GDNF and neoplasm: The main stimulation for macrophages and microglia accrual around the tumor is mediated by the same glioblastoma cells through the production of CCL2 (C-C Motif Chemokine Ligand 2), CCL7 (C-C Motif Chemokine Ligand 7), GDNF (Glial Cell-Derived Neurotrophic Factor), SDF1 (stromal cell-derived factor 1), TNF (tumor necrosis factor), VEGF (vascular endothelial growth factor), ATP (adenosine triphosphate), CSF-1 (Colony-stimulating factor 1), GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor), and expression of OLIG2 (Oligodendrocyte transcription factor 2) (48, 51, 53).