Psammoma bodies in meningocytic whorls were investigated by electron microscopy. 
In some whorls, connective tissue fibers were seen and membrane-bound vesicles 
were contiguous to degenerated cells. Some small vesicles, 0.1 to 0.5 micron in 
diameter, were outlined by plasma membrane (matrix vesicles), other larger ones, 
about 1 to several micron in diameter, were invested by a thick wall (matrix 
giant bodies). Mineralized deposits were frequent in these vesicles and 
occasionally large masses of mineralized connective tissue fibers (psammoma 
bodies) were seen. Analysis of the material in the mineralized vesicles and 
fibers, using an energy dispersive X-ray microanalyzer, showed that both calcium 
and phosphorous were evident and hydroxyapatite was substantiated using an X-ray 
diffractometer. Psammoma body formation in the meningocytic whorls may represent 
degeneration in some whorls of the central cells which contain connective tissue 
fibers, producing cell debris such as membrane invested vesicles. Subsequently, 
calcification occurs in these vesicles, and the mineralization process extends 
to neighboring connective tissue fibers. The calcified mass forms a psammoma 
body.