[1]
Ronan O'Rahilly and
Fabiola Muller, Human Embryology & Teratology (New York:
Wiley-Liss, 2001), p. 12.
[2]
Wilhelm His, Anatomie
menschlicher Embryonen (Leipzig: Vogel,
1880-1885).
[3]
FULL REFERENCES:
"Although life is a continuous process, fertilization ...
is a critical landmark because, under ordinary circumstances, a new,
genetically distinct human organism is formed when the chromosomes of the
male and female pronuclei blend in the oocyte.
This remains true even though the embryonic genome is not actually
activated until 2-8 cells are present at about 2-3 days.
... 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. ...
Fertilization takes place normally in the ampulla (lateral end) of the
uterine tube. (p. 31); ...
Coalescence of homologous chromosomes results in a one-cell embryo.
...The zygote is characteristic of the last phase of fertilization
and is identified by the first cleavage spindle.
It is a unicellular embryo and is a highly specialized cell.
The combination of 23 chromosomes present in each pronucleus results
in 46 chromosomes in the zygote. Thus
the diploid number is restored and the embryonic genome is formed. The embryo now exists as a genetic unity." (p. 33);
"... [I]t is now accepted that the word embryo, as
currently used in human embryology, means 'an unborn human in the first 8
weeks' from fertilization'. Embryonic
life begins with the formation of a new embryonic genome (slightly prior to
its activation)." (p. 87) [O'Rahilly and Muller, 2001]
"Human
pregnancy begins with the fusion of an egg and a sperm, but a great deal of
preparation precedes 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 [fallopian 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) [Bruce M.
Carlson, Human Embryology & Developmental Biology (St. Louis:
Mosby, 1999)].
"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). [William J. Larson, Essentials of Human Embryology (New York:
Churchill Livingstone, 1997)]
"Human
development is a continuous process that begins when an oocyte (ovum) from a
female is fertilized by a sperm (or spermatozoon) from a male." (p. 2);
" ... but the
embryo begins to develop as soon as the oocyte is fertilized. "
(p. 2); " ... Zygote:
this cell results from the union of an oocyte and a sperm.
A zygote is the beginning of a new human being (i.e., an embryo).
" (p. 2); "...
Human development begins at fertilization, the process during which a
male gamete or sperm ... unites with a female gamete or oocyte ... to form a
single cell called a zygote. This
highly specialized, totipotent cell marks the beginning of each of us as a
unique individual.". (p. 18)
"... The usual site
of fertilization is the ampulla of the uterine tube [fallopian tube], its
longest and widest part. If the
oocyte is not fertilized here, it slowly passes along the tube to the
uterus, where it degenerates and is reabsorbed.
Although fertilization may occur in other parts of the tube, it does
not occur in the uterus. ... Human development begins when a oocyte is fertilized.
Fertilization ... begins with contact between a sperm and a oocyte
and ends with the intermingling of maternal and paternal chromosomes of the
zygote, a unicellular embryo." (p.
34) [Keith L. Moore and T.V.N. Persaud, The Developing Human:
Clinically Oriented Embryology (use 6th ed. only) (Philadelphia:
W.B. Saunders Company, 1998)]
"Of
verified pregnancies that have survived the first 4 postovulatory weeks, it
is generally maintained that 15-20% are lost through spontaneous abortion.
Under 4 weeks, however, the number is far larger and may be as high
as 40%. Many fertilized oocytes
fail to become implanted, and as many as one-third of those implanted may be
lost without being recognized. The total loss of conceptuses from fertilization to birth is
believed to be considerable, perhaps even as high as 50% to nearly 80%.
A high percentage of abortuses (30-80%, depending on the study) are
structurally abnormal, and it is maintained that all abortuses under 4
postovulatory weeks have abnormally formed embryonic tissue.
Most malformed conceptuses (more than 90%) are spontaneously aborted,
compared with the normal 18%. Thus,
spontaneous abortion greatly reduces the number of malformed fetuses
born." (O'Rahilly and
Muller 2001, pp. 92-93).
"Early
spontaneous abortions occur for a variety of reasons, one being the presence
of chromosomal abnormalities in the zygote.
Carr and Gedeon (1977) estimated that about half of all known
spontaneous abortions occur because of chromosomal abnormalities.
Hertig et al. (1959), while examining blastocysts recovered from
early pregnancies, found several clearly defective dividing zygotes ... and
blastocysts. Some were so
abnormal that survival would not have been likely.
The early loss of embryos, once called pregnancy wastage, appears to
represent a disposal of abnormal conceptuses that could not have developed
normally, i.e., there is a natural screening of embryos.
Without this screening, about 12% instead of 2 to 3% of infants would
likely be congenitally malformed (Warkany, 1981)."
(p.p. 42 - 43) [Moore and Persaud 1998].
[4]
For extensive scientific
references for these processes of gametogenesis and fertilization, see D.N.
Irving, "When do human beings begin? 'Scientific' myths and scientific
facts", International Journal of Sociology and Social Policy
1999, 19:3/4:22-47.
[5]
"Gametogenesis is the
production of germ cells (gametes), i.e., spermatozoa and oocytes. ... The
gametes are believed to arise by successive divisions from a distinct line
of cells (the germ plasm), and the cells that are not directly concerned
with gametogenesis are termed somatic. ... The 46 human chromosomes consist
of 44 autosomes and two sex chromosomes:
X and Y. In the male the
sex chromosomes are XY; in the
female they are XX. Phenotypic
sex is normally determined by the presence or absence of a Y chromosome.
... During the differentiation of gametes, diploid cells are termed primary,
and haploid cells are called secondary, e.g., secondary oocyte.
Diploid refers to the presence of two sets of homologous
chromosomes: 23 pairs, making a
total of 46. This is
characteristic of somatic and primordial germ cells alike.
Haploid is used for a single set of 23 chromosomes, as in
gametes." [O'Rahilly and
Muller 2001, p. 19].
"A
subset of the diploid body cells constitute the germ line. These give rise
to specialized diploid cells in the ovary and testis that can divide by
meiosis to produce haploid gametes (sperm and egg). ... The other cells of
the body, apart from the germ line, are known as somatic cells ... most
somatic cells are diploid ... ." [Strachan and Read 1999, p. 28].
"Meiosis
is a special type of cell division that involves two meiotic cell divisions;
it takes place in germ cells only. Diploid germ cells give rise to haploid
gametes (sperms and oocytes)." [Moore
and Persaud 1998, p. 18].
"In a mitotic
division, each germ cell produces two diploid progeny that are genetically
equal." [Carlson 1999, p. 2].
"Like
all normal somatic (i.e., non-germ cells), the primordial germ
cells contain 23 pairs of chromosomes, or a total of 46. " [Larsen
1998, p. 4].
[6]
"Future somatic cells
thereby lose their totipotency and are liable to senscence, whereas germ
cells regain their totipotency after meiosis and fertilization."
[O'Rahilly 2001, p. 39]. "Early primordial germ cells are spared;
their genomic DNA remains very largely unmethylated until after
gonadal differentiation and as the germ cells develop whereupon widespread de
novo methylation occurs." [Tom
Strachan and Andrew Read, Human Molecular Genetics 2 (2nd ed.) (New
York: Wiley-Liss, 1999), p.
191]
[7]
"Cells differentiate
by the switching off of large portions of their genome."
[O'Rahilly 2001, p. 39]. "Gene
expression is associated with demethylation.
Methylation of DNA is one of the parameters that controls
transcription. This is one of
several regulatory events that influence the activity of a promoter;
like the other regulatory events, typically this will apply to both
copies of the gene." [Benjamin Lewin, Genes VII (New York:
Oxford University Press, Inc., 2000), p. 678;
also p. 603]. "Gene
regulation as the primary function for DNA methylation:
DNA methylation in vertebrates has been viewed as a mechanism for
silencing transcription and may constitute a default position."
[Strachan and Read, pp. 193 ff]
[8]"
The expression of genes is
determined by a regulatory network that probably takes the form of a
cascade. Expression of the
first set of genes at the start of embryonic development leads to expression
of the genes involved in the next stage of development, which in turn leads
to a further stage, and so on until all the tissues of the adult are
functioning." [Lewin, p.
63; also pp. 914, 950].
[9]
"Early 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." (pp. 44-49). ...
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) [Carlson 1999]
[10]
"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) "... 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." [O'Rahilly
and Muller 2001, p.37]
"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); " ... 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) [Carlson 1999]
"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) [Larsen 1998]
[11]
"[O]ther events are possible after this time [segmentation -- 14
days] which indicate that the notion of "irreversible
individuality" may need some review if it is to be considered as an
important criterion in human life coming "to be the individual human
being it is ever thereafter to be".
There are two conditions which raise questions about the adequacy of
this notion: conjoined twins,
sometimes known as Siamese twins, and fetus-in-fetu.
... Although conjoined
twins and fetus-in-fetu have rarely been documented, the possibility of
their occurring raises several points related to the notion of irreversible
individuality. Conjoined twins
arise from the twinning process occurring after the primitive streak has
begun to form, that is, beyond 14 days after fertilization, or, in terms of
the argument from segmentation, beyond the time at which irreversible
individuality is said to exist.
... Similar reasoning
leads to the same confusion in the case of fetus-in-fetu.
... One case recorded
and studied in detail showed that the engulfed twin had developed to the
equivalent of four months gestation and consisted of brain, bones,
nerve tissue, muscle and some rudimentary organs.
Microscopic study showed that engulfment had occurred at about four
weeks after fertilization, in terms of the argument for segmentation long
after the time when it is claimed that individuality is resolved."
[Her reference is: Yasuda,
Y., Mitomori, T., Matsurra, A. and Tanimura, T., "Fetus-in-fetu:
report of a case", Teratology 31 (1985), 337-41.]
[Karen Dawson, "Segmentation and moral status", in Peter
Singer, Helga Kuhse, Stephen Buckle, Karen Dawson, and Pascal Kasimba, Embryo
Experimentation (New York: Cambridge
University Press, 1990), pp. 57-59].
"MZ
[monozygotic] twinning usually begins in the blastocyst stage, around the
end of the first week (before formation of the germ disc starting at 8
days).... Uncommonly,
early separation of embryonic blastomeres, (e.g., during the 2 - 8 cell
stages) results in MZ twins with two amnions, two chorions, and two
placentas that may or may not be fused. (p. 159);
... About 35% of MZ twins result from early separation of the
embryonic blastomeres; i.e.,
during the first 3 days of development.
The other 65% of MZ twins originate at the end of the first week of
development; i.e., right after
the blasocyst has formed [5-7 days]. Late
division of early embryonic cells, such as division of the embryonic disc
during the second week, results in MZ twins that are in one amniotic sac and
one chorionic sac." (p.
159); ...
If the embryonic disk does not divide completely, or adjacent
embryonic discs fuse, various types of conjoined MZ twins may form. ...
the incidence of conjoined (Siamese) twins is 1 in 50,000- 100,000
births." [Moore and
Persaud 1998, p. 161].
"Partial
duplication at an early stage and attempted duplication from 2 weeks onward
(when bilateral symmetry has become manifest) would result in conjoined
twins." (p. 30);
... Once the primitive streak has appeared at about 13 days,
splitting that involves the longitudinal axis of the embryo would be
incomplete and would result in conjoined twins."
[O'Rahilly and Muller 1994, p. 30]. ...
Similarly, after the appearance of the primitive streak and
notochordal process, any attempt at longitudinal division would be
incomplete and would result in conjoined [Siamese] twins. " (ibid,
2001, p. 55)
[12]
"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); "...
The relationship between the position of the blastomeres and their ultimate
developmental fate was incorporated into the inside-outside hypothesis.
The outer blastomeres ultimately 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); " ...
Classic strategies for 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 material into 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). [Carlson 1999]
[13]
Many women, and men, assume that the "pre-embryo" myth is true,
and thus unfortunately believe contraceptive providers that swear that their
products could not possibly be abortifacient. However, it is a scientific fact that several so-called
"contraceptives" could possibly sometimes be abortifacient:
"Inhibition of Implantation:
The administration of relatively large doses of estrogens ("morning-after
pills") for several days, beginning shortly after unprotected
sexual intercourse, usually does not prevent fertilization but often
prevents implantation of the blastocyst.
Diethylstilbestrol, given daily in high dosage for 5 to 6 days, may
also accelerate passage of the dividing zygote along the uterine tube
(Kalant et al., 1990). Normally,
the endometrium progresses to the secretory phase of the menstrual cycle as
the zygote forms, undergoes cleavage, and enters the uterus.
The large amount of estrogen disturbs the normal balance between
estrogen and progesterone that is necessary for preparation of the
endometrium for implantation of the blastocyst.
Postconception administration of hormones to prevent implantation
of the blastocyst is sometimes used in cases of sexual assault or
leakage of a condom, but this treatment is contraindicated for routine
contraceptive use. The
'abortion pill' RU486 also destroys the conceptus by interrupting
implantation because of interference with the hormonal environment of the
implanting embryo.
"An intrauterine device (IUD) inserted into the uterus through the vagina and cervix usually interferes with implantation by causing a local inflammatory reaction. Some IUDs contain progesterone that is slowly released and interferes with the development of the endometrium so that implantation does not usually occur." (p. 58); ... [Question Chapter 2, #5 for students:] "#5. A young woman who feared that she might be pregnant asked you about the so-called "morning after pills" (postcoital birth control pills). What would you tell her? Would termination of such an early pregnancy be considered an abortion?" (p. 45); ... [Answer #5 for students:] "Chapter 2, #5. Postcoital birth control pills ('morning after pills') may be prescribed in an emergency (e.g., following sexual abuse). Ovarian hormones (estrogen) taken in large doses within 72 hours after sexual intercourse usually prevent implantation of the blastocyst, probably by altering tubal motility, interfering with corpus luteum function, or causing abnormal changes in the endometrium. These hormones prevent implantation, not fertilization. Consequently, they should not be called contraceptive pills. Conception occurs but the blastocyst does not implant. It would be more appropriate to call them 'contraimplantation pills'. Because the term 'abortion' refers to a premature stoppage of a pregnancy, the term 'abortion' could be applied to such an early termination of pregnancy." (p. 532); ... [Question chapter 3, #2 for students]: "Case 3-2: A woman who was sexually assaulted during her fertile period was given large doses of estrogen twice daily for five days to interrupt a possible pregnancy. If fertilization had occurred, what do you think would be the mechanism of action of this hormone? What do lay people call this type of medical treatment? Is this what the media refer to as the "abortion pill"? If not, explain the method of action of the hormonal treatment. How early can a pregnancy be detected?" (p. 59); [Answer Chapter 3, #2 for students:]: "Chapter 3-2 (p. 532): Diethylstilbestrol (DES) appears to affect the endometrium by rendering it unprepared for implantation, a process that is regulated by a delicate balance between estrogen and progesterone. The large doses of estrogen upset this balance. Progesterone makes the endometrium grow thick and succulent so that the blastocyst may become embedded and nourished adequately. DES pills are referred to as "morning after pills" by lay people. When the media refer to the "abortion pill", they are usually referring to RU-486. This drug, developed in France, interferes with implantation of the blastocyst by blocking the production of progesterone by the corpus luteum. A pregnancy can be detected at the end of the second week after fertilization using highly sensitive pregnancy tests. Most tests depend of the presence of an early pregnancy factor (EPF) in the maternal serum. Early pregnancy can also be detected by ultrasonography." [Moore and Persaud 1998, pp.