The cellular events associated with regression and progression of murine (moloney) sarcomas

Tumors were induced in adult and neonatal mice by intramuscular injections of either 10 4 or 10 6 cells from a cultured murine (Moloney) sarcoma line. Neoplasms that progressed were induced in neonates by either dose, and in adults only by the larger dose; adult mice receiving 10 4 cells usually developed tumors that regressed. Light microscopic examinations were performed at 2–3 day intervals throughout the course of tumor development and subsequent regression or progression. Initially all tumors became infiltrated with polymorphonuclear leukocytes—mainly neutrophils—and edema was extensive. By the end of the second week post inoculation, this acute inflammatory infiltrate had been replaced in adult mice by one consisting of mononuclear cells; neonatal mice never developed significant numbers of these inflammatory cells in their tumors. Of particular significance, since mononuclear inflammatory cells were associated intimately with tumors during the process of regression, was the disappearance of these cells 12–14 days post inoculation from tumors destined to progress in adult mice. Hyperplastic changes were found in the cortices and medullae of regional lymph nodes draining both progressing and regressing sarcomas. Secondary neoplasms developed commonly, and the distribution of these lesions was related to the ages of mice at the time of inoculation.