Biological analysis using pathological and immunohistochemical methods was performed to charify the mechanism underlying the resistance of chronic apical lesions to conventional therapy. Eighty cases of chronic lesions were classified into fourtypes: type I (granuloma surrounded by firm collage bundles), type II (granuloma with abscess formation), type III (granuloma with infiltration of numerous neutrophils) and type IV (scar tissue consisting of collagens). The collagenous capsulation of granuloma and scar tissue are considered to cause the intractable behavior. The collagenous tissues are obviously produced by the fibroblasts, which are a main component of the granuloma. The immunohistochemistry indicated that the proliferation of such fibroblasts is enhanced by basic fibroblast growth factor, which is mainly produced by macrophages infiltrated in the granuloma. Animal experiments using rats injected with Actinomyces suspension strongly suggested that the finfiltration of macrophages is promoted by Actinomyces components in the infected root canal. In addition, it was also suggested that the formation of foam cells, which are observed in chronic apical lesions, is related to Actinomyces components in the lesion.
