Stem cell myths

Abstract

Stem cells, although difficult to define, hold great promise as tools for understanding development and as therapeutic agents. However, as with any new field, uncritical enthusiasm can outstrip reality. In this review, we have listed nine common myths that we believe affect our approach to evaluating stem cells for therapy. We suggest that careful consideration needs to be given to each of these issues when evaluating a particular cell for its use in therapy. Data need to be collected and reported for failed as well as successful experiments and a rigorous scientific approach taken to evaluate the undeniable promise of stem cell biology.

Figure 1

A model of how pluripotent embryonic stem cells undergo maturation and progressive restriction in their developmental potential. Initially, tissue-specific stem cells, e.g. neural, haemopoietic, hepatic, etc., are generated. These stem cells proliferate and may mature during development but retain their differentiation ability. Tissue-specific stem cells do not generate fully differentiated cells directly, but do so by first generating intermediate precursors or blast cells that undergo limited proliferation and ultimately mature to generate fully differentiated progeny.

Figure 2

Different models of stem cell division. In model A, each stem or progenitor cell gives rise to a different cell type requiring a different stem cell population for each tissue. Models B and C demonstrate the possibility of a multipotent stem cell generating different types of precursors by asymmetric division. Model B presupposes stem cell maturation over time in response to extrinsic signals, thus it does not generate truly identical daughter cells. Model D displays a model of symmetric cell division without the necessity of asymmetric division.

Figure 3

Age-related changes in stem cells. Although stem cells persist in different tissues and maintain certain traits throughout life, several changes can be found dependent upon the age at which the cells are examined. Environmental stressors and increasing failure of the support system can account for some of the observed changes.

Figure 4

Transdifferentiation and dedifferentiation. A change in phenotype could happen in two ways: a direct transformation (transdifferentiation) or by a regression to earlier developmental stages (dedifferentiation).

Figure 5

Homing in the stem cell niche. The process of homing to a stem cell niche involves a complex orchestration of different soluble, cellular and extracellular signals, which eventually lead to a stable stem cell population capable of self-renewal and differentiation. Stem cells follow a chemokine gradient across an endothelial cell barrier to home to a niche provided by many different cell types. These niches in the adult are usually limited to very distinct areas (e.g. the SVZ). Under less than ideal circumstances, this process can fail at multiple levels (some of them indicated by crosses).