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Medulloblastomas are embryonal neoplasms. 

These tumors are all composed of small round blue cells.

Medulloblastoma arise in the posterior fossa, generally in the cerebellar midline.


Cellular Origin:

  • The cellular origin of medulloblastoma remains controversial.
  • There are currently two dominant hypotheses as to the origin (see table below).


Postulated cellular origins of medulloblastoma:

Proposed cellular origin


External granular layer of the cerebellum

  • medulloblastoma cells are cytologically similar to those of the external granular layer
  • this layer persists for the first year of life, before involuting

Posterior medullary velum

  • undifferentiated cells migrate to the external granular layer and persist for a short time after birth



The commonest specific abnormality in medulloblastoma (present in about 50 %), is isochromosome 17q [i(17q)].

See Atlas of Genetics and Cytogenetics in Oncology and Haematology



Tumor Histology: 


Cerebellar vermis or cerebellar hemispheres

Cell density

Densely cellular

Nuclear characteristics

Round, oval or angulated (‘carrot-shaped’)

Vascular features

Low vascular density

Cellular patterns


  1. rows
  2. lobules
  3. intertwined fascicles of cells
  4. Homer-Wright rosette

Cell differentiation


  1. glial lines
  2. neuronal lines

Less common:

  1. mesenchymal lines



Classic medulloblastoma is a highly cellular tumor

  • Composed of cells with round, oval or angulated (‘carrot-shaped’) nuclei and minimal cytoplasm.
  • The tumor cells are in sheets, rows or nodules.
  • Homer-Wright rosettes are present in <40% of cases and consist of nuclei of rosette forming cells arranged in a circular fashion around fibrillary processes.
  • Medulloblastomas are mitotically active tumors, often show single cell necrosis and less commonly geographical areas of necrosis.
  • Occasionally multinucleated giant cells or cells with large nuclei are seen within the tumor.


Classic hypercellular medulloblastoma with several mitotic figures:



Tumor cells in higher magnification. There are cells with angulated hyperchromatic nuclei. There is cell moulding:



Desmoplastic form of medulloblastoma can be histologically differentiated from the classic form.

  • Desmoplastic medulloblastoma is defined by the presence of several prominent nodules or “pale islands” of tumor.
  • These areas are of lower cellularity, reticulin–free, show nuclear uniformity and are in a background of collagen-rich, highly-proliferative tumor.
  • This form also tends to be more discrete than the classic variety and is often located in the cerebellar hemispheres.
  • The degree of anaplasia of the tumor cell may be a predictor of the patient’s outcome.
  • Tumors that are well-differentiated and extensively nodular are found to have better clinical outcomes than those with large-cell anaplasia.


Nodular desmoplastic medulloblastoma:



Reticulin staining in the same desmoplastic medulloblastoma. The reticulin surrounds the nodules and is in the internodular areas:



Tumor cells infiltrating through the molecular layer of the cerebellum. Tumor is present in the subarachnoid space:


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