CD44 and central nervous system cancer: Microglia cells are also able to favour glioma invasion by releasing of many growth factors and ECM proteins [79] and thus activating, in glioma cells, several pro-invasive signalling pathways, including: protein tyrosine kinase 2 beta (Pyk2) signalling [80,81,82], osteopontin-CD44 signalling [83], epidermal growth factor (EGF) signalling [84] and transforming growth factor-β (TGF-β) pathways [85,86,87,88,89].