Americans to Ban Cloning

What Embryologists Are Saying About the Terms Pre-embryo and Primitive Streak

Date: 01/13/2002

RONAN O’RAHILLY AND FAIOLA MULLER, Human Embryology & Teratology (3rd ed.)(New York: Wiley-Liss, 2001): … The term ‘pre-embryo’ is not used here for the following reasons: (1) it is ill-defined because it is said to end with the appearance of the primitive streak or to include neurulation; (2) it is inaccurate because purely embryonic cells can already be distinguished after a few days, as can also the embryonic (not pre-embryonic!) disc; (3) it is unjustified because the accepted meaning of the word embryo includes all of the first 8 weeks; (4) it is equivocal because 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)

RONAN O’RAHILLY AND FABIOLA MULLER, Human Embryology & Teratology (3rd ed.)(New York: Wiley-Liss, 2001): (Chart) TABLE 2-1 Examples of Discarded and Replaced Terms: “Undesirable terms in Human Embryology”: “Pre-embryo”; ill-defined and inaccurate; use “embryo”. “Egg”; best confined to the hen and to cuisine; use “oocyte”. “Ovum”; does not exist in human; use “oocyte”, “ootid”, “embryo”. “Tail”; incorrect interpretation; use “caudal eminence”. “Brancial”; a fishy term not applicable to species without gills; use “pharyngeal” or “visceral”. “Anterior and posterior”; best avoided for the early embryo; use “rostral and caudal”. “Blastocoel”; avian term for space between epiblast and primary endoderm; use “blastocystic cavity”. “Blastopore”; an opening in early amphibian embryo; not used [do not use for human embryo]. “Blastula, blastulation”; not applicable to mammals; use “blastocyst”. “Cerebral vesicles”; based on interpretation of avian species; use “forebrain”, “midbrain”, and “hindbrain”. “Choda dorsalis”; acceptable but clumsy term; use “notochord”. “Gastrulation”; a gastrula is not formed [in human embryo]; not used [in human embryology]. “gestational age”; ambiguous, and not age; use “menstrual weeks”. “Head process”; obsolescent; use “notochordal process”. “Horizon”; no longer in use; use “stage”. “Medullary folds and groove”; medulla has other implications; use “neural folds and groove”. “Menstrual age”; not age; use “menstrual weeks”. “Midline, midsagittal”; unofficial and unnecessary terms; use “median”. “Morula”; an early amphibian embryo that gives rise to embryonic tissue only; term useful [only]. “Parasagittal”; misinterpretation of term “sagittal”; use “sagittal”. “Pervitelline space”; vitelline best avoided; use “subzonal space”. “Protodeum”; does not exist in human; use “anal pit”. “Pronephros”; does not exist in mammals; use “rostralmost part of mesonephros”. “Ultimobranchial”; branchial best avoided; use “telopharyngeal”. “Viteline vein”; viteline best avoided; use “omphalomesenteric”. “Vitello-intestinal duct”; vitelline best avoided; use “omphalo-enteric duct”. “Vitellus” no yolk is involved; use “ooplasm”, “cytoplasm”. “Yolk sac”; no yolk is involved; use “umbilical vesicle” [END OF CHART] (p. 12)

RONAN O’RAHILLY AND FABIOLA MULLER, Human Embryology & Teratology (3rd ed.)(New York: Wiley-Liss, 2001): Pregnancy is frequently suggested by the absence of one or more menstrual periods. In obstetrics, use is usually made of the first day of the last menstrual period (LMP), on the assumption that fertilization occurred 2 weeks later. It is further assumed that birth is likely to occur some 280 days after the LMP. The predictive accuracy of the LMP in estimating the time of birth can involve an error of at least plus or minus 3 weeks. Irregularities in menstruation and ovulation as well as inaccurate recall and reporting are key factors in the lack of reliability. Reference to menstrual days or menstrual weeks (i.e., the length of time from day 1 of the last menstrual period) is useful clinically, but the term menstrual “age” in incorrect. The use of the LMP measures the period of amenorrhea (from when no embryo existed), but not prenatal age. The confusing terms “gestational age” and “gestational weeks” should be discarded. They either are not defined or are used indiscriminately both for menstrual weeks and for postfertilizational age. Moreover, gestation, which means pregnancy is variously defined as beginning at the LMP, at fertilization, or at implantation. (pp. 88-92)

RONAN O’RAHILLY AND FABIOLA MULLER, Human Embryology & Teratology (3rd ed.)(New York: Wiley-Liss, 2001): The convenient term morula (Latin, mulberry) can be used for embryos when about a dozen or more cells are present and until the blastocystic cavity appears. The term is not ideal, because it was used originally for amphibians, in which it gives rise to embryonic tissues only and not, as in mammals, to both embryonic and non-embryonic (e.g., chorion, amnion) structures. (p. 37-38)

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)

RONAN O’RAHILLY AND FABIOLA MULLER, Human Embryology & Teratology (3rd
ed.)(New York: Wiley-Liss, 2001): 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. … During the embryonic period proper, milestones include fertilization, activation of embryonic from extra-embryonic cells, implantation, and the appearance of the primitive streak and bilateral symmetry. Despite the various embryological milestones, however, development is a continuous rather than a saltatory process, and hence the selection of prenatal events would seem to be largely arbitrary.(p. 8); … 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, resulting in a one-cell embryo. The two pronuclei do not fuse but their nuclear envelopes break down and form vesicles. The two groups of homologous chromosomes then move together and become arranged on the first cleavage spindle. [[[ i.e., the embryo begins before syngamy.]]] …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); … Prenatal life is conveniently divided into two phases: the embryonic and the fetal… [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)

RONAN O’RAHILLY AND FABIOLA MULLER, Human Embryology & Teratology (New York: Wiley-Liss, 1994): The ill-defined and inaccurate term pre-embryo, which includes the embryonic disc, is said either to end with the appearance of the primitive streak or … to include neurulation. The term is not used in this book. (p. 55); … The term conception, however, may refer either to fertilization or to implantation and hence (like gestation) is best avoided. (p. 19).