**4. Properties of gastric cancer stem cells**

Tumor cells are heterogeneous in terms of mutations carried, susceptibility to drugs, markers expressed or morphology, and not all are tumorigenic. This genetic heterogeneity would come not only from intrinsic factors such as genetic mutations acquired progressively and amplified within new clones but also from extrinsic factors related to the variation of the tumor microenvironment [54, 55]. To explain these observations, two concepts have been proposed: the cancer stem cell (CSC) theory, also named the hierarchical model, and the stochastic model. In the stochastic model, all cancer cells have similar tumorigenic properties, with cancer arising after a series of genetic and epigenetic events leading to successive waves of clonal selections depending on the proliferative and survival benefits acquired. In the hierarchical model, there is a cellular hierarchy between cancer cells inside the tumor, with CSCs being at the origin of the more or less differentiated cells, not all proliferative and tumorigenic, composing the tumor mass. CSCs represent a small percentage of tumor cells and possess particular properties compared to non-CSCs (**Figure 3**): (1) the first and most important is their capacity to self-renew and divide asymmetrically and to generate a new CSC and a non-CSC progenitor cell, a property that maintains a constant CSC pool; (2) CSCs are able to initiate tumor growth when injected in low cell numbers in immunocompromised mice; (3) CSCs display differentiation properties giving rise to the more or less differentiated cells composing the tumor mass, reconstructing the tumor heterogeneity observed within the primary tumor; (4) CSCs have increased resistance to current chemo- and radiotherapies; and (5) CSCs express

**Figure 3.** Hierarchical model illustrating the heterogeneity of the tumors. (a) Representative images of CD44 detection by immunohistochemistry on tumor tissue section (left panel) and of the detection of CD44 by immunofluorescence and ALDH activity (Aldefluor™ reagent) with Hoescht 33342 dye (middle panel) on tumor spheres of MKN45 gastric cancer cells (right panel, phase contrast microscopy). Scale bars, 50 μm. (b) Hypothetical strategies to target CSC to cure gastric cancer. The main gastric CSC markers are CD44 and ALDH activity. Cancer stem cells represent a subpopulation of cells implicated in tumor initiation, growth, metastasis, and chemo-/radio-resistance.

specific markers [55]. This hierarchical model is not exclusive but is now the most accepted model with the recent identification of CSCs in most cancers since their first discovery in acute myeloid leukemia in 1995, then in solid tumors in 2003, and more recently in gastric carcinoma. However, we must keep in mind that this hierarchical model is also subjected to clonal evolution even if it has not been clearly demonstrated for gastric carcinoma [56].
