Connective tissue reactions in early phases of spontaneous regression of experimental malignant histiocytomas. An electron microscopic study.

Interactions between cancer and host are said to comprehend immunologic processes and stromal reactions including an angiofibroblastic response. We studied the latter phenomenon in spontaneously regressing experimental malignant histiocytomas produced by inoculation of an established macrophage-like cell line (WEHI-3) into baby rats. Light and electron microscopic investigations of the tumor tissue showed that there was a clear proliferation of fibroblasts and capillary vessels beginning from the 8th-10th day of the experiment. The vascular structures seemed to be somewhat changed and fibroblasts often displayed close contacts to tumor cells. The significance of these connective tissue reactions is discussed and, finally, the observations led to the conclusion that the angiofibroblastic reaction in the malignant histiocytoma model presented here may actively contribute to tumor breakdown and should not be interpreted as being only a secondary phenomenon after irreversible damage of tumor cells.