The biology of head and neck cancer stem cells.

Emerging evidence indicates that a small population of cancer cells is highly tumorigenic, endowed with self-renewal, and has the ability to differentiate into cells that constitute the bulk of tumors. These cells are considered the "drivers" of the tumorigenic process in some tumor types, and have been named cancer stem cells. Epithelial-mesenchymal transition (EMT) appears to be involved in the process leading to the acquisition of stemness by epithelial tumor cells. Through this process, cells acquire an invasive phenotype that may contribute to tumor recurrence and metastasis. Cancer stem cells have been identified in human head and neck squamous cell carcinomas (HNSCC) using markers such as CD133 and CD44 expression, and aldehyde dehydrogenase (ALDH) activity. The head and neck cancer stem cells reside primarily in perivascular niches in the invasive front where endothelial-cell initiated events contribute to their survival and function. In this review, we discuss the state-of-the-knowledge on the pathobiology of cancer stem cells, with a focus on the impact of these cells to head and neck tumor progression.