Mammary Tumor Regression. I. Physiopathologic Characteristics of Hormone-Dependent Tissue

Summary Primary 7,12-dimethy lbenz[a]anthracene-induced and transplantable MTW9 mammary carcinomas of the rat were used to study the regression process of hormone-dependent tumors. Both tumors were half their initial size within a few days after the host was deprived of hormone. Chemical composition, nuclear/cytoplasmic ratios of DNA, RNA, and protein, and average DNA content per nucleus were similar in regressing and growing tumors. Vascular space, blood flow, and regulation of blood supply were similar in regressing and growing tumors. Regression could not be identified by histologic analysis until it was far advanced and tumor size was significantly reduced. Extensive invasion by immunologic-type cells was not seen in regressing tumors, except for isolated foci of macrophages and lymphocytes, observed mostly around necrotic areas. In a group of “spontaneous” mammary tumors, no relation was found between tumor regression and functional differentiation of neoplastic cells, as measured by lactose, phosphoprotein, and triglyceride production. Transplantation of regressing MTW9 fragments into hosts with the appropriate hormonal milieu was as successful as transplantation of fragments from growing tumors. The tumor models indicated that regression consisted of a coordinated sequence of events, which occurred in the whole tumor and involved rapid destruction of single cells, but did not change significantly the relative size of the vascular, interstitial, and cellular compartments or the efficiency of the vascular system of the tumor.