The fine structure of psammoma bodies was examined in four cases of fibroblastic 
meningioma. In general, large numbers of various-sized calcified bodies 
(psammoma bodies) were scattered among the interstitial fibers. In these bodies, 
the smallest calcific site was found in the extracellular membrane-bound matrix 
vesicles, which measured approximately 0.1 to 0.2 mu in diameter. In addition, 
extracellular "matrix giant bodies," with or without hydroxyapatite aggregates 
and measuring up to 3 mu in diameter, were frequently encountered. These bodies 
were apparently invested with single, double, or multiple concentric walls 
averaging nearly 0.1 mu thick. The presumably originated from the neoplastic 
cells as a consequence of cytoplasmic residuals associated with cellular 
degeneration or necrotic cell processes. Hydroxyapatite crystals precipitated 
repeatedly within the bodies. The precipitate may gradually aggregate within the 
bodies, and gather in clusters, resulting in a large psammoma body. Finally, 
collagen fibers around the calcified giant bodies accrued deposits of apatite 
crystals to make a huge psammoma body. These findings suggest that both matrix 
giant bodies and matrix vesicles may serve as initial nidus of calcification of 
psammoma bodies in fibroblastic meningioma. Consequently, this mineralization 
process may represent a certain dystrophic calcification of meningocytic cells.