Medulloblastoma is very densely cellular, with sheets of anaplastic cells. Individual tumor cells are small, with scant cytoplasm and hyperchromatic nuclei that are frequently elongated or crescent shaped. Mitoses are abundant, and markers of cellular proliferation, such as Ki-67, are detected in a high percentage of the cells. The tumor may express neuronal (neurosecretory) granules, form Homer-Wright rosettes and express glial markers (e.g., GFAP).