Glioblastoma, light micrograph. Glioblastoma is a high-grade (WHO Grade IV) aggressive astrocytoma primarily seen in adults. Microscopically, it shows hypercellularity, greater nuclear pleomorphism and hyperchromasia, and increased mitotic activity. The爌resence of necrosis爄s one of the histologic hallmarks of glioblastoma. Necrotic areas are usually seen in爏erpentine pattern?shown here) or geographic pattern in hypercellular foci. Tumor cells often aggregate around the periphery of necrotic areas creating爌seudopalisading.