2. Cancer stem cells
Cancer is characterized by the excessive and uncontrolled expansion of abnormal, malignant cells that display morphological, proliferative, and functional heterogeneity. Morphological heterogeneity is further manifested in tumor cells of variegating size, shape, thickness, nucleus/cytoplasm ratio, etc. In order to explain this tumor cell heterogeneity, two models have been proposed, one being the cancer stem cell (CSC) concept [47] and [48]. This model postulates that, akin to growth of normal proliferative tissues, growth of tumors or expansion of a tumor clone is driven by a population of cells endowed with both self-renewal and differentiation capabilities [48]. CSCs, as with normal stem cells, are long-lasting and have self-renewal capabilities. Both human cancers (or tumor clones) and regenerating normal tissues are organized in a hierarchical manner according to stages of differentiation and proliferative potential with stem cells as the common denominator. However, this does not necessarily imply that CSCs are always derived from normal stem cells. Stem cells are often the target of genetic events that are necessary or sufficient for malignant transformation; however, restricted progenitors, due to their cycling feature, oftentimes represent the preferred transformation targets [47]. Even differentiated cells can undergo oncogenic reprogramming and de-differentiation and be transformed [47]. Both normal stem cells and CSCs share the ability to self renew and produce differentiated progeny, and thus parallels can be found between signaling pathways that regulate these attributes. A CSC is set apart from a normal stem cell in that it has acquired the capacity for indefinite proliferation through accumulated genetic mutations and epigenetic alterations. In this case, when signaling pathways that regulate normal stem cell self-renewal are dysregulated, tumorigenesis occurs. Multiple approaches have been employed to identify, enrich, purify, and characterize CSCs in different tumor systems [47].