Inverted papilloma: observation with scanning and transmission electron microscopy.

Three cases of inverted papilloma of the urinary bladder were studied by transmission electron microscopy. Scanning electron microscopic observation was made in one of these. The surfaces of the outer-most tumour cells were covered with short stubby microvilli. Multiple bud like proliferations of the tumour cells were compatible with a trabecular type of inverted papilloma. The tumour cells of the trabeculum mimicked the intermediate and basal cells of the epithelium which covered the surface. Microcysts are believed to be formed by epithelial migration into pits, creating an epithelial inversion, and do not represent central necrosis. Ultrastructure suggests that inverted papilloma is a very well differentiated tumour.