Cancer cells within “micro-regions” of the tumor containing either high or low ratios of CXCL12-secreting to non-endothelial CXCR7+ cells are more migratory than the tumor bulk.

We assume tumors are mostly composed of immobile or weakly mobile cells due to a balance of CXCL12-secreting and CXCR7+ cells and a weak CXCL12 gradient. We predict that cancer cells located in environments containing a high number of CXCR7+ cells and few CXCL12-secreting cells will likely migrate towards vasculature, whereas cancer cells in environments with high secreting cell numbers and few CXCR7+ cells will likely migrate deeper into the tissue.