Abstract

The rate of tumor cell shedding into efferent tumor blood was measured in growing and regressing MTW9 rat mammary carcinomas. The hormone-dependent tumor, grown as an 'isoession was induced by reduction of mammotropin level in the host. Tumor cells were differentiated from normal leukocytes by indirect immunofluorescence. Growing tumors shed 3.2 x 10-6 and regressing tumors shed 4.1 x 10-6 cells per 24 hr per g tissue. Cell shedding rates of growing versus regressing tumors were not siginificantly different over a tumor size range of 2 to 4 g. The number of tumor cells in the arterial blood was 12-fold smaller than in the efferent tumor blood. It is concluded that: (ay cell shedding via blod probably plays only a minor role in the total cell loss by gtowing MTW9 carinomas; (b) hormone-induced tumor regression does not depend on increased cell shedding; (c) tumor cells are rapidly cleared from circulating blood; and (d) a 2-g MTW9 carcinoma pours enough cells into the host circulation to transplant the tumor every 24 hr.