The therapeutic potential of stem cells

Abstract

In recent years, there has been an explosion of interest in stem cells, not just within the scientific and medical communities but also among politicians, religious groups and ethicists. Here, we summarize the different types of stem cells that have been described: their origins in embryonic and adult tissues and their differentiation potential in vivo and in culture. We review some current clinical applications of stem cells, highlighting the problems encountered when going from proof-of-principle in the laboratory to widespread clinical practice. While some of the key genetic and epigenetic factors that determine stem cell properties have been identified, there is still much to be learned about how these factors interact. There is a growing realization of the importance of environmental factors in regulating stem cell behaviour and this is being explored by imaging stem cells in vivo and recreating artificial niches in vitro. New therapies, based on stem cell transplantation or endogenous stem cells, are emerging areas, as is drug discovery based on patient-specific pluripotent cells and cancer stem cells. What makes stem cell research so exciting is its tremendous potential to benefit human health and the opportunities for interdisciplinary research that it presents.

Figure 1.

Origin of stem cells. Cells are described as pluripotent if they can form all the cell types of the adult organism. If, in addition, they can form the extraembryonic tissues of the embryo, they are described as totipotent. Multipotent stem cells have the ability to form all the differentiated cell types of a given tissue. In some cases, a tissue contains only one differentiated lineage and the stem cells that maintain the lineage are described as unipotent. Postnatal spermatogonial stem cells, which are unipotent in vivo but pluripotent in culture, are not shown (Jaenisch & Young 2008). CNS, central nervous system; ICM, inner cell mass.

Figure 2.

The cancer stem cell hypothesis. The upper tumour is shown as comprising a uniform population of cells, while the lower tumour contains both cancer stem cells and more differentiated cells. Successful or unsuccessful chemotherapy is interpreted according to the behaviour of cells within the tumour.

Figure 3.

The stem cell niche. Stem cells (S) are shown dividing symmetrically to produce two stem cells (1) or two differentiated cells (D) (2), or undergoing asymmetric division to produce one stem cell and one differentiated cell (3). Under some circumstances, a differentiated cell can re-enter the niche and become a stem cell (4). Different components of the stem cell niche are illustrated: extracellular matrix (ECM), cells in close proximity to stem cells (niche cells), secreted factors (such as growth factors) and physical factors (such as oxygen tension, stiffness and stretch).