Cancer stem cells in solid tumors: an overview.

It has long been appreciated that significant functional and morphologic heterogeneity can exist within the individual cells that comprise a tumor. Increasing evidence indicates that many solid tumors are organized in a hierarchical manner in which tumor growth is driven by a small subset of cancer stem cells (CSCs) or tumor-initiating cells. Although these cells represent a small percentage of the overall tumor population, they are the only cells capable of initiating and driving tumor growth. Emerging evidence indicates that these cells are also resistant to chemotherapy and radiation therapy, which has led to much speculation and interest surrounding the potential clinical applicability of CSCs.