Immunoelectron microscopic study on the histogenesis of epidermoid metaplasia in respiratory epithelium

To elucidate the histogenesis of epidermoid metaplasia with an immunoelectron microscopic method, we studied the localization of secretory component (SC), a differentiation marker of the epithelial cell, in human bronchial epithelium. In the area of normal epithelium, SC was synthesized by mucous cells, particularly by small mucous granule cells. It was also found with small mucous granules in both superficial columnar or cuboidal cells in the area of stratification and superficial, less flattened cells in the area of epidermoid metaplasia, but SC was not identified in polygonal cells under the superficial SC-positive cells in both of the above areas. These superficial cells containing SC and small mucous granules possessed increasing number of tonofilaments parallel with the degree of flattening of the surface cells. On the other hand, in the advanced epidermoid metaplasia, in which surface cells were composed of very flattened cells, SC and small mucous granules were not found in the bronchial epithelium. Tonofilaments increased in number in the cytoplasm parallel to morphologic changes from basal cells to outermost flattened cells, via polygonal cells. These findings suggest that small mucous granule cells containing SC could gradually change to flattened cells through a quantitative change of a cytoplasmic component, such as SC or tonofilaments, and constitute the superficial cells of initial epidermoid metaplasia, in which surface cells are composed of less flattened cells. Moreover, these findings suggested that basal cells play an important role in epidermoid differentiation.