COL4A1 and glioblastoma: Reactive astrocytes and α‐SMA‐expressing cells overexpress ECM molecules such as collagen I/IV.[36, 43, 84] Co‐immunostaining of whole glioblastoma tissues with COL4A1 and COL1A1 revealed that the main basement membrane marker collagen IV[31, 32, 33, 34] is surrounded by a collagen I rim in MVPs and MVs (Figure 1D).