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Transcriptional Characterization of Glioma Neural Stem Cells Diva ...

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3.1 The Cancer <strong>Stem</strong> Cell Hypothesis Introduction<br />

Figure 3.1: <strong>Stem</strong> cell differentiation hierarchy, in which a possible increase in plasticity<br />

is highlighted that may be present within cancer populations. Such plasticity<br />

would be the enabler for the bidirectional interconvertibility between cancer stem<br />

cells and non-cancer stem cells. Adapted from Gupta et al 2009 [182].<br />

tissues. This type <strong>of</strong> cell division is referred to as "asymmetrical division" or<br />

"asymmetrical self-renewal" and is specifically characterised by adult stem cell<br />

divisions that produce a new adult stem cell together with a non-stem cell sis-<br />

ter that becomes the progenitor for short-lived, differentiating, functional cells<br />

that in most cases mature to a terminal division arrest [420]. For the vast ma-<br />

jority <strong>of</strong> adult tissues, it remains unclear how stem cells succeed in maintaining<br />

a precise balance between proliferation and differentiation in steady-state. To<br />

explain their long-term viability, it has been argued that tissue stem cells are<br />

maintained in a long-lived quiescent state, with most divisions supported by<br />

differentiating progenitor cells that ultimately exit the cell cycle and are re-<br />

placed by stem cell progeny, which provides a mechanism for protecting stem<br />

cells from damage and loss throughout adult life (Fig 3.2). Clear evidence<br />

for asymmetric stem cell divisions is found in invertebrates C. elegans and D.<br />

melanogaster, although in recent lineage-tracing studies in mammals it was<br />

shown that stem cells behave as an equipotent population, in which the bal-<br />

ance between proliferation and differentiation is achieved through frequent and<br />

stochastic stem cell loss and replacement [240]. While the asymmetrical self-<br />

renewing properties <strong>of</strong> adult stem cells are particularly important for homeo-<br />

static control in adult tissues that undergo continuous cellular turnover such<br />

as epithelium and blood [457,486], the non-stem cells <strong>of</strong> most adult tissues go<br />

through a rapid cycle in which they are born, mature, expire, and are removed<br />

from the tissue by apoptosis. The fact that the rate <strong>of</strong> adult cellular turnover<br />

is much faster than that <strong>of</strong> tumour development is the basis for the hypothesis<br />

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