Fig. 3.18. a-b Microcysts are frequently seen in oligodendrogliomas, usually owing to mucinous degeneration, as evidenced by the faintly stained contents. NP19529, H&E. a Low magnification. b High magnification.

Rosenthal Fibers, Cytoid Bodies and Eosinophilic Hyaline Bodies Eosinophilic hyaline bodies of variable shapes and thicknesses (commas, sausages or just thick bands) are known as "Rosenthal fibers". These structures are usually densely red with a mild purplish tinge and sometimes resemble columns of red blood cells. They are found typically in pilocytic astrocy-tomas and gliotic white matter surrounding craniopharyngiomas. The origin of Rosenthal fibers has been debated but they probably represent a degenerated form of glial fibers on which crystalline is deposited. They stain variably with GFAP but not so intensely as do the usual astrocytic fibers. Eosinophilic hyaline bodies, cytoid bodies and eosinophilic granular bodies are found in low-grade gliomas, such as pilocytic astrocytomas and oligodendrogliomas, but their origin has not been clarified.

Keratin (Dry Keratin) and Parakeratin (Wet Keratin) Multilaminated desquamated epithe lial membranes appear as thin parallel lines to hexagonal plates, depending on the plane of section. They are known as "keratin" or "dry keratin" ("dandruff") and are typically seen in epidermoid and dermoid cysts (Fig. 3.19a), where the dehydrating pattern of maturation is to be expected on exposure to air. By contrast, dead, swollen, mucosal epithelium ("wet keratin", where the swelling represents maturation of cells exposed to moisture) appears as eosinophilic masses with pale membranous septa representing cell walls and ghosts of nuclei (Fig. 3.19b). Wet keratin is characteristic of craniopharyngiomas, which develop from Rathke's duct and nasopharyngeal mucosa.

Desmoplasia and Fibrosis Excessive fibrous connective tissue sometimes forms the stroma of tumors and at times divides tumors into numerous small nodules or cords. This phenomenon is seen especially when a tumor invades and infiltrates the leptomeninges, even

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