By Dr. David Fleischacker
[This springs from a series of blogs titled “When does the human being begin to exist?” which I had written starting in December, 2007. I had drafted this piece in 2008, but just now finished it.]
Some will argue that an embryo becomes a human being at implantation in the uterine wall. In the business world, this became one of the arguments for those selling the contraceptive pill as well as the morning after pill. These companies could argue that their pill was not an abortifacient, because though it might prevent implantation (as one of the ways for preventing a pregnancy), this did not kill a human being because that “this” was not yet human.
To a philosopher who reads through a biology text on implantation, the argument may sound a bit arbitrary at first glance. Why is something different after implantation? However, there are some biological reasons for saying this. This becomes apparent if one examines the various developmental stages of the embryo.
Early Stages of Development
In any mammal, the first stage of development begins at the moment of fertilization. Sperm entered into an oocyte through a protective layer originally created by the mother called the zona pelucida (or the ZP as it is usually labeled). For many types of animals, the entry location of the sperm then determines a polarity to the cell. Polarity refers to different layouts of the biochemical schemes and constituents of the cell, such that as it begins to divide, these materials begin to cause differences in the way subsequent daughter cells function. Such differences in subsequent daughter cells that are created through mitosis are called cell differentiation. As the zygote begins to divide, the daughter cells form a mass of cells scattered within the housing of the ZP. This is called the morula stage. As cell division continues, some of the cells begin to form a ring called the trophoblast just on the inside of the ZP. Other cells come to fill the inner ring and a blastocoel forms pushing these inner cells to one half of the ring, leaving the fluid called the blastocoel on the other half. This is the beginning of the “blastocyst” stage.
Many texts will identify the “inner cell mass” as that which becomes the adult organism, because it is from this that the matured cell systems of the organism develop. The cells that form the trophoblast are not the source of cells that continue into adulthood. However, not all of this inner cell mass will become the adult either. Once the blastcyst bursts the ZP, it is now possible for the blastocyst to unite with the wall of the mother, which usually occurs in the uterus (though if it bursts from the ZP in the fallopian tube, it could bond at that location causing an ectopic pregnancy). Some of these cells will form part of the placenta with some of the cell schemes attaching to the uterine wall, others to form the amniotic cavity. In other words, only some of these cells of the inner cell mass will become the matured adult systems.
When implantation occurs, there is a further determination or differentiation of these cells such that one can then identify specific cells that will become the adult. This process leads to the gastrula stage where some of the cells then form into a primitive streak and into a node of cells that become important for the induction of further differentiation of cells. Because of this differentiation that determines cell fates, twinning is no longer possible, and hence, this is the reason that some will argue that life begins at this point of differentiation.
The basis for saying human life begins at implantation
The notion implicit in this search for the beginning of life is the search for the determined originating cells that will lead to the matured systems of the adult, such as the circulatory and immune systems. In the zygote, the cell is not yet determined, it could be split multiple times and thus form twins or triplets. Likewise for the morula and blastocyst stages. So in many standard textbooks, a particular life or thing does not yet exists at these earlier stages.
Hence in the language of many textbooks, the “real embryo” is that which arises from those cells which have reached a stage of determined fate.
Shifting the Basis to the beginning of the Unity-Identity-Whole (see Chapter 8 of INSIGHT for more on Unity-Idenity-Whole)
The argument needs to shift seeking the origin of human life from that of the “fate determined cells directly leading to the adult organism” to the origin of the “unity-identity-whole.” One way to think through this is in the following way: At different stages of existence, one and the same being has different relationships to its environment. This is rooted in Lonergan’s point that the unity is a unity in changes (INSIGHT, chapter 8). One sees this after birth. In early stages, young infants nurse from the mother’s milk which has nutrients suited to these first post-natal stages. As the infant grows into a child, a young calf, a kit, or some other pre-adult creature, its abilities to relate and interact with the environment expand in such activities as the food it eats and its mobility. This ongoing horizontal and vertical differentiation and expansion of the creature to the environment is no less true in human beings who have the lengthiest sequence of stages of growth from infancy through childhood, adolescence, and into various phases of adulthood. It is the same unity-identity-whole through all these developmental changes.
This is true as well for the unborn. A zygote is largely related to the zona pleucida, that protective coat of cells formed by the mother when the oocyte was first formed. As the zygote divides within the context of the ZP, it grows until the ZP bursts, at which stage (during the blastocyst stage) it has interiorly differentiated sufficiently to become related to the uterine lining and the environment of the uterus. Hence, differentiations of cells and cells systems at each stage are really part of one and the same being, but having different purposes. The embryonic stage, for example, includes the cell systems that form the trophoblasts, which will then form the amniotic cell system as well as the placenta cell system. These are not distinct from the being of the embryo, but rather “parts” of that whole, in the same way that the immune system and the circulatory system are “parts” of the whole adult organism. The functional meaning of these cell systems at this early stage are grounded upon unity with the entire growing embryo and fetus. Separate these from the whole, and these will loose their wholistic properties. The plancenta for example is not merely an aggregate of cells, but rather it is like the digestive and respiratory systems tied together. The cells of the digestive system and the cells of the respiratory system collaborate in a functional whole which is quite different from each of the properties of the individual cells composing these systems (no one cell digests and no one cell respires). Likewise for the placenta and its role in exchanging nutrients and respiration. Each cell has a function that is part of a whole order of cells. Disconnect the placenta from its relationships both to the mother and to the other cells in the embryo, and it looses this higher intelligible meaning. The cells may still survive for a time, but not in a united way that makes them part of a nutritive scheme. And this nutritive scheme is one that belongs to the unity called the embryo. It is not a scheme of the mother even though it is related to the mother. Again, this is much like the lungs which have a relationship to the atmosphere that is breathed. The lungs are schemes not of the atmosphere, but of the creature that breaths.
Thus, though implantation does bring about some differentiating cell schemes, it is not the beginning of a new unity-identity-whole, but rather the continued differentiation of an already existing unity-identity-whole. The somewhat confusing language in the world of developmental biology and thus in many text books has led to these ideas that the embryo was distinct from some of these temporary “parts” of the embryo. However upon closer examination, the “embryo” as a unity is not one distinct thing and the trophoblastic set of cells another. Furthermore, though this confusion suggests that implantation might be a valid starting point for the organism, the argument here is to eliminate that confusion and shift to the origin of the unity-identity-whole that develops. [Note: Though this point is more or less correct, to be more precise, it is a shift that looks not merely at fated originating cells that will develop into adult schemes, but to a unity-identity-whole that is differentiated both by its current integration and unfolding through its operators into its next stages, and then asks, what is the first stage of “this thing”]. Hence one is moving away from defining this thing and its starting point in terms of a developmental stage, and shifting really to a search for the initial stage with its finality for all the subsequent stages. It does not mean that cell fate is not relevant, but it puts it within the different functions of the cells and cell systems that relate the intrinsic cell schemes to the chemical and cellular world of the creature. Hence, in this larger functional set of relationships in which the unity-identity-whole thrives, the cells that form the placenta and other support functions are just as much a part of the unity as are the cells that form the primitive ectoderm that come to constitute the systems of the adult creature. The fact that they “disappear” at latter stages does not change the central form to which they belong at these earlier stages.
This shift results in turning not to implantation as the starting point of a living thing, but the zygote, since the zygote has the real finality to develop into a mature adult organism (even if twinning occurs). This argument was made in an earlier blog with greater precision however and does not need to be made here.