Neuroblastic medulloblastoma with abundant cytoplasmic actin filaments

Routine light microscopic examination of a midline cerebellar medulloblastoma with unusual morphologic features disclosed features identical to those of classic medulloblastoma. Immunoperoxidase staining for glial fibrillary acidic protein (GFA), neurofilament protein (NF), and skeletal muscle actin showed diffuse cytoplasmic staining for actin, but not for GFA or NF. Ultrastructurally, the cells had many blunt cytoplasmic processes with abundant, often parallel, 4- to 5-nm filaments. Other ultrastructural features included scattered, clear, and dense-core vesicles (80 to 90 nm), microtubules, rare intermediate filaments, polyribosomes, rough endoplasmic reticulum, and cell junctions. Plasmalemmal densities and invaginations, occasionally associated with clusters of 40- to 50-nm clear vesicles, were encountered infrequently and were interpreted as immature or abortive synapses. The blunt cytoplasmic processes containing abundant actin filaments, several microtubules, and large clear vesicles resemble growth cones of developing nerve cells.