Ultrastructural study of granular cell ameloblastoma.

A granular cell ameloblastoma occurring in the anterior mandible of a 49-year-old woman has been studied both histologically and ultrastructurally. Histologically the tumor in this case showed a mixed type of follicular, granulal cell, and acanthomatous type of ameloblastoma. Many granular cells seemed to gather around the periphery of each tumor island. Electron-microscopically the cytoplasmic granules were measured as approximately 0.6 mu in diameter, and were surrounded by limiting membrane. Most of these granules revealed high electron density and their contents showed various patterns--osmiophilic homogeneous, fingerprint, and small particles--and were identified as lysosomes. However, no enamel matrix precursor was identified in either lysosome or cytoplasm. All the granular cells showed regular nuclei and no degeneration. Though lysosomes might play a role of autophagy and of remodeling the cytoplasm, its presence seems to have an active function in tumor cells, as opposed to the aging or degenerating function.