Eugenia Allegra, Serena Trapasso

Cancer stem cells (CSCs), also called "cells that start the tumor," represent in themselves one of the most topical and controversial issues in the field of cancer research. Tumor stem cells are able to self-propagate in vitro (self-renewal), giving rise both to other tumor stem cells and most advanced cells in the line of differentiation (asymmetric division). A final characteristic is tumorigenicity, a fundamental property, which outlines the tumor stem cell as the only cell able to initiate the formation of a tumor when implanted in immune-deficient mice. The hypothesis of a hierarchical organization of tumor cells dates back more than 40 years, but only in 1997, thanks to the work of John Dick and Dominique Bonnet, was there the formal proof of such an organization in acute myeloid leukemia. Following this, many other research groups were able to isolate CSCs, by appropriate selection markers, in various malignancies, such as breast, brain, colon, pancreas, and liver cancers and in melanoma. To date, however, it is not possible to isolate stem cells from all types of neoplasia, particularly in solid tumors. From a therapeutic point of view, the concept of tumor stem cells implies a complete revision of conventional antineoplastic treatment. Conventional cytotoxic agents are designed to target actively proliferating cells. In the majority of cases, this is not sufficient to eliminate the CSCs, which thanks to their reduced proliferative activity and/or the presence of proteins capable of extruding chemotherapeutics from the cell are not targeted. Therefore, the theory of cancer stem cells can pose new paradigms in terms of cancer treatment. Potential approaches, even in the very early experimental stages, relate to the selective inhibition of pathways connected with self-renewal, or more specifically based on the presence of specific surface markers for selective cytotoxic agent vehicles. Finally, some research groups are trying to induce these cells to differentiate, thus making them easier to remove. For all these reasons, we have collected existing literature on head and neck cancer stem cells that correlate the biological characteristics of this subpopulation of cancer cells with the clinical behavior of tumors.