Inverted papillomas of nasal cavity and paranasal sinuses. Ultrastructural investigations on epithelial-stromal...

10 cases of inverted papillomas of nasal cavity and paranasal sinuses were examined electron microscopically with particular regard to the epithelial-stromal interface. The papilloma cells were clearly demarcated from the stromal tissue by a basement membrane-like material. However, this structure mainly consisted of two or more layers, occasional breaks in the basement membrane were visible. Within the basal epithelial cells resting on the basement membrane accumulations of actin-like microfilaments could sometimes be observed. Furthermore, in the stromal tissue some myofibroblasts were present in the vicinity of the papillomatous cell complexes and capillary vessels also demonstrated several layers of basement membrane. All the changes presented here are considered to be the result of a permanent interaction between aggressive forces of papilloma cells and the defence mechanism of stromal tissue. The results allow the conclusion that inverted papillomas of nasal cavity and paranasal sinuses are true neoplasms with very low malignancy.