The fine structure of psammoma bodies in meningocytic whorls.

Psammoma bodies within meningocytic whorls in five cases of meningioma were examined by electron microscopy. The cells comprising the meningocytic whorls showed the conventional ultrastructural features of meningothelial cells with interdigitations and interconnections by means of junctional complexes. In some whorls, collagen fibers, elastic fibers, microfibrils, as well as finely reticulated material were observed. Calcified foci of various sizes were seen within many of these whorls. Extracellular matrix vesicles, with or without hydroxyapatite crystals, were occasionally noted at the periphery of extracellular psammoma bodies in the whorl. Furthermore, many small aggregates of crystals were present adjacent and parallel to the collagen and elastic fibers in the meningocytic whorls. Substantial amounts of collagen fibers were also observed within the large psammoma bodies. These findings suggest that the matrix vesicles, presumably produced by the meningothelial cell, may serve as the initial nidi of calcification of psammoma bodies as originally suggested by Lipper et al. Collagen fibers as well as elastic fibers in the whorls may also play an important role in orienting apatite crystal deposition during the growth of extracellular psammoma bodies and serve as early sites of calcification.