Classic medulloblastoma
Classic medulloblastoma (the majority) is a highly cellular tumor composed of diffuse masses of small, undifferentiated oval or round cells.
Hyperchromatic nuclei and scant cytoplasm (and inconstant cell clusters in Horner Wright rosettes) 1). (sometimes called blue tumor (monotoneous appearance).
Some medulloblastomas show neuronal, glial and other differentiation. Neuronal differentiation is manifested by neuropil and rosette formation. Rosettes are groups of tumor cells arranged in a circle around a fibrillary center. Mature neurons may also be found infrequently. Glial differentiation in some tumors is reflected by GFAP-positive cells. There may also be differentiation along oligodendroglial or ependymal lines. More unusual lines of differentiation result in formation of striated muscle cells (medullomyoblastoma) and melanin-producing cells.