LIGHT AND ELECTRON MICROSCOPIC STUDIES OF AMELOBLASTOMA

Abstract

Histopathological studies of ameloblastoma of the jaw were made in a total of 41 cases in connection with clinical aspects. In addition, electron microscopy was carried out in limited cases for exploration of the cell origin.Based on the predominant pattern of growth and the grade of differentiation of parenchymal epithelial cells, cases were classified histologically into two categories; plexiform and follicular. Of 41 cases examined, 18 were of plexiform type and 13 of follicular type. The remaining 10 cases displayed mixed follicular and plexiform patterns and were classified as mixed category.In the plexiform type, the epithelial cells formed anastomosing cords or strands with a variable amount of stroma. The cells appeared undifferentiated and irregular in shape. In the follicular type, the epithelium formed more or less discrete islands. The cells making up the peripheral zone of the islands were columnar in shape with the nuclei appearing to be polarized away from the basement membrane. The central area was composed of a loose network of cells resembling stellate reticulum of enamel organs. Squamous metaplasia, formation of keratinized pearls and colloid cystic degeneration were frequently noticed in the center of the follicular islands. In addition to the presence of both plexiform and follicular patterns in the same tumor, epithelial islands with transitional features were also found in the mixed type. Squamous metaplasia and cystic degeneration were often noticed both in the follicular and transitional islands.Cases with follicular and mixed variants were detected in adolescence and adult, whereas those with plexiform type occured mostly in childhood and adolescence. Ten cases were found to develope at less than 20 years of age. Microscopically, all of them were of plexiform type.Grossly the plexiform type of ameloblastoma appeared as a solid mass or sometimes wholly cystic. In follicular and mixed types the lesions were mostly multilocular. It appeared that the size of the lesions was generally larger in plexiform type than in other two types at the time of surgery. Among 41 cases examined, there were 7 cases with recurrent ameloblastoma. All of them were grossly multilocular. Microscopically, 5 of them showed the features of follicular type and the other two were classified as mixed type.In electron microscopy, the epithelial islands of follicular type displayed a distinct zonal arrangement and were composed of three layers, peripheral, intermediate and central. The peripheral layer consisted of a single row of columnar cells with the nuclei displayed away form the basement membrane, which the cells rested upon. The cytoplasm contained variable amounts of mitochondria, granular endoplasmic reticulum and aggregates of glycogen granules. Tonofilaments were few in number. Intercellular spaces between the lateral surfaces of the cells were generally narrow. The desmosomal connections were seen only occasionally. The intermediate and central zones were composed of cells, stellate or polyhedral in shape, with elongated anastomosing cell processes. Desmosomes were demonstrated where the cells were in close contact with each other. The cytoplasm contained bundles of tonofilaments. The peripheral cells had a superficial resemblance to the ameloblasts of enamel organs, but the features characterising the fine structure of the ameloblast in the stage of matrix production, such as Tomes process, well developed Golgi apparatus, complex granular endoplasmic reticulum and of secretory granules were not evidenced. The peripheral cells comprising the epithelial islands with transitional features in the mixed type similar displayed fine structure to those found in follicular type.