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. Some glioblastomas contain爂iant tumour cells with one or more bizarre-shaped, hyperchromatic nuclei. They are referred to as爂iant cell glioblastomas.