The bad seed: Cancer stem cells in tumor development and resistance.

Over the past two decades cancer stem cells (CSCs) have emerged as essential players in the pathogenesis of cancer, with the capacity to initiate, maintain and repopulate different tumors. Within the tumor bulk, CSCs represent a small subpopulation, bestowed with the capacity to self-renew and yield heterogeneous lineages of cancer cells. In many scenarios, CSCs exhibit increased resistance toward irradiation and chemotherapy, and given their spectacular ability to replenish the tumor, they constitute a substantial therapeutic challenge. In this review, we provide a brief overview of the concept of CSCs and the experimental methodology utilized for identifying and isolating these unique cells. We discuss how CSCs are regulated within the tumor microenvironment as well as the role they portray in seeding fresh tumors. Finally, we explore the mechanisms that enable CSCs to evade modern therapeutic approaches and the possible strategies that can be utilized to prevent CSCs from resurrecting the disease.