Two or three months ago, a question was asked on a forum that I track intermittently. It was from a student taking a biology course who was surprised by the amount of controversy elicited on the subject of stem cells. (Wait until he gets to evolution!) He wanted some input into what it really means to be alive—and swerved from there to “What is Reality?” which the student thought might be a related topic.
The responses in the forum—a spiritual forum hoping to fabricate a new myth for our time—seemed rather vaporous, largely evoking the “Mystery.” They all seemed unsatisfactory to me, or would have been, had I been the student. At the time, I wanted to contribute something substantive but was far too busy to give the issue the attention and thought it deserved. The printout of question and answers has remained in my to-do pile however, and I’ve recently dug down to it (task archeology). So I’ll try to write a more substantive answer. At least I’ll try to focus on unpacking the issues as I see them, with the aim of offering something more concrete than “mystery.”
Ultimately, of course, mystery will figure in, because we cannot truly know the way the world is constructed, given our limited senses and brains. But we can use the microscope of science and reason to probe those three topics in considerable depth. Would it be possible to do so in the space of a blog entry? Of course not. It will probably require two or three entries just to sketch in the bones of the issue. But let’s try. The logical structure of such an exploration would probably proceed from Reality to Life to Stem Cells, in order of decreasing comprehensiveness, like boxes within boxes. However, since the original topic was stem cells, and that issue gave rise to the other two topics, I will proceed in that direction.
First of all: “What are stem cells?”
Stem cells are cells with a full complement of genetic material that are able to differentiate into several different types of cells with different functions. In this definition, I’ve used words and phrases that are not part of standard everyday usage but are well understood by biologists. Full complement of genetic material means a cell has all the genetic information necessary for producing a whole organism; in the case of humans and most other animals, this means a diploid cell, or a cell that has chromosomes (and genes) from two parents. Differentiate is a verb that means to transform from one, rather general functional capacity to another, more specialized functional capacity. This involves permanently turning off some genes and turning on others. Different types of cells means cells with different functions, such as liver cells compared with cells of the pancreas. Most cells in the body are not stem cells because they are already fully differentiated or specialized. And differentiated cells rarely divide or reproduce themselves.
Stem cells may be “pluripotent,” that is, they may be able to differentiate into many different cell types. This sort of stem cell is plentiful in embryos and is rare in adult individuals. Other stem cells may be partially differentiated; for example, they may be able to develop into one type of blood cell or another type of blood cell, but they cannot produce a skin cell. Other stem cells are highly differentiated, such as those cells that produce only skin cells or muscle cells when stimulated to do so.
The signals for cellular differentiation come from the surrounding environment. Chemical signals may arrive through the blood stream, but more often they come from surrounding cells. Physical signals such as stretch and pressure may also play a role. Thus, if you transplant a pluripotent stem cell into a tissue, it will receive instructions from surrounding cells, which form a sort of cellular cultural milieu, telling the stem cell what traits it should develop.
So how does the issue of stem cells impact on the question, “What is life?” A cell is alive if it is able to survive in its environment by taking in nutrients, expelling wastes, and producing biological molecules. In other words, all cells capable of independent metabolizm are alive. This is commonly accepted by biologists. (And by this definition, viruses are not alive because they do not metabolize on their own.)
However, I think the philosophical or ethical question implied originally was: “Are human stem cells actually micro-humans?” The answer to this question has to be “No.” Only a developed human being is actually a human, with the rights and privileges (and ethical consideration) due to a person. A cell is not a person. Our bodies contain many stem cells, but we do not thereby consider that we are several persons in one body.
It is an absurdity and a failure of education in this country that so many Americans think a single cell—or even a group of cells—should be considered a human being.
To be continued: What is life?
BTW, the original questioner and questions were at: