Principles of stem cell biology and cancer: future applications and therapeutics. Edited by T. Regad, T. J. Sayers and R. C. Rees. John Wiley & Sons (2015)
Part II. Cancer stem cells
Adult stem cells are characterized by their tissue longevity (stem cells persist for the lifetime of their owner) and multipotency (the ability to produce all cell types of the tissue to which they belong) (Barker et al., 2012). Early studies by Cheng and Leblond (1974) revealed the presence of a slender cell wedged in between Paneth cells in the small intestine, which they called crypt base columnar (CBC) cells (Figure 9.1). By labelling phagocytosing CBC cells with tritiated thymidine, Cheng and Leblond were able to demonstrate radioactive labelling of more differentiated cells of the crypt. This and other experiments later led Bjerknes and Cheng (1999, 2002) to claim that CBC cells are self-renewing and multipotent ISCs and reside within the stem cell zone of the intestinal crypt. Such findings have paved the way for the development of methodologies that allow for the specific analysis of the cell fates of CBC cells and other cells in the intestine.
Figure 9.1. Schematic of the architecture of the crypt bottoms of the small intestine and colon. (A) Cycling CBC stem cells are located interspersed between Paneth cells in the small intestine, which provide the stem cell niche in this segment of the gut. CBC cells express high levels of Lgr5 (Lgr5Hi ) and function as ancestral cells by generating progenitor cells that form the transit-amplifying zone. CBC functions as a stem cell for all epithelial lineages in the small intestine under conditions of unstressed homeostasis. Stem cells with a lower rate of cell proliferation can be found around the +4 position in the small intestine, and such cells may express or possess elevated activity of Bmi1/Hopx/mTERT/Lrig1+ . Stem cells at the +4 position become actively dividing following conditions of disrupted epithelial homeostasis after tissue injury. (B) In the descending colon, cycling and noncycling stem cells are intermingled between crypt base goblet cells, which are believed to provide the stem cell niche. Frequently cycling stem cells express Lgr5, whereas quiescent stem cells are positive for Lrig1. Cell division generates an anisotropic movement of progeny, which gradually lose contact with the stem cell niche and successively becomes differentiated lineages.
More recent ISC markers have been used to identify such cells in GEMs, primarily in the small intestine but also in the colon. The discovery of such markers through the use of GEMs (typically expressing either fluorescent reporter proteins or LacZ controlled by the promoters of putative ISC-specific genes) has boosted our understanding of ISCs in terms of identifying their spatial location and the role they might play in tissue homeostasis of the gut. In the past, knowledge of ISCs was largely based on combination radiation-exposure experiments and DNA label retention to elucidate their location in the organ. More recent experiments have revealed some novel observations about ISCs under normal homeostasis: they do not divide asymmetrically, they are not quiescent and they can self-sustain their own cellular niche within the epithelial cell population of the small intestine and colon. In some cases, it is possible to isolate intestinal cells expressing stem cell markers and expand them to organoids in vitro using specific culture conditions. The growth pattern of such organoids resembles the tissue architecture of the small-intestinal epithelium and indicates that ISCs may be maintained in the absence of other nonepithelial cells that were previously considered to be critical for the expansion of the intestinal epithelium. These findings have functional relevance to human epithelial tissues. Human colon cells can grow as spheroids in vitro. The colonocytes grow especially well when they express high levels of EPHB2, a receptor tyrosine kinase for the colonies (Jung et al., 2011). This type of culture condition may serve as a tool for the study of mechanisms that control the differentiation and proliferation of ISCs.
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