The Promise and Science of Stem Cells

The human body is made up of millions of cells. The better part of these cells can be described as “highly differentiated.” That is, they have different, highly specialized, functions. Neural cells, heart cells, and liver cells are examples of highly differentiated cells. Each of these differentiated cells has a limited life span. During that span it is capable of dividing a certain number of times before it dies, that is, before it undergoes senescence. With each division, each differentiated cell produce two cells identical to it, two cells with the same specialized function.

Stem cells are unlike highly differentiated cells in three important respects. First, stem cells are less differentiated than other cells; their function is less highly specialized. Second, stem cells do not have such limited life spans; these cells are capable of prolonged self-renewal. Third, when stem cells divide, they not only reproduce themselves, but they produce “daughter cells” that are more highly specialized than they are. Stem cells are vital to the human body. These cells regenerate certain organ systems of the human body through the generation of new cells within those organ systems. For example, hematopoietic (i.e. blood) stem cells replenish our blood supply. When we donate blood, hematopoietic stem cells respond by accelerating the production of replacement blood cells. These are the cells from which others stem, so to speak. Healing and growth depend on these stem cells. All people have stem cells in their bodies which are native to specific organ systems. For quite some time scientists have investigated the role these stem cells play in regenerating the tissues of those systems. In the last decade, however, something new has happened. Scientists have been able to derive stem cells in the laboratory that are not specific to particular organ systems. These stem cells offer the promise of regenerating any tissue in the body.

In August of 1998, for the first time, scientists derived stem cells from a human zygote (a fertilized egg). Working in vitro (“in glass”), the scientists allowed the zygote to develop to the blastocyst stage - about five days.At this stage the zygote becomes a sphere of cells consisting of an outer layer of cells (the trophectoderm), hollow fluid filled cavity (the blastocoel) and a cluster of cells inside this cavity (the inner cell mass). Breaking open the trophectoderm, scientists were able to isolate the inner cell mass, and, placing those cells into a culture medium, cause them to proliferate. Scientists named these isolated cells human embryonic stem cells (or hES cells).

This naming has had some unfortunately consequences. The term “embryonic stem cells” can be misleading in at least two ways. First, by reifying the term embryo, it implies that there is such as thing as “an embryo.” However, as a recent publication of the President’s Council on Bioethics has pointed out, in a strictly technical sense, “there is no such thing as ‘the embryo,’ if by this is meant a distinctive being (or kind of being) that deserves a common, reified name - like ‘dog’ or ‘elephant.’”From the Greek meaning “to grow,” the term “embryo,” used precisely, describes a stage of development - from fertilization until approximately the eighth week of gestation. Hence, properly speaking, in 1998 scientists derived stem cells from the zygote or the blastocyst, not the embryo. They might more properly have been called human blastocyst stem cells.

The term “embryonic stem cells” can be misleading for a second reason. The term “embryo” often evokes an image of an infant-formed creature in miniature, a creature with a head, arms, legs, etc. In fact, at the blastocyst stage the cells of the zygote are virtually undifferentiated, consisting only two types of cells, the cells of the trophectoderm and the inner cell mass. That the blastocyst stage zygote does not yet “look like” a more developed human individual does not, in itself, constitute an salient ethical fact. An organism’s form does not determine its moral status. However, precise concepts are vital to sound and fair ethical reasoning. When forming our ethical arguments we should strive to work with precise language, even though the phrase “human embryonic stem cells” is frequently unavoidable.

Having noted these difficulties with the term “embryo” and with it “embryonic stem cells” how should we proceed? These terms have become ubiquitous in the current discussion, as such, it is virtually impossible to avoid using them. However, when we do use them, we should be aware of their imprecision, remaining attentive to the value of using alternative terms.

Now, let us return to our discussion of the promise and science of human embryonic stem cells. Recall that stem cells found in the body are native to specific organ systems. They have the potential to regenerate the tissues of that organ system; they have the potential to generate multiple different daughter cells. Because of this characteristic, these cells are described as “multipotent.” Recall also, that the stem cells found in the body can renew themselves for a prolonged period. It is thought that in vivo, or “in the body,”these cells can renew themselves and produce daughter cells throughout the life of the organism.

Human embryonic stem cells differ from stem cells found in the body in two important ways.First, whereas stem cells found in the body can self-renew throughout the life of the organism in vivo (the life-span of these cells is considerably shorter in vitro), hES cells appear to be capable of unlimited self-renewal in their pre-differentiated state without genetic deterioration. This characteristic is referred to in scientific literature as “immortality.” Second, whereas stem cells found in the body are multipotent, hES cells are “pluripotent.” They have the potential to produce all cell and tissue types in the human body. Hence, unlike stem cells found in the body, which regenerate the tissues of specific organ systems, hES cells hold out the promise of being able to rejuvenate any and all tissues.

If a scientist could guide stem cells to become specific tissues and develop means of transplanting these cells into the body, stem cell therapy would not merely stop deterioration of a part of our body due to disease or injury; it would regenerate tissue to levels of health and strength that represent our ‘normal’ expectations. Medical researchers hope to discover regenerative therapies for heart disease, liver disease, diabetes, spinal cord injury and paralysis, Parkinsons, Alzheimers; and in related research, nearly every type of cancer. The potential global impact of regenerative medicine is staggering.

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