[Desmoid fibroma of bone. An histoenzymological and ultrastructural study of the mandible in one case (author's transl)].

A case of post-traumatic desmoid tumor of the mandible is reported in a 4-year old girl. This tumor was considered typical clinically and by light microscopy. Four years later, the patient was symptomless, without evidence of recurrence. Histoenzymology showed a high level of leucine aminopeptidase activity. On electron microscopy, a part from a few primitive mesenchymal cells, abnormal fibroblasts represented the principal cellular component. These cells showed evidence of intracytoplasmic collagen fibers synthesis and of myoid differentiation (numerous filaments and endocytosis vacuoles). Myofibroblasts, with a microfibrillary network showing areas of dense osmiophilic condensation in the vicinity of cell membrane were also present. This atypical ultrastructural feature suggests, in this case, the non-neoplastic nature of this lesion. It seems to reflect an abnormal reaction to a traumatic aggression, characterized by proliferation of myofibroblasts (which normally disappear in the late stages of healing) and to have, thus, the signification of pathologic scar.