Research indicates that a small population of cancer cells is highly tumorigenic, endowed with the capacityfor self-renewal, and has the ability to differentiate into cells that constitute the bulk of tumors. These cells areconsidered the ‘‘drivers’’ of the tumorigenic process in some tumor types, and have been named cancer stemcells (CSC). Epithelial-mesenchymal transition (EMT) appears to be involved in the process leading to theacquisition of stemness by epithelial tumor cells. Through this process, cells acquire an invasive phenotype thatmay contribute to tumor recurrence and metastasis. CSC have been identified in human head and neck squamouscell carcinomas (HNSCC) using markers such as CD133 and CD44 expression, and aldehyde dehydrogenase(ALDH) activity. Head and neck cancer stem cells reside primarily in perivascular niches in the invasive frontswhere endothelial-cell initiated events contribute to their survival and function. Clinically, CSC enrichment hasbeen shown to be enhanced in recurrent disease, treatment failure and metastasis. CSC represent a novel targetof study given their slow growth and innate mechanisms conferring treatment resistance. Further understandingof their unique phenotype may reveal potential molecular targets to improve therapeutic and survival outcomesin patients with HNSCC. Here, we discuss the state-of-the-knowledge on the pathobiology of cancer stem cells,with a focus on the impact of these cells on head and neck tumor progression, metastasis and recurrence due totreatment failure.
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