Gliomagenesis: a game played by few players or a team effort?

Glioblastoma multiforme (GBM) represents the most aggressive and deadliest brain tumor of adults. To date, cell heterogeneity within GBM has been explained by the "hierarchical" model of tumorigenesis, aka the "cancer stem cell" hypothesis. In agreement with this model, only rare tumor cells, namely the cancer stem cells (CSCs), are responsible for GBM initiation and, as such, are considered the favored target of therapy. However, multiple evidence has recently indicated that tumor-initiating cells (TICs) may not represent a restricted and infrequent GBM component; rather, they might constitute most of the cells within the tumor bulk. Here we review several studies that recently shed new light on the process of gliomagenesis. We critically analyze the methodological inconsistencies and drawbacks that are causing protracted controversy in the field. Finally, we discuss the clinical implications and the novel therapeutic scenarios that have been put forward by the presence of functionally and molecularly distinct subpopulations of GBM-initiating cells within the same tumor.