Distinctly Human: The When, Where & How of Life's Beginnings
Ever since President George W. Bush announced his decision regarding federal funding of research on human embryonic stem cells last year, newspapers worldwide have reported on various issues surrounding such research. Many people are confused about the status of the embryo. Is it a human being? Is it a person? Is it "one of us"? These are questions not only of biology but also of philosophy.
From experience, all know that a newborn baby is quite different from a child. It is different in size, form, physical ability, mental acuity, and in countless other ways. Everyone realizes also that a child is different from an adolescent, an adolescent from an adult, and an adult from an octogenarian. These differences reflect developmental changes in a person. They are part of the cycle of life.
Neuroanatomists have established that the brain is not completely formed in the womb. The brain of the newborn baby continues to develop anatomically for the next two years. New brain cells appear, differentiate, and migrate to different areas of the brain during this period. Neuroanatomists have established also that when an individual reaches eighteen, about fifty thousand cerebral-cortical cells wither and die each day during the rest of life. Recently we learned that the human brain has the capacity to develop new cells to replace dead ones. Thus the process of growth, decline, and replacement in the brain is a part of the natural cycle of life. Furthermore, the upper part of the brain, the cerebral cortex, serves as a biological substrate for our "personhood."
The ancient Greek philosophers taught that all existing things have substance composed of form and matter. Aristotle, in his Metaphysics, noted that every thing that is has both "being" and "essence." Being indicates an existence--that a thing "is." Essence indicates a nature--which makes a thing "what" it is. The Western philosophical tradition received another central metaphysical/ontological concept from the tradition of Aristotle and St. Thomas Aquinas. This is the notion of being that can account for both the reality of a particular essence and the changes that a particular being may undergo.
In order to understand and to appreciate this dual dimension inherent in the structure of reality, we need to appreciate the importance of another pair of categories, namely potentiality and actuality. Potentiality and actuality bespeak the meaning both of the substance a particular being possesses and the changes a particular being undergoes. A being can be defined in its essential form by its belonging to a particular genus or species. Yet within that reality, the form defined by its particular species and genus, a being may undergo profound changes. Thus, it should be seen not statically, as a being defined once and for all, but dynamically, full of potentialities to be developed historically. These basic metaphysical categories are helpful in understanding the status of the embryo.
Boethius, a Roman Christian philosopher of the sixth century, taught another important philosophical concept. He defined the person as "an individual substance of a rational nature." Philosophers through the ages have found this definition most useful.
Biologists generally define the fundamental unit of life as the cell. Anatomically, the cell has an outer wall, the cell membrane, which surrounds protoplasm where all the life processes of the cell occur (burning oxygen, absorbing food, excreting waste, producing energy, etc.). In the center of the protoplasm is a distinct small mass of material surrounded by another membrane. This is the nucleus of the cell. The nucleus contains paired lines of chemical material, dna--genes--fashioned into many double helixes, the chromosomes. These chromosomes are unique for every species of cell. The nucleus is the "brain" of the cell and directs cell activities through the chemical actions of its genes.
Independent life may exist in a unicellular organism such as the paramecium. Reproduction in unicellular organisms takes place asexually--by simple cell division, mitosis. During this process, the cell body, nucleus, and its chromosomes simply divide, forming two daughter cells complete in every way. Each daughter cell gets a full set of chromosomes containing its species's total genetic material. These genes direct all the life processes in each cell of the succeeding generations. Chromosomes of different species differ in total number but often share genes of the same chemical composition with other species.
But independent life also exists in multicellular organisms. These organisms may have millions upon millions of cells. In humans, the cells of the body develop from a single original cell generated by the union of the sperm cell from the father and the egg cell from the mother. The development of the sperm and egg cells in plants and animals like ourselves who reproduce by sexual union is quite a different process from simple mitosis. The reproductive cells--germ cells contained in a given species's sexual organs--divide to produce the sperm and egg cells by a complicated process called meiosis. Meiosis reduces the species's number of chromosomes in the sperm and eggs to half. Since human cells possess forty-six chromosomes, each human sperm and egg has only twenty-three chromosomes.
When a human sperm penetrates the cell membrane of the human egg, the sperm's twenty-three chromosomes migrate through the egg's protoplasm and pair up with its corresponding twenty-three chromosomes. This process, called syngamy, produces a nucleus containing a full complement of forty-six human chromosomes, the first cell of a unique new living human being with its own entirely distinct chromosomal pattern. All further cell divisions from this unicellular stage of the human being occur by simple mitosis. Every subsequent developed cell, no matter the type, contains forty-six chromosomes. The process of syngamy is popularly called conception. Syngamy takes place in the distal oviduct of the female and is complete only twelve to eighteen hours after sexual intercourse.
During the fertilized egg's passage down the oviduct toward the uterus, it divides rapidly by simple mitosis, becoming a cluster of sixty-four cells. At this stage of its development it is called the morula. Under the microscope the morula resembles a tiny raspberry. The cells of the morula continue to divide by mitosis, and the morula passes down into the uterus. Here the ball of cells folds in on itself to form a double cellular-layered hemisphere, called the blastocyst. The blastocyst forms into a hollow ball that implants itself in the uterine lining.
All this development takes place in an orderly pattern directed by the DNA and proteins that make up the chromosomes, and the chemical interactions between the chromosomes and other proteins which are produced in the blastocyst. Additionally, as some embryologists, but not all, believe, chemical interactions between the cells of the blastocyst and maternal cells in the uterine lining also contribute to embryonic development.
Over the next three or four days, the cells of the inner mass continue to divide by mitosis, now numbering between 128 and 256. Some six to eight mitotic cell divisions from the original fertilized egg, these older cells are no longer toti-potent. Now they have the potential to develop into only the cell types of the various tissues and organs of the fully developed human body, and for this reason they are called pluri-potent cells. Meanwhile, the other mass of cells in the blastocyst forms its outer layer, and ultimately becomes part of the placenta.
By the fourteenth day after conception, the inner mass has grown larger and become pear-shaped. At this time a "primitive streak" appears in the oval part of the pear-shaped mass of cells. This primitive streak is the first recognizable anatomical differentiation of cells. These cells subsequently form the brain. The thinner "neck" of the pear becomes the body of the individual. Soon tissue buds appear, which become the limbs. Further differentiation of other cells occurs and structures begin to appear which later may be clearly identified as gut, spine, and heart. By the ninth week of gestation, the embryo has developed into the fetus, which in appearance resembles a miniature adult.
Some scientists wish to harvest human embryonic stem cells for research purposes. They foresee promising possibilities for cure of many different human diseases by utilizing modified cell lines developed from primitive human stem cells. Some scientists create embryos in vitro from commercial egg and sperm donors, specifically to harvest embryonic stem cells. Other scientists harvest such stem cells from embryos developed, but not implanted, during treatment of childless couples by in vitro fertilization and embryo transfer. In this treatment, four to twelve eggs are harvested at one time. All are fertilized in vitro, but at the most only three or four embryos are transferred back into the womb. This is to avoid the serious obstetrical problems that occur with multiple pregnancies. The remaining embryos are then stored in liquid nitrogen for implantation at a later time.
If the parents decide against another transfer and implantation, the created embryos remain in suspended animation for as long as they remain frozen. Scientists who want to harvest stem cells from these embryos call them "spare embryos." They sometimes point to the natural "wastage" of the process of conception, ascribing moral import to the fact that in all living species the processes of mitosis and meiosis are not always perfect. More often than not during these processes, mistakes occur both in the transcription of the genes and in the division of chromosomes. In species where reproduction takes place sexually, the mistakes may be disastrous, even lethal. Sometimes, instead of developing into "one of us," the cells develop into a rare form of cancer called a chorionic epithelioma or hydatidiform mole. Other such products of conception die at either the zygote, morula, or blastocyst stage. They never reach the implant stage but are discharged in the menstrual flow of the next period. In such occurrences, a woman may never even know that she has been pregnant.
It is estimated that such death occurs in the human species in more than 50 percent of conceptions. Therefore, the question arises: "Do all products of conception fit with our concept of the human embryo?" If not, those who wish to harvest embryonic stem cells from human embryos argue, pre-implantation embryos ought not to command the respect accorded to fetal life at later stages of development. And this move allows proponents of embryonic stem-cell research to conclude that pre-implantation embryos are not inviolable, providing advocates a morally justified source of stem cells for research and for the therapeutic treatment of disease in others.
Such moral reasoning, however, is a prime example of the naturalistic fallacy: One cannot derive moral conclusions from factual information. "What is" can never be the norm for "what ought" to be.
Some ethicists, but not all, believe that it is perfectly moral to obtain human stem cells for research in either of the two ways described above. These advocates give additional reasons for their approval apart from the "pre-implantation argument." Some argue that before the primitive streak appears, the zygote is not a human being and therefore cannot make the same moral claims on us as human persons do. They insist that the zygote does not fit Boethius's definition of a person, since at this stage of development it does not have an anatomically identifiable brain. (They do agree that human embryos are single substances, even those that "bud" from the blastocyst and form monozygotic twins or triplets.) They argue that without a brain, the human embryo cannot be said to have a "rational" nature; thus the zygote lacks the biological substrate for personhood. Consequently, it is permissible to destroy these embryos in order to harvest their stem cells.
Other ethicists take a utilitarian position. They justify killing human embryos at any stage of development to obtain human stem cells, cells needed to correct any number of pathological conditions affecting large numbers of people. They argue that the great goods society will obtain by the use of such cells will far outweigh the evils of such actions.
These arguments are not defensible.
First, the human embryo in its stages of zygote, morula, and blastocyst is a live developing "thing." As such, it has being and essence. Since it is living, its essence--that is, its nature or "whatness"--is determined by its human chromosomal constitution. Its forty-six helical-structured chromosomes are human, not bovine, avian, or reptilian. Thus, it has a human nature and as such is a living human being. The substrata for the "rationality" of the embryo in its early stages of development are found in its chromosomes. In fact, it is the activity of some of these that directs the anatomical and physiological development of the brain. This unfolding process of the whole organism and its individual organs, the cycle of life, is a continuous one. It only begins within the womb.
From the stage that syngamy is completed in the egg, a distinctly unique living human being has come into existence. This being, in its very first stage of anatomical and physiological development, exists as a human unicellular organism, the first form of the human embryo. This development from embryo to fetus, newborn babe, child, adolescent, adult, and eventually octogenarian, is simply an ordered serial development of different stages in the complete cycle of human life. It is potentiality becoming actuality. Each unique living human being is constantly changing every single second throughout his or her cycle of life. A human being is unchangeable and complete only at the moment of death.
The human embryo is, indeed, a living human being from the moment of conception. It is a single substance with a rational nature, of course, unfolding in an ordered fashion from embryo to octogenarian. It has an inviolable dignity. As such, it may never be considered only an object or used as a means to an end. It is an end in itself. It may not be killed so that parts of its body may be used for the good of another person. It merits our full respect, the very same respect with which we hold both socially and legally the newborn babe, the child, the adolescent, the adult, and the octogenarian. Indeed, it is "one of us"!
Dr. Harvey is professor of medicine, emeritus, at Georgetown University Medical Center, Washington, D.C.
A version of this article was published as "Distinctly Human: The when, where and how of life's beginnings" by John Collins Harvey (February 8, 2002) ) 2002 Commonweal Foundation, reprinted with permission. For Subscriptions, call toll free: 1-888-495-6755.