Several theories exist. Stevenson and Echlin proposed that medulloblastoma arises from remnants of the fetal external granular layer present in the infant cerebellum and then moves through the molecular layer to form the internal granular layer. This theory can be reconciled with that of Raaf and Kernohan who suggested that the tumor arises from cells rests in the inferior medullary velum . There is evidence that the external granular cells originate in the neuroepithelial roof of the fourth ventricle and migrate laterally to form the external granular layer. Recent immunohistochemistry results suggest that medulloblastomas arise from more than one cell of origin.
Older patients tend to have a higher frequency of lateraly placed neoplasms.
The tumor most often arises in the midline and arises from the inferior medullary velum of the cerebellum. It tends to grow to occupy the fourth ventricle and infiltrate adjacent posterior fossa structures. The tumors are soft and friable, and may have central areas of necrosis. Since the tumor can spread along cerebrospinal fluid pathways, metastatic spinal lesions have been identified in over half the cases seen at autopsy. Extraneural metastases can occur, possibly related to preoperative shunting, and remote metastases may be found in the bone marrow, and cervical lymph nodes.