the nuclei of meningioma cells tend to be less spindle-shaped. In addition, streaming cells forming parallel rows with little interlacing can be seen in pilocytic astrocytomas, fibrillary astrocytomas and oligodendrogliomas.

Whorls, Loops, Onion-skin Pattern and Psam-moma Bodies These patterns are characteristic of meningiomas in which there are concentric layers of tumor cells around a center, which may contain no structure or which may show a small blood vessel, a hyaline body or even a calcified granule. Psammoma bodies are calcified granules that are usually laminated and non-specific, unless one can see the whorling meningioma cells around them (Fig. 3.9a, b). Whorls can also be seen in schwannomas but they are not as distinctly outlined by a thin fibrous membrane as in meningiomas and tend to be larger with an indistinct border (Fig. 3.9c). Larger whorls may look more like oval loops. Cross-sections of neurofibromas may show a somewhat similar pattern, known as an "onion skin" or "onion bulb". These structures are more numerous and loosely spaced, typically with a demonstrable axon in the center of the onion bulb.

Nodular, Lobular and Alveolar Patterns Tumors consisting of nodules and lobules of various sizes are numerous. Small nodules and large lobules separated by thin fibrous membranes are typical of meningiomas, separated by capillaries or hypocellular gliotic tissue frequently in oligodendrogliomas and separated by connective tissue septa in pituitary adenomas. Similar lobular patterns are also seen in chordomas, chemodectomas, metastatic carcinomas and alveolar soft-part sarcomas. Alveolar and follicular patterns are more or less synonymous with a lobular pattern (Fig. 3.10).

Palisades and Pseudo-palisades Nuclei that form parallel rows are known as "nuclear palisades". Anuclear eosinophilic cytoplasmic bands between nuclear rows in a schwannoma are known as "Verocay bodies" (Fig. 3.11). The combination of nuclear palisades, Verocay's bodies and interlacing bundles of spindle-shaped cells (see Fig. 3.8) is relatively specific for schwannoma, although an almost identical pattern can be seen in occasional astrocy-tomas (bipolar spongioblastomas or "central schwannoma"). Palisading of nuclei can also occur in PNET, again as a clone of the rare primitive spongioblastoma. An area of coagulative necrosis surrounded by rows of nuclei is often called "pseudopalisading", more accurately called "perinecrotic palisading", and is often seen in glioblastoma multiforme. Adjacent areas of dense (Antoni A) and loose (Antoni B) tissue is typical of schwannoma (Fig. 3.11).

Rosettes and Pseudo-rosettes There are four types of these patterns: two types of true rosettes and two types of pseudo-rosettes. All show cells radiating around a center. One true rosette is an ependymal rosette that has a small or large lumen in the center, resembling the central canal of the spinal cord with cilia or blephalo-plasts around the lumen (Fig. 3.12a). Such rosettes are found in some ependymomas (Fig. 3.12b). Another true rosette is composed of rods and cones and is seen in some retinoblastomas. One type of pseudo-rosette includes a blood vessel in the center, a perivascular pseudorosette (Fig. 3.13a, b), the cell processes from the surrounding cells tapering toward the vascular wall. These can be seen with H&E stain but more easily seen with van Gieson (VG) stain (Fig. 3.13c). Perivascular pseudo-rosettes are very common in ependymomas, and are much more common than true rosettes. When the center consists of anuclear eosinophilic cytoplasm (on H&E stain), it is known as a "Homer Wright pseudo-rosette", a pattern typical of neuroblasts growing in culture. They are found in some PNETs but more frequently in neuroblastomas (Fig. 3.14), central neurocytomas and pineocy-tomas. The eosinophilic amorphous areas tend to be larger and more irregular in pineocytomas and neuroblastomas.

Cartwheels and Perivascular Crowns Difficult to distinguish from perivascular pseudorosettes, a cartwheel formation has been described as characteristic of astroblastomas in which radially arranged tumor cells show cell feet attached to the vascular wall, whereas only tapering cytoplasmic processes are demonstrable in ependymomas. When no cellular processes are present around the blood vessel, the pattern is simply called a "perivascular crown", as seen in numerous types of tumors, including astrocytomas (Fig. 3.15), adenomas and carcinomas .

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