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. Vascular (endothelial) proliferation and necrosis are two histologic hallmarks of glioblastomas. The image shows glomeruloid vessels (vessels with multiple lumina) and endothelial multilayering as a result of endothelial hyperplasia. These changes are driven by Vascular Endothelial Growth Factor (VEGF) secreted by the tumour in response to hypoxia. Note the focus of necrosis on the lower left.