Development of malignant fibrous histiocytoma induced by 7,12-dimethylbenz[a]anthracene in the rat: characterization of early atypical cells.

Morphologically atypical cells were first detected in the adjacent connective tissue 98 days after implanting a paraffin pill containing 2 mg of 7,12-dimethylbenz[a]anthracene (DMBA) into the subcutaneous tissues of rats. These cells subsequently formed groups and finally produced gross malignant fibrous histiocytomas (MFH). Early atypical cells were located between proliferating fibroblasts and histiocytes in the center of a fibrous capsule surrounding the DMBA pill. They exhibited a smooth cell surface, dilated rough endoplasmic reticulum, multiple Golgi complexes, and were often associated with newly formed collagen. These cells incorporated [3H]thymidine and [3H]proline intensively, and showed weak acid phosphatase activity but no features diagnostic of macrophages (microvilli, numerous lysosomes, high acid phosphatase and non-specific esterase activities, antigens recognized by monoclonal antibodies ED1 and OX-42 and vital staining with trypan blue). There was no evidence that atypical cells differentiated into muscle cells (no expression of desmin or the alpha-sarcomeric form of actin) or Schwann cells (no expression of S-100 protein). No point mutation in the neu gene at nucleotide 2007, specific for N-ethyl-N-nitrosourea- and DMBA-induced malignant rat schwannomas, was detected by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) analyses. These results support the view that malignant fibrous histiocytoma is derived from immature fibroblasts exhibiting pronounced phenotypic diversity during the later stages of carcinogenesis.