syncytiotrophoblasts have a very basophilic, vacuolated cytoplasm and multiple hyperchro-matic nuclei which stain positive for HCG. Hemorrhage and necrosis are common.

Teratoma is composed of well differentiated tissues from all three germ cell layers (endo-derm, mesoderm and endoderm). Mature teratoma may contain solid or cystic foci of squamous epithelium, cartilage or glandular or tubular structures, lined by tall columnar mucus-secreting cells. Teratoma will often contain neuroepithelial tissue showing varying degrees of glial and neuronal differentiation. It is important to differentiate any primitive features, typically in only one of the three germinal layers, which define an immature ter-atoma. Immature teratomas contain poorly differentiated non-neuroepithelial cells in high density, often staining positive for CEA, cytok-eratins or epithelial membrane antigen (EMA) [17].

Pineoblastoma macroscopically is a soft tumor that is pinkish-gray in color and may contain hemorrhagic, necrotic or cystic components. It can be relatively well circumscribed or ill defined and invasive into local surrounding structures, often destroying the pineal gland with growth. By contrast, pineocytoma is well defined and has a pale gray color and lobulated surface. Necrosis, hemorrhage and invasion into adjacent structures are rare findings in pineocytomas. Calcification may be grossly obvious on inspection or on imaging.

Microscopically, pineoblastoma cells are highly cellular, with small, round nuclei resembling cerebellar medulloblastomas. Mitotic figures may be numerous and the tumor cell cytoplasm is scanty. The cells are usually in amorphous sheets, but ill defined Homer-Wright rosettes may be present. Occasionally, perivascular orientation of the cells may be seen. Pineoblastomas also have a propensity to seed the subarachnoid space. Pineocytoma, on the other hand, can be cellular but less so than pineoblastoma. There is a strong resemblance to normal pineocytes, with tumor cells arranged in sheets or diffuse lobules. Giant cells may be present, but mitotic figures are uncommon. Pineocytoma cells may differentiate into mature astrocytes or neurons, or both [17]. Mixed tumors may contain features of both pineocy-toma and pineoblastoma, although no consensus exists on where the transition from well differentiated pineoctyoma to undifferentiated pineoblastoma occurs. Some mixed tumors have been designated when divergent differentiation exists along neuronal, glial or both lines; however, more recent designation of mixed tumors relates to the presence of pineoblastoma and pineocytoma elements [19].

The pathological appearances of other tumors (astrocytoma, ependymoma and menin-gioma) in the pineal region are not site-specific and are histologically similar when found within other areas of the brain.

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