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Morphology, Natural History, and Enzyme Patterns in Mammary Tumors of the Rat Induced by 7,12-Dimethylbenz(a)anthracene¹

Department of Pathology, University of Chicago, Chicago, Illinois 60637

The natural history, morphology, and hormone responsiveness of 218 7,12-dimethylbenz(a)anthracene-induced breast tumors in the rat were observed and classified. The cell distribution and biochemical activity of nonspecific phosphatases and β-glucuronidase were studied. One-third of all tumors grew progressively, one-third regressed spontaneously, and one-third remained stationary in size over a prolonged period. Three histologic patterns were distinguished which reflected the degree of differentiation, and a fourth category was characterized by secretion-like changes which appear to be a response to estrogen stimulation. Histologic type was related to growth pattern at either end of the growth spectrum, but in the large middle group of moderately well-differentiated tumors no definite correlation was present. In an additional group of tumors, ovariectomy caused regression of all tumors. This effect was prevented in some, but not all, by administration of exogenous estrogen. Similar histologic features were present in those tumors which regressed following ovariectomy and those which regressed spontaneously.

Alkaline phosphatase was localized in myoepithelial elements of the tumors, and the distribution of the enzyme was not altered after ovariectomy or estrogen administration. The activity of alkaline phosphatase was significantly reduced in static as compared to actively growing tumors. β-Glucuronidase was increased in a small group of tumors showing secretion-like activity histologically.

¹ This investigation was supported by USPHS Research Grant No. CA-06981 from the National Cancer Institute.

Received 6/26/67. Accepted 10/ 4/67.

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