eventually compress their envelopes where they lie adjacent near the center of the cell. Stage 1c is the last phase of fertilization and exists for a relatively short period . The pronuclear envelopes disappear and the parental chromosomes that were contained in separate pronuclei come together in a process called syngamy thereby establishing the genome of the embryo. The one-cell Stage 1c embryo is named the syngamic embryo orzygote. The chromosomes assume positions on the rapidly formed first mitotic spindle in preparation for cleavage.”
II. Additional Resources
A. Online URLs for the International Committee on Human Embryology
The most recent updating of theCarnegie Stages(Jan. 2012) by the international nomenclature committee on human embryology, i.e., the Terminologia Embryologica Committee (FIPAT) , is now also online and accessible on the internet (although not as “user friendly”).
The embryo sexually reproduced begins to exist at the beginning of the process of fertilization:
To use this new website for the Terminologia Embryologica online go to FIPAT, at: http://www.unifr.ch/ifaa/. Click on “Free access to published terminologies”, “Enter” to get to: http://www.unifr.ch/ifaa/Public/EntryPage/HomePublic.html. You are now on the Public Entry Page; Click into “Source terminologies as originally published”, to get to: http://www.unifr.ch/ifaa/Public/EntryPage/ViewSource.html. This page lists the 3 Terminologias. To the right of the page, under “Terminologia Embryologica, from internal document (2009)”, click onto “General Terms”, that takes you to:http://www.unifr.ch/ifaa/Public/EntryPage/ViewTE/TEe01.html. At the bottom of the page see “Footnote #5: E188.8.131.52.0..0.2 – Aetas a fecundiatione– Fertilization age begins at the time of fertilization with the sperm penetrating the oocyte … . It is the age of the conceptus and the preferred measure.”
There is no such thing as a “pre-embryo”:
Again, go to FIPAT, at: http://www.unifr.ch/ifaa/. Click on “Free access to published terminologies”, “Enter” to get to: http://www.unifr.ch/ifaa/Public/EntryPage/HomePublic.html. You are now on the Public Entry Page; Click into “Source terminologies as originally published”, to get to: http://www.unifr.ch/ifaa/Public/EntryPage/ViewSource.html. This page lists the 3 Terminologias. To the right of the page, under “Terminologia Embryologica, from internal document (2009)”, click onto e2.0: “Ontogeny”in order to get to: http://www.unifr.ch/ifaa/Public/EntryPage/ViewTE/TEe02.html. You are now viewing “Page 8”. This is a bit tricky: Now use button-arrows at top right of web page to move to Page 10to arrive at the description of Carnegie Stages 1-5 in the Chart. The right side of the Chart provides the following documentation of the first 5 Stages; see especially “Single cellEMBRYO [St. 1] [[“Stage One”]]. At the bottom of Page 10, in a footnote, you can find their rejection of the false scientific term “pre-embryo”: Footnote #32 – E184.108.40.206.0.0.3 – Embryo praegastrulationis [St. 1 ad 6a] The term pregastrulation embryo is useful because such an embryo has distinctive attributes (see footnote 35). The foreshortened term “pre-embryo”, which has been used in legal and clinical contexts, is not recommended.”[[Note: “St. 1 ad 6a” means “Stages 1 to 6a“]]
B. The Virtual Human Embryo Project
The Virtual Human Embryo Project[https://www.prenatalorigins.org/virtual-human-embryo/; Stage 1, at: https://www.prenatalorigins.org/virtual-human-embryo/stage.php?stage=1]
was originally developed as a collaboration between embryologist Dr. Raymond Gasser at LSUHSC and the HDAC in Washington DC. The overall aim of the project is to make the Carnegie collection, which is housed at the HDAC, accessible for research and teaching of human embryology. Dr. John Cork at LSUHSC joined the project at its inception as the software developer with a special interest in 3D-reconstruction. The project has two components, both of which are supported by grants from the National Institutes of Health. Anyone can access the various stages of the new sexually reproduced developing human embryo by going to the Virtual Human Embryo’s “DREM DEMOS” page, click into “Enter”, then click into “Demo” on the left of the page. Click into “Stage One: Introduction”: https://www.prenatalorigins.org/virtual-human-embryo/stage.php?stage=1
Stage 1 is the unicellular embryothat contains unique genetic material and is an individually specific cell that has the potential to develop into all of the subsequent stages of a human being. It is the beginning of embryonic life and ontogenetic development that starts when an oocyte, arrested in metaphase of meiosis II, is penetrated by a sperm. This is the first event of fertilization.The embryo has a postovulatory age of approximately one day, is between 0.1 to 0.15 mm in diameter and weighs approximately 0.004 mg.
Fertilization is a series of events that begins when a sperm makes contact with an oocyte and ends with the intermingling of paternal (male) and maternal (female) chromosomes on the spindle at metaphase of the first mitotic division of the single cell. The events of fertilization require just over 24 hrs. to complete and normally take place in the ampulla of the uterine tube. Stage 1 is divided into three substages; a, b and c. Stage 1a is referred to as the ‘primordial embryo’since all the genetic material necessary for the new individual, plus some redundant chromosomes, is now within a single plasmalemma (cell membrane). From the perspective of the female gamete it has also been named the penetrated oocyte. The fertilizing sperm has passed through the zona (capsula) pellucida and its plasmalemma has fused with that of the oocyte. Penetration activates the embryo into resuming its arrested meiosis II and after anaphase it enters telophase with the expulsion of the redundant chromosomes as a second polar body. This marks the beginning of Stage 1b in which the single-cell is referred to as the ‘pronuclear embryo’. From the perspective of the female gamete it has also been named the ootid because its female component is haploid like a spermatid. However, in the pronuclear embryo there are two separate haploid components: one maternal, or female, pronucleus and one paternal, or male, pronucleus. The pronuclei move toward each other and eventually compress their envelopes where they lie adjacent near the center of the cell. Stage 1c is the last phase of fertilization and exists for a relatively short period.The pronuclear envelopes disappear and the parental chromosomes that were contained in separate pronuclei come together in a process called syngamy thereby establishing the genome of the embryo. The one-cell Stage 1c embryo is named the ‘syngamic embryo’ or zygote.The chromosomes assume positions on the rapidly formed first mitotic spindle in preparation for cleavage. [http://virtualhumanembryo.lsuhsc.edu/demos/Stage1/Intro_pg/Intro.htm]
C. The iPhone App – “Embryo”[*** in process of being changed to a new URL]
Thanks to USA.gov, anyone can access the new iPhone appentitled, “Embryo”, from the National Library of Medicine, documenting the same facts of human embryology. The iPhone app is now available at: http://apps.usa.gov/embryo/. It is necessary to have iTunes for this application, but you can download it free from this same site (also downloads onto all computers). As noted on this government website:
Scientists and educators have used the Carnegie Embryo Collection, housed at the National Museum of Health and Medicine, to define normal human embryo development for decades. A database, called the Virtual Human Embryo, has been created to provide digital serial sections of human embryos from the collection….
This project is a collaboration between:
- National Library of Medicine
- Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Louisiana State University Health Sciences Center
- National Museum of Health & Medicine, Human Developmental Anatomy Center [TheCarnegie Stages of Early Human Embryonic Development]
D. The Carnegie Stages can also be found onDVD’spurchasable from the Endowment for Human Development, at:http://www.ehd.org/shoppingcart/products/The-Biology-of-Prenatal-Development.html .
III. A Few Additional References for Sexual and Asexual Human Reproduction:
Human Sexual Reproduction:
** Ronan O’Rahilly and Fabiola Muller, Human Embryology & Teratology, 3rd ed. (New York: Wiley-Liss, 2001):
— Recapitulation, the So-Called Biogenetic Law.The theory that successive stages of individual development (ontogeny) correspond with (“recapitulate”) successive adult ancestors in the line of evolutionary descent (phylogeny) became popular in the nineteenth century as the so-called biogenetic law. This theory of recapitulation, however, has had a “regrettable influence on the progress of embryology”(G. de Beer)…. According to the “laws” of von Baer, general characters (e.g., brain, notochord) appear in development earlier than special characters(e.g., limbs, hair). Furthermore, during its development an animal departs more and more from the form of other animals. Indeed, the early stages in the development of an animal are not like the adult stages of other forms but resemble only the early stages of those animals.The pharyngeal clefts of vertebrate embryos, for example, are neither gills nor slits. Although a fish elaborates this region into gill slits, in reptiles, birds, and mammals it is converted into such structures as the tonsils and the thymus (p. 16).
… (Fertilization is)the procession of events that begins when a spermatozoon makes contact with a secondary oocyte or its investments, and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote. (p. 19).
— Although life is a continuous process, fertilization… is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed… (p. 31).
— Fertilization takes place normallyin the ampulla (lateral end) of the uterine tube (p. 31).
— “The term ‘pre-embryo’ is not used herefor the following reasons: (1) it is ill-definedbecause it is said to end with the appearance of the primitive streak or to include neurulation; (2) it is inaccuratebecause purely embryonic cells can already be distinguished after a few days, as can also the embryonic (not pre-embryonic!) disc; (3) it is unjustifiedbecause the accepted meaning of the word embryo includes all of the first 8 weeks; (4) it is equivocalbecause it may convey the erroneous idea that a new human organism is formed at only some considerable time after fertilization; and (5) it was introduced in 1986 ‘largely for public policy reasons’(Biggers).”… Just as postnatal age begins at birth, prenatal age begins at fertilization,” (p. 88).
— “Undesirable terms in Human Embryology”: “Pre-embryo”; ill defined and inaccurate;
Use “embryo”(p. 12). [Note: O’Rahilly is one of the originators of The Carnegie Stages of Early Human Embryological Development, and has sat on the international committee for human embryology for decades]
** BRUCE M. CARLSON, Human Embryology and Developmental Biology (St. Louis, MO: Mosby, 1994): Human pregnancy begins with the fusion of an egg and a sperm. (p. 3); … finally, the fertilized egg, now properly called an embryo, must make its way into the uterus (p. 3) ; … The sex of the future embryo is determined by the chromosomal complement of the spermatozoon … Through the mingling of maternal and paternal chromosomes, the zygote is a genetically unique product of chromosomal reassortment .. (p. 31); … “After the eighth week of pregnancy the period of organogenesis (embryonic period) is largely completed and the fetal period begins. (p. 407)
** BRUCE M. CARLSON, Human Embryology & Developmental Biology (St. Louis, MO: Mosby, 1999): “Human pregnancy begins with the fusion of an egg and a sperm, but a great deal of preparation [recedes this event. First both male and female sex cells must pass through a long series of changes (gametogenesis) that convert them genetically and phenotypically into mature gametes, which are capable of participating in the process of fertilization. Next, the gametes must be released from the gonads and make their way to the upper part of the uterine tube, where fertilization normally takes place. … Finally, the fertilized egg, now properly called an embryo, must make its way into the uterus ….”. (p. 2); … Fertilization age: dates the age of the embryo from the time of fertilization. (p. 23) … In the female, sperm transport begins in the upper vagina and ends in the ampulla of the uterine tube [fallopian tube] where the spermatozoa make contact with the ovulated egg. (p. 27) … After the eighth week of pregnancy the period of organogenesis (embryonic period) is largely completed, and the fetal period begins.” (p. 447). … The sex of the future embryo is determined by the chromosomal complement of the spermatozoon. (If the sperm contains 22 autosomes and an X chromosome, the embryo will be a genetic female, and if it contains 22 autosomes and a Y chromosome, the embryo will be a male.) … Through the mingling of maternal and paternal chromosomes, the zygote is a genetically unique product of chromosomal reassortment, which is important for the viability of any species. (p. 32)
** WILLIAM J. LARSEN, Human Embryology (New York: Churchill Livingstone, 1997): In this text, we begin our description of the developing human with the formation and differentiation of the male and female sex cells or gametes, which will unite at fertilization to initiate the embryonic development of a new individual. … Fertilization takes place in the oviduct [not the uterus]… resulting in the formation of a zygote containing a single diploid nucleus. Embryonic development is considered to begin at this point. (p. 1); … “These pronuclei fuse with each other to produce the single, diploid, 2N nucleus of the fertilized zygote. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development. (p. 17).
** Geoffrey Sher, Virginia Marriage Davis, Jean Stoess, In Vitro Fertilization:The A.R.T. of Making Babies (New York: Facts On File, 1998): The moment a sperm penetrates the egg’s zona pellucida, a reaction in the egg fuses the zona and the perivitelline membrane into an impermeable shield that prevents other sperm from entering. … Propelled by contractions of the fallopian tube, the dividing embryo begins its three- or four-day journey back to the uterus and continues to divide after it reaches the uterus. (The fertilization process occurs near the middle of the fallopian tube — not in the uterus.) (p. 18)
** Lewin, Benjamin, Genes VII(New York: Oxford University Press, 2000):
— “A genome consists of the entire set of chromosomes for any particular organism, and therefore comprises a series of DNA molecules, each of which contains a series of many genes. The ultimate definition of a genome is to determine the sequence of the DNA of each chromosome. (p. 4)
… “Genes not residing within the nucleus are generally described as extranuclear; they are transcribed and translated in the same organelle compartment (mitochondrion or chloroplast) in which they reside. By contrast, nuclear genesare expressed by means of cytoplasmic protein synthesis.” (p. 81)
** Read, Andrew P., and Tom Strachan, Human Molecular Genetics 2, 2nd ed. (New York: John Wiley & Sons, Inc., 1999):
— “In animal cells, DNA is found in both the nucleus and the mitochondria.” (p. 10)
— “The mitochondria also have ribosomes and a limited capacity for protein synthesis.” (p. 18)
— “The human genome is the term used to describe the total genetic information (DNA content) in human cells.It really comprises two genomes: a complexnucleargenome…, and a simple mitochondrial genome…Mitochondria possess their own ribosomes and the few polypeptide-encoding genes in the mitochondrial genome produce mRMAs, which are translated on the mitochondrial ribosomes.” (p. 139)
Human Asexual Reproduction:
** Tom Strachan and Andrew P. Read, Human Molecular Genetics 2 (New York: John Wiley & Sons, Inc., 1999), pp. 508-509].
— The term ‘clones’ indicates genetic identityand so can describe genetically identical molecules(DNA clones), genetically identicalcellsor genetically identical organisms. Animal clones occur naturallyas a result of sexual reproduction. For example, genetically identical twins [momozygotic twins] are clones… A form of animal cloning can also occur as a result of artificial manipulationto bring about a type of asexual reproduction. The genetic manipulation in this case uses nuclear transfer technology: a nucleus is removed from a donor cell then transplanted into an oocyte whose own nucleus has previously been removed…. The individual providing the donor nucleus and the individual that develops from the ‘renucleated’ oocyte are usually described as “clones”, but it should be noted that they share only the same nuclear DNA; they do not share the same mitochondrial DNA, unlike genetically identical twins…. Wilmut et al (1997) reported successful cloning of an adult sheep. For the first time, an adult nucleus had been reprogrammed to becometotipotent once more, just like the genetic material in the fertilized oocytefrom which the donor cell had ultimately developed…. Successful cloning of adult animals has forced us to accept that genome modifications once considered irreversible can be reversed and that the genomes of adult cells can be reprogrammed by factors in the oocyte to make them totipotent once again.
** Carlson, Bruce M., Human Embryology and Developmental Biology, 2nd ed. (St. Louis, MO: Mosby, 1999):
— Early mammalian embryogenesis is considered to be a highly regulative process.Regulationis 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. (p. 44).
— 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)
— 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).
— The relationship between the positionof the blastomeres and their ultimate developmental fate was incorporated into the inside-outside hypothesis. The outer blastomeresultimately differentiate into the trophoblast, whereas the inner blastomeres form the inner cell mass, from which the body of the embryo arises. Although this hypothesis has been supported by a variety of experiments, the mechanisms by which the blastomeres recognize their positions and then differentiate accordingly have remained elusive and are still little understood. If marked blastomeres from disaggregated embryos are placed on the outside of another early embryo, they typically contribute to the formation of the trophoblast. Conversely, if the same marked cells are introduced into the interior of the host embryo, they participate in formation of the inner cell mass.Outer cells in the early mammalian embryo are linked by tight and gap junctions… Experiments of this type demonstrate that the developmental potential or potency (the types of cells that a precursor cell can form) of many cells is greater than their normal developmental fate (the types of cells that a precursor cell normally forms). (p. 45).
— Another means of demonstrating the regulative properties of early mammalian embryosis 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 normalappearing 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).
— Classic strategiesfor investigating developmental properties of embryos are (1) removing a part and determining the way the remainder of the embryo compensates for the loss(such experiments are called deletion experiments) and (2) adding a part and determining the way the embryo integrates the added materialinto its overall body plan (such experiments are called addition experiments). Although some deletion experiments have been done, the strategy of addition experiments has proved to be most fruitful in elucidating mechanisms controlling mammalian embryogenesis. (p. 46).
** Elder, Kay T. “Laboratory techniques: Oocyte collection and embryo culture,” in ed. Peter Brinsden, A Textbook of in vitro Fertilization and Assisted Reproduction, 2nd ed. (New York: The Parthenon Publishing Group, 1999):
— Surprisingly, fragmented embryos, repaired or not, do implant and often come to term. This demonstrates the highly robust nature of the human embryo, as it can apparently lose over half of its cellular mass and still recover. (p. 197)
** Larsen, William, Essentials of Human Embryology(New York: Churchill Livingstone, 1998):
— If the splitting occurred during cleavage– for example, if the two blastomeres produced by the first cleavage division become separated – the monozygotic twin blastomeres will implant separately, like dizygotic twin blastomeres, and will not share fetal membranes. Alternatively, if the twins are formed by splitting of the inner cell mass within the blastocyst, they will occupy the same chorion but will be enclosed by separate amnions and will use separate placentae, each placenta developing around the connecting stalk of its respective embryo. Finally, if the twins are formed by splitting of a bilaminar germ disc, they will occupy the same amnion. (p. 325)
** Ronan O’Rahilly and Fabiola Muller, Human Embryology & Teratology (New York: Wiley-Liss, 2001):
— Biopsy of an embryo can be performed by removing one cell from a 4-cell, or two cells from an 8-cell, embryo. This does not seem to decrease the developmental capacity of the remaining cells. (p. 37).
— The embryo enters the uterine cavity after about half a week… 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).
** National Bioethics Advisory Commission. Cloning Human Beings: Report and Recommendations. (Rockville, MD: June 1997):
— The Commission began its discussions fully recognizing that any effort in humans to transfer a somatic cell nucleus into an enucleated egg involves the creation of an embryo, with the apparent potential to be implanted in utero and developed to term. (p. 3).
** National Institutes of Health. Background Paper: Cloning: Present uses and Promises, Jan. 29, 1998.
— This experiment [producing Dolly] demonstrated that, when appropriately manipulated and placed in the correct environment, the genetic material of somatic cells can regain its full potential to direct embryonic, fetal, and subsequent development. (p. 3).
Dr. Dianne N. Irving (1994), a former research biochemist at the NIH/NCI in radiation biology and Professor of Philosophy at the Dominican House of Studies, also gave a relevant testimony before HERP.
Read more of her articles HERE