The Influence of Complete Blockage of the Nipple on the Incidence and Location of Spontaneous Mammary Tumors in Mice

Abstract

It is generally admitted that constitutional factors predispose towards the development of neoplastic growths. Given a genetic complex permitting the development of a tumor, whether or not the potentiality becomes an actuality is known to be influenced by non-genetic or, in a wide sense of the word, environmental factors. Among such factors, chronic irritation is believed to be one of the most efficacious and hence to play an important part in the etiology of neoplastic growths. An evaluation of the effect of one type of chronic irritation—milk stagnation—on the incidence of mammary gland tumors has twice been attempted experimentally in mice. Bagg (1925) ligated the mammary ducts on one side of pregnant females of a strain in which about 5 per cent normally developed carcinoma of the breast. This operative procedure was followed by suckling of the litter and remating the female. Although very small numbers were involved, at least two females developed carcinoma on the ligated side. This, in conjunction with other methods of bringing about dysfunction of the breast, indicated that milk stagnation might be important in producing tumors. Bogen (1935) similarly ligated the mammary ducts of 18 females, of which 6 developed mammary cancer while 18 normal breeding controls died without tumors. Three circumstances tend to render these investigations inconclusive: (1) the very small number of animals in each; (2) the possible effect of operative injury on mature glands; (3) the low normal tumor incidence, together with the unknown genetic constitution of the mice in the second experiment, making an adequate control difficult. To test the possible effects of occlusion of mammary ducts with resulting stagnation of milk, and at the same time to overcome the objections cited above, the following experiment was designed.