The Pre-Implantation Embryo Revisited: A Two-Celled Individual or Two Individual Cells?

Cibelli Jose B. , Kiessling Annn A. , Cunniff Kerrianne , Richards Charlotte , Lanza Robert P. , and West Michael D. “Somatic cell Nuclear Transfer in Humans: Pronuclear and Early Embryonic Development,” e-biomed; The Journal of Regenerative Medicine, Volume 2; November 26, 2001 at: http://www.liebertpub.com/EBI/defaultl.asp

Ford Norman “The Early Human Embryo as Person in Catholic Teaching,” National Catholic Bioethics Quarterly 1 (2001): 155–160, p. 160.

For representative formulations of this proposal, see Ford N. , When Did I Begin? (Cambridge: Cambridge University Press, 1988), pp. 116–131, and H. Kuhse and P Singer, “Individuals, Humans and Persons: The Issue of Moral Status,” in Singer P. , Kuhse H. , Buckle S. , Dawson K. , Kasimba P. , Eds., Embryo Experimentation (Cambridge: Cambridge University Press, 1990), pp. 65-75.

For example, this position was endorsed in an unsigned opinion piece in the prestigious scientific journal Nature as one reason for the licitness of embryonic stem cell research. In fact, as the essay points out, the British Parliament in 1990 was persuaded to permit experimentation on pre-gastrulation embryos precisely by this argument. See “The Meaning of Life,” Nature 412 (2001): 255.

For details and citations to the relevant literature, see Johnson Martin H. “Mammalian Development: Axes in the Egg?” Current Biology 11 (2001): R281–R284.

Piotrowska Karolina , and Zernicka-Goetz Magdalena “Role for Sperm in Spatial Patterning of the Early Mouse Embryo,” Nature 409 (2001): 517–521.

Piotrowska Karolina , and Zernicka-Goetz Magdalena “Role for Sperm in Spatial Patterning of the Early Mouse Embryo,” Nature 409 (2001), p. 521.

Gardner R.L. “Specification of Embryonic Axes Begins Before Cleavage in Normal Mouse Development,” Development, 128 (2001): 839–847.

For details, see Ciemerych M.A. , Mesnard D. , Zernicka-Goetz M. “Animal and Vegetal Poles of the Mouse Egg Predict the Polarity of the Embryonic Axis, Yet are Nonessential for Development,” Development 127 (2000): 3467–3474.

Even more recent lineage tracing studies have confirmed this discovery. See Piotrowska K. , Wianny F. , Pederson R.A. , and Zernicka-Goetz M. “Blastomeres Arising from the First Cleavage Division have Distinguishable Fates in Normal Mouse Development,” Development 128 (2001): 3739–3748.

Goodwin B. “Development as a Robust Natural Process,” in Stein W. , and Varela F.J. , eds., Thinking About Biology (Reading, MA: Addison Wesley Publishing Co., 1993), pp. 123–149.

For instance, theologian Lisa Sowle Cahill suggests that since scientists have shown that fertilization is a process, it cannot be the moment when individuality is specified. See her “Abortion, Sex and Gender: The Church's Public Voice,” America 168 (1993): 6–11. A similar argument is made by Shannon T.A. , and Wolter A.B. in their “Reflections on the Moral Status of the Pre-Embryo.” Theological Studies, 51 (1990): 603-626, pp. 606-608.

No one single study reports this result. The calculation for this is as follows. Studies of body composition show that the human body is made up primarily of eleven elements. However, 99% of the total atoms are H. O. C. and N. Further, nearly all of these atoms are found as water, protein, fat and carbohydrates which are the four largest categories of molecular components of the human body. Approximate replacement rates for these components (assuming for simplicity that there is no weight gain or loss) can be calculated for the 70-kg reference man. Water: In the reference man, 42 kg is water with a replacement rate of approximately 10% per day. Consequently, 99% of the total water in the human body will be replaced de novo in approximately 60 days. Protein: In the reference man. 10.5 kg is protein with a replacement rate of approximately 1% per day (Turnover rate: 250g/day with absorption rate of 184 g/d.). Consequently, 99% of the total protein in the human body will be replaced de novo in approximately 450 days. Fat: In the reference man, 12 kg is fat with a replacement rate of approximately 0.71% per day (Turnover rate: 210g/day with absorption rate of 142 g/d). Consequently, 99% of the total fat in the human body will be replaced de novo in approximately 650 days. Carbohydrates: In the reference man, 0.5 kg is glycogen (carbohydrates) with a replacement rate of approximately 35% per day (Turnover rate: 220g/day with absorption rate of 800 g/d). Consequently. 99% of the total carbohydrates in the human body will be replaced de novo in approximately 11 days. In sum. the component with the limiting replacement rate is fat. We can conclude therefore that 99% of 99% (that is, 98%) of the total atoms of the human body are replaced in approximately 650 days, which is less than two years. In fact, this would be an overestimate, given the higher turnover rates of the other bodily components. Note that less than 1% of the total atoms in the body (composed primarily of minerals and rare elements) have a negligible replacement rate. If one held the non-substantial view of the human body, could one claim that conservation of these few atoms is responsible for maintaining personal identity? In response, I return to the rowboat analogy. Would the conservation of a few nails in the transformation of the rowboat into the speedster be sufficient to maintain that the original boat remains? I think not. Finally, it is noteworthy that atomic replacement also occurs throughout the neural system which for many materialists is responsible for maintaining personal identity in the human being. This replacement of atoms takes place during normal metabolism within cells and occurs even in the absence of cellular replacement or replication. Body composition data was obtained from Report of the Task Group on Reference Man, International Commission on Radiological Protection (New York: Oxford University Press, 1975). Absorption/digestion rates were taken from R.M. Berne and M.N. Levy, eds. Physiology, 4th edition (St. Louis: Mosby, 1998) cited in “Digestion and Absorption: Lecture 61,” presented by Prof. Wechsler at: http://www.mmi.mcgill.ca/Unit4/Wechsler/lect61digestionabsorption.htm. accessed on November 8, 2001. Maximum values of the indicated range were used in the calculation. Turnover rates were communicated to me by (water) P. Spanel and D. Smith [See their paper with S. Davies, “Rapid Measurement of Deuterium Content of Breath Following Oral Ingestion to Determine Body Water,” Physiol. Meas. 22 (2001): 651-659]; (protein) D. Matthews [See his paper, “Proteins and Amino Acids,” in Modern Nutrition and Disease. 9th edition. Shils M.E. , Olson J.A. , Shike M. , and Ross A.C. , eds. (Baltimore: William & Wilkins. 1999), pp. 11-48]: and (carbohydrates and fats) R. Wolfe [See his book. Radioactive and Stable Isotope Tracers in Biomedicine: Principles and Practice of Kinetic Analysis. (New York, Wiley-Liss, 1992)]. I thank these individuals for their assistance.

For details, see Agata K. , and Watanabe K. “Molecular and Cellular Aspects of Planarian Regeneration,” Semin. Cell Dev. Biol. 10 (1999): 377–383, and W.A. Muller, “Pattern Formation in the Immortal Hydra,” Trends. Gen. 12 (1996): 91-96.

For details, see Layde P.M. , Erickson J.D. , Falek A. , and McCarthy B.J. “Congenital Malformation in Twins,” Am. J. Hum Genet. 32 (1980): 69–78, and B. Luke and L.G. Keith. “Monozygotic Twinning as a Congenital Defect and Congenital Defects in Monozygotic twins,” Fetal Diagn Ther 5 (1990): 61-69.