Histopathology of leptomeningeal dissemination. A, Aggregates of medulloblastoma cells on the pial surface of the thoracic spinal cord. B, Tumor cells (arrow) attached to the conus medullaris in the lumbar spine. C, Classic medulloblastoma cytoarchitecture showing tumor cells with molded nuclei and mitotic figures. D, Medulloblastoma in the cerebellum spreading into the adjacent fourth ventricle and leptomeninges of the brain stem (arrow). E, Disseminating tumor cells in the subependymal space of the lateral ventricle. Ciliated ependymal cells are visible at the top. F, Nodule of disseminating tumor in the wall of the temporal horn of the lateral ventricle. The dentate gyrus of the hippocampus is visible below. A–E, Shh + Arnt–induced tumors. F, Shh + Gdi2–induced tumors. Scale bar, 25 μm ( A – C ), 16 μm ( E ), 500 μm ( D and F ).