According

to Zhang et al[21] because of loss of specific small molecular inhibitors screening markers that govern degree of differentiation, thyroid CSCs undertake aberrant differentiation pathways and suffer maturation arrest. If this arrest is seen late in the differentiation process, they give rise to well differentiated carcinoma[21], when encountered early in the process, poorly differentiated carcinoma results. Therefore, different oncological pathways are responsible for providing diverse histological and morphological patterns to thyroid cancer. Other studies demonstrate that stem cells can be recruited to the site of tumor and probably can acquire tumor-like properties and acting as parental tumor cells. Moreover, these cells have internal driven-force for supporting tumor progression and metastasis and they have the power to communicate with other cells through exosomes[22,23]. ISOLATION AND IDENTIFICATION OF CANCER STEM CELLS Various pre-clinical in vivo and in vitro models have been designed by the researchers to determine thyroid cancer progression and their response to treatment. According to the American Association for Cancer Research, ‘cancer stem cell can only be defined experimentally by their ability to recapitulate the generation of a continuously growing tumor’, proving the term TIC’s[6,24].

The commonest and most definite way to confirm their presence is by isolating cells and then serially injecting them into immuno-deficient, for example non-obese diabetic mice or severe combined immunodeficiency (SCID) mice, to identify tumor initiation. CSCs isolated by flowcytometry are sorted according to CSC-specific surface markers, thyrosphere formation assay, aldehyde dehydrogenase activity (ALDH) and ATP-binding cassette sub-family G member 2 (ABCG2) efflux-pump mediated Hoechst 33342 dye exclusion[6,9,10]. The sphere-forming assays are the best in vitro strategy to study clonal behavior and multi-potential of thyroid stem cells. There are different CSC-specific markers proposed by different authors such as side population (SP), CD-133+,

CD-44, POU5F1, ALDH, insulin and insulin-like factor (IGF). The existence of embryonic remnants with stem-cell properties in mature thyroid gland Dacomitinib has already been hypothesized using Oct-4, ABCG-2, GATA-4, HNF-4α, α-fetoprotein and p63 markers[10,25,26]. Malguanera et al[25] demonstrated expression of various stemness markers (Oct-4, NANOG, Sox-2, CD44, and CD133) in follicular thyrospheres. However, the sphere cultures displayed very low levels of thyroid differentiation markers (Tg and TPO). Additionally, their findings also displayed higher expression of IGF components in the stem cells suggesting their important role in the regulation of precursor cells in follicular cancer[25]. Specific genetic alterations such as RET/PTC and PAX8/PPARγ rearrangements play a crucial role in thyroid carcinogenesis.