Basic Human Embryology
Human Embryology is the scientific study of the material (biological, anatomical, physiological) aspect of the developing human embryo and fetus; specifically from the beginning of fertilization through eight weeks. Human embryology has been systematically documented in the Carnegie Stages of Early Human Embryonic Development (CSEHED) since 1942.
In other words, the field of human embryology has been an internationally recognized and defined field of study with set standards and definitions since 1942.
Human beings can reproduce:
Sexually — both natural and artificial (IVF) fertilization
Asexually — as in naturally occurring monozygotic twinning (identical twins) in vivo (inside the womb) as well as in many different kinds of cloning and genetic engineering techniques in vitro (“in glass” outside the womb)
The immediate products of both human reproductive processes are new genetically unique individual living human beings.
These new human beings immediately produce very specific human proteins and enzymes and continuously form very specific human cells, tissues and organs throughout development.
These objective scientific facts should be the starting point for any discussion, debate, policy or legislation on the life issues; unfortunately, they often are not. The fine details of human embryology have been corrupted for political purposes.
SEXUAL HUMAN REPRODUCTION
The GENOME of a species is determined by the total amount of DNA in both the nuclear and the cytoplasmic chromosomes (total genetic material).
However, the SPECIES (biological class or category) to which an individual belongs is determined only by the number of nuclear chromosomes per cell. For humans, that species number is 46 (plus or minus).
When a cell in the human body contains 46 chromosomes, it is called diploid; there are two basic categories of diploid cells: somatic, or body, cells and germ line cells, or future sex gametes. When a human cell contains only 23 chromosomes, it is called haploid.
Gametogenesis is the process in which the number of chromosomes in each diploid germ line cell is cut in half; this must take place before fertilization can occur. The final effect of gametogenesis is the production of haploid sex gametes: the sperm and oocyte, which have only 23 chromosomes in each cell.
NOTE: the terms egg and ovum (rather than oocyte) are rejected as unscientific.
Fertilization is the fusion of sperm and oocytes. It has been empirically known for over a hundred years that the beginning of the process of fertilization is the beginning of sexually reproduced human beings (it is obviously not the beginning of asexually reproduced human beings).
NOTE: the terms fertilized oocytes and fertilized egg are rejected as unscientific.
The diploid number of chromosomes (23 + 23 = 46) is restored and a new single-celled genetically unique living human being is reproduced. This is also the beginning of: the human embryo, the human organism, the human individual, the genetic sex of the embryo, the embryonic period, and normal pregnancy.
A normal pregnancy begins at fertilization which occurs in the fallopian tube (or ovaduct) of the mother, NOT at implantation in her womb (as was redefined by the American Congress of Obstetricians and Gynecologists, or ACOG, in 1965).
This new single-cell human being immediately directs his/her own further continuous human growth and development by producing specifically human proteins and enzymes.
Totipotent means that the cells are capable of forming all of the other cells, tissues and organs of the later embryo, fetus and adult (the most ’un-specialized’ and/or undifferentiated cell in the organism)
The newly formed single-cell embryo is totipotent — he or she has the capability of developing further. The cells (blastomeres) of the early developing human embryo will also exhibit a range of totipotency (until 2.5 weeks).
If one or more of the blastomeres is separated from the developing embryo, they too are capable of forming a new human organism(s). This is one way in which twins develop (both natural and artificial monozygotic twinning.)
Carnegie Stages of Human Development (CSEHED)
The first scientist to systematically study human embryos was Wilhelm His and the first to stage them was Franklin Mall in 1914. Later, George Streeter laid down the basis for the currently used Carnegie staging system, which was completed by Ronan O’Rahilly in 1973 and revised by O’Rahilly and Müller in 1987.
The Carnegie Stages are often referred to as the “Bureau of Standards” of human embryology. Today, they continue to be verified and documented by the international Terminologia Embyologica, which consists of more than twenty experts academically credentialed specifically in human embryology from around the world.
According to the CSEHED, the embryonic period is composed of twenty-three stages. Approximately 90 percent of the more than 4,500 structures of the adult body become apparent in the embryonic period.
In other words, 90 percent of all of adult body parts or functions are already present in the developing child in just the first weeks of life in the womb!
Of special note is STAGE ONE — which begins when the sperm penetrates the oocyte and continues until just before the zygote starts it first cleavage cell division at syngamy.
Much human cloning and human genetic engineering takes place during Stage One of the developing human embryo, even before the formation of the zygote, or slightly later while the cells of the very early human embryo are still totipotent.
ASEXUAL HUMAN REPRODUCTION
Understanding human sexual reproduction can aid in understanding human asexual reproduction, especially in terms of the natural biological processes of methylation and regulation.
Following fertilization (sexual reproduction), the early human embryo grows and develops by multiplying its cells and by means of various biochemical processes.
One of these critical, natural and biochemical processes is called regulation. Regulation involves methylating and demethylating the DNA in each of the cells.
That is, the DNA in each cell is either “allowed to speak” or is “silenced” by adding or removing these methylation bars — depending on what products, tissues or organs the embryo or fetus needs to grow and develop at any point in time.
These products then cascade throughout growth and development. The more specialized (or differentiated) a cell, the more methylated or silenced its DNA becomes. This is one way that the programming of the DNA of a cell is naturally accomplished. By adulthood, the DNA in many of the cells of the human being have been almost completely silenced by the insertion of methylation bars.
methylation -> add methylation bars to the DNA -> silence DNA in cells -> cell becomes more specialized (differentiated) -> allowed to “speak” only certain characteristics
In human asexual reproduction, many of these processes operate in reverse to reprogram, or de-differentiate, the DNA in a cell.
specialized cell -> demethylation -> remove methylation bars -> allow DNA to “speak” -> cell becomes less specialized (undifferentiated) -> return to state of single cell zygote where it can take on, or “speak”, many different characteristics
For example, in cloning by somatic cell nuclear transfer (SCNT), one can begin with a highly specialized human cell (like a skin cell), remove the nucleus and inject it into an enucleated oocyte (oocyte with nucleus removed).
Then elements in the cytoplasm of the enucleated oocyte incrementally remove the methylation bars on that DNA to allow it to speak until the DNA in that cell is in the same state of differentiation as the single-cell zygote — resulting in a new single-cell human organism — a single cell embryo — a human being!
This is roughly what happened with the reproduction of Dolly the sheep: “For the first time, an adult nucleus had been reprogrammed to be totipotent … just like the genetic material in the fertilized oocyte from which the donor cell had ultimately developed.”
LET’S NOT FORGET: a fertilized oocyte is a single-celled being — an embryo — a single celled organism at Stage One of embryonic development.
Despite certain claims to the contrary, the technique referred to as nuclear transfer just described is always cloning — regardless of why it is performed.
Some attempt to make a distinction between reproductive cloning and therapeutic cloning.
In reproductive cloning, the cloned organism is implanted in the womb and given the opportunity to further develop and be birthed, or born.
In therapeutic cloning, the cloned organism is purposely destroyed (killed) and no further development is permitted.
Both are cloning; both processes begin the same way; the only difference is the purpose for which the organism is created and whether or not he or she is allowed to continue growing and developing.
A cloned human embryo would not, as some try to claim, be “virtually genetically identical to the donor cell.” The new organism would have a different genome (genetic material) due to the presence of foreign DNA from the extra-nuclear mitochondrial chromosomes left over from the enucleated oocyte used and due as well to the lack of original mitochondrial chromosomes from the donor cell used.
In other words: the skin cell taken from John and placed into the enucleated oocyte of Jane does not produce an exact genetic match of John. This is due to the presence of foreign DNA in the cytoplasm of the oocyte Jane donated and because of the fact that DNA was not taken from the cytoplasm of John’s cell, only the nucleus.
(The only exception to this is if a specialized, “adult” cell from a female human being was injected into the enucleated egg of that same human female.)
THUS, using stem cells from such cloned human embryos in therapies could cause serious immune rejection reactions, even in human donor recipients.
In addition to cloning by means of nuclear transfer, one may also clone by means of twinning, for example, as happens in naturally occurring monzygotic twinning (both naturally and in vitro).
Twinning can also take place with the separated cells of the early embryo (through blastocyst stages) because such cells still exhibit a range of totipotency.
Twinning is also a common, and the most exact, form of cloning because the chromosomal DNA in the mitochondria in cells of twins are the same.
There are many different kinds of cloning techniques that can be used to clone human beings. Many of these cloning techniques are being considered or have already been used in IVF as infertility treatments.
In sum, the immediate product of both sexual and asexual human reproduction is a
new, living, genetically unique, single-cell
human being, human embryo, human organism, human individual, who immediately directs his/her own
specifically human functions, activities and development.
Thus, there is a great deal at stake in debates involving the human embryo:
abortion, the use of abortifacients (drugs and devices that kill the new embryo before implantation in the womb), IVF, prenatal genetic diagnosis, human embryo research, human genetic engineering, unethical production of vaccines and drug and biological/chemical testing and development.
Source: New Catholic Encyclopedia Supplement 2009