Fig. 1

Schematic of tumor cell interactions in the brain. An intensive direct and indirect cross-talk between the resident cells and tumor cells needs to occur for circulating tumor cells to pass through the BBB and grow in the brain. These interactions, in addition to the genetic predisposition of the tumor cells, result in a multitude of pathway activations in both tumor and host cells. BBB blood–brain barrier, EGFR epidermal growth factor receptor, GABA gamma-aminobutyric acid, HER human epidermal growth factor receptor, IL interleukin, JAG jagged, MMP matrix metalloproteinase, TGF transforming growth factor, VEGF vascular endothelial growth factor