Fine structure of psammoma bodies in meningocytic whorls. Further observations.

We examined psammoma bodies at the ultrastructural level within meningocytic whorls in nine cases of meningioma. The cells comprising the meningocytic whorls displayed the typical fine structural characteristics of meningothelial cells. The extracellular spaces contained amorphous substances in some whorls and interstitial fibers in others. Presumed residues of degenerated cells were frequently observed within the whorls that had interstitial fibers, and they sometimes contained various amounts of dense crystals identical with those labeled hydroxyapatite crystals. Large calcified masses involving cellular debris and interstitial fibers with apatite deposits were also observed. These findings suggest that the initial foci of the calcification of psammoma bodies in meningocytic whorls are not only matrix vesicles but also are remnants of degenerated cells.