Definitions of a "Human Organism" and a "Cell"

Dianne N. Irving
October 2004
Reproduced with Permission

Central to the current debates concerning human cloning and human genetic engineering in general, and "stem cell research" in particular (e.g., California Proposition 71), is how to define the difference between a "human organism" and a "cell". Desperate - even amusing - claims to the contrary, there is no confusion in biology as to what a "human organism" is, and how it differs from just a "cell" - regardless if one is talking about sexual or asexual human reproduction. Nor is it difficult to obtain this information. Indeed, all one would have to do is go to the library or internet and look it up. We are talking Biology 101 here (not "physics"1). Yet some will persist in the scientifically ludicrous claim that the immediate product of any human reproductive process is just a "cell", rather than an "organism". Thus, they argue, there is no human being there. So one can reproduce and kill it for its body parts, etc., and pretend one is just doing "stem cell research". Such claims are nothing more than professionally irresponsible scientific "fairy tales" concocted in order to fool the public and unwary politicians and legislators.

There is a critical difference between an "organism" and its constituent cells. Even common dictionaries understand this, e.g., "In biology and ecology, an organism is a living being. The phrase complex organism describes any organism with more than one cell. Characteristics common to many organisms include: movement, feeding, respiration, growth, reproduction, sensitivity to stimuli."2 (emphases added) "In biology, the cell is the fundamental structural and functional unit of all living organisms. The cell theory, first developed in the 19th century, states that all organisms are composed of one or more cells."3 (emphases added)

Thus an "organism" is a whole being, an individual (terms used in the scientific textbooks) - even if that being is comprised of just one cell.

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.4 (emphases added)

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.5 (emphases added)

... Coalescence of homologous chromosomes, resulting 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. (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.6 (emphases added)

In multi-cellular organisms, each of the cells that comprise it are only "parts" of that whole being. An organism is inherently capable of its growth and reproduction as a being; a cell can only multiply more cells, not more beings (unless they are totipotent, separated from the whole organism, and the state of differentiation of their DNA is reversed to "zero" by the process of "regulation" - the basis of cloning by "twinning".7).

O’Rahilly addresses "human organisms", their distinction from just "cells", and their growth and development in his first chapter dealing with the science of human embryology. As O’Rahilly documents, the immediate product of human sexual reproduction is a single-cell organism:

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.8 (emphases added)

It is precisely because the immediate product is an organism that the international Nomina Embryologica Committee formally rejected the fake term "pre-embryo". As O’Rahilly put it:

(4) it [the term "pre-embryo"] is equivocal because it may convey the erroneous idea that a new human organism is formed at only some considerable time after fertilization;9 (emphases added)

Rather, the single-cell human organism - the human being, human embryo, human individual - simply then proceeds to grow bigger:

Human embryology ... is the study of the human embryo and fetus. ... Development includes growth (an increase in the mass of tissue) and differentiation, by which is meant increasing complexity. Although early development, particularly that of the embryo, is the main focus of embryology, development continues after birth as well as before. ... Development is under the control of the genome, which operates at several levels of organization. A reductionist approach, however, needs to be complemented by descriptive embryology so that the end products of genetic and environmental interaction, mainly organs and systems, can be clearly discerned. (p. 7) ... Growth, strictly speaking, is an increase in the size of an organism or of its parts. ... The chief cause of prenatal growth is cellular division. Growth is ordinarily accompanied by the specialized cellular changes that constitute differentiation. Although growth is very marked prenatally and during the first two decades postnatally, it continues throughout life.10 (emphases added)

This is the real science, not the fairy tale science.


1 See Irving, Comments: ‘Review: Blueprint for the Earth - and Junk Physicists’" (Sept.5, 2004), at: [Back]

2 Open Encyclopedia, at:; also Wikipedia, at: [Back]

3 Open Encyclopedia, at: [Back]

4 Keith Moore and T. V. N. Persaud, The Developing Human: Clinically Oriented Embryology (6th ed. only) (Philadelphia: W.B. Saunders Company, 1998), p. 18. [Back]

5 William J. Larsen, Human Embryology (New York: Churchill Livingstone, 1997), p. 1. [Back]

6 Ronan O'Rahilly and Fabiola Muller, Human Embryology & Teratology (New York: Wiley-Liss, 2001), p. 87. [Back]

7 For a scientific explanation (with extensive scientific references) of the role of "regulation" in both sexual and asexual reproduction, see Irving, "Playing God by manipulating man: Facts and frauds of human cloning" (October 4, 2003), presented twice at the Missouri Catholic Conference Annual Assembly Workshop, Jefferson City, MO, at:, and at; see also "Stem cells that could become embryos: Implications for the NIH Guidelines on stem cell research, the NIH stem cell report, informed consent, and patient safety in clinical trials" (July 22, 2001); written as consultant on human embryology and human embryo research as Fellow of The Linacre Institute (CMA), The Catholic Medical Association (USA), and The International Federation of Catholic Medical Associations (FIAMC), at: [Back]

8 Ronan O'Rahilly and Fabiola Muller, Human Embryology & Teratology (New York: Wiley-Liss, 2001), p. 31. [Back]

9 Ibid., p. 88. [Back]

10 Ibid, p. 98. [Back]