Stem cells

What is a stem cell? The term is a combination of ‘cell’ and ‘stem’. A cell is a major category of living thing, while a stem is a site of growth and support for something else. In science today, a stem cell is defined as a cell derived from a multicellular organism, which is able to both self-renew (produce more stem cells of the same kind) and differentiate (produce cells corresponding to later developmental stages of the source organism). So the concept of a stem cell is somewhat complex, bearing on questions of biological individuality, relations between cells and organisms, and our understanding of development. Stem cell phenomena range from everyday to extraordinary laboratory products. On the everyday side: hair, skin, and blood cells are shed and replaced by ongoing stem cell activities. Stem cells help maintain organs and tissues in mature multicellular organisms. Regeneration in wound-healing is also, often, stem-cell mediated. The hydra’s mythic regeneration potential is due to its plentiful stem cells; similarly for plants. Looking to earlier developmental stages, embryonic cells also exhibit stem cell capacities. If such cells are removed from an early embryo and grown in artificial cell culture, this produces an embryonic stem cell line – an indefinitely renewable source of cells that can, under appropriate conditions, develop to produce many (even all) cell types found in a mature organism. Other experimental products of stem cells include embryoid bodies, organoids, and embryo-like structures. Stem cells are thus found in living organisms (in vivo) and grown artificially (in vitro).

Stem cells raise several important metaphysical questions for philosophers of biology. One concerns biological individuality. Multicellular organisms are paradigmatic biological individuals. There are strong reasons to think cells are individuals. Stem cells are cells that divide and develop into other kinds of cell, tissues, organs, and even analogues of whole organisms. Are stem cells individuals? One way to answer this question is in terms of cell lineages. Complicating matters, stem cells mediate between cell and organismal levels of biological organisation. This raises questions about individuality and development for organisms and constituent cell lineages. Metaphysical theories about the nature of stem cells – natural kinds, causal mechanisms, processes – are also unsettled, as is the science. Different metaphysical theories about the nature of stem cells present a problem of theory choice. Alternatives include: stem cells as entities, stemness as a state, disposition to develop, and cell-environment systems.

Our knowledge about stem cells is incomplete, based on many different kinds of experiment. The main ways of identifying stem cells are to find, grow, or make them: cell-sorting, in vitro culture, and reprogramming, respectively. The basic design is to remove cells from an organismal source and place them in an environment where they can self-renew. After measuring cell traits in this environment, some cells are moved to a new environment to encourage differentiation. Cell traits in the new environment are then measured. The results correlate traits of an organismal source, candidate stem cells, and differentiated cells. Collectively, these experiments yield many different varieties of stem cell. Characterisation of these varieties is closely tied to technologies and experimental methods for culturing, visualising, and manipulating cells. Uncertainty is a constant, however. It’s impossible to experimentally show that a single cell is a stem cell; all methods of identifying stem cells require populations of homogeneous stem cells. But homogeneity for cells that by definition transform into other things is a fragile assumption. Consequently, stem cells are identified relative to particular experimental methods. Our knowledge of stem cells accumulates by multiplying experimental contexts and relating their outcomes to one another.

In practice, knowledge about stem cells has the form of a proliferating network of models. In vitro stem cells are a prominent example: concrete approximations of early developmental stages of a multicellular organism of a particular species. Other important stem cell-based models are organoids and human-animal chimeras. Different stem cell models complement one another, highlighting different aspects of development. More generally, stem cell biology is replete with abstract and concrete models. Social organisation of experiments and resultant models is important for understanding the epistemology of stem cell research. Abstract models play a less prominent role in stem cell research, although lineage tree models are important representations of stem cells and their potential. Classifying stem cells is an unsettled and messy affair, with many different cross-cutting or overlapping distinctions used in practice. There are many varieties of stem cell, but no single agreed-upon system for classifying them. Lineage tree models offer one prospect for such a system.

In popular culture, stem cells are associated with medical promise on the one hand, and embryo destruction on the other. Stem cells are tokens of medical promise and hope; the idea being to use their potential to cure a wide range of injuries and diseases. This promise motivates stem cell ‘clinics’ alongside scientific research. The former peddle cures for many ailments unencumbered by scientific evidence or regulatory approval. The latter challenged by ethical questions about human embryo research. Tension between medical hopes and objections to human embryo research has produced a large bioethics literature. Key ethical debates are about research using human embryos, creating human–animal chimeras, and how to balance hope and hype in regulating and funding stem cell research. Broad anti-science cultural movements encourage proliferation of stem cell ‘clinics’ that market alleged cures directly to consumers, bypassing scientific and medical standards.