FRMs vary from sparse to moderately cellular and are composed of immature cells with little cytoplasm and small, round to spindled nuclei, with or without nucleoli. Strap cells with eosinophilic cytoplasm, occasionally with cross stria-tions, may be seen, but are few in number. Mitoses, necrosis and nuclear pleo-morphism are typically absent. ARMs contain large, closely packed polygonal cells with light pink to bright eosinophilic cytoplasm and one or more round to oval nuclei, sometimes with prominent nucleoli. The cytoplasm contains abundant glycogen, often appearing as vacuoles and producing a characteristic "spider cell". Rod-shaped cytoplasmic crystals ("jackstraws") may also be apparent. Cross striations are infrequent and, mitoses and necrosis are absent.
JRMs contain a large number of strap-shaped muscle cells with abundant eosinophilic cytoplasm with centrally located nuclei. Cytoplasmic vacuoles are common. The tumour often co-exists with typical areas of FRM.
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