Localized changes in methylcholanthrene-treated epidermis.

Since Yamagiwa and Itshikawa in 1915 (14) succeeded in inducing cancer in rabbits by means of coal tar, investigators have been aware that the induced cancers appear within fairly sharply defined loci. Most data secured by chemical analysis do not supply direct information about these foci; because, in order to obtain enough material for analysis the treated epidermises of several mice are usually pooled so that with the foci is included a large volume of epidermis that does not undergo local malignant transformation. Methods are needed that will reveal the chemical composition of areas of tissue of microscopic size. The immediate objective of this investigation is to learn more about these focal tissue changes by microscopic examination of whole mounts of epidermis-a tech-nic more effective for this purpose than the study of stained sections. Ultimately it is hoped that the histochemical method can be applied to such whole mounts. MATERIAL AND METHODS The backs of young female Swiss mice 6 to 8 weeks old were shaved, care being taken to avoid mechanical injuries. Three days later 0.6 per cent methylcholanthrene in benzene was applied to the shaved areas with a camel's hair brush regularly at 10 A.M. (St. Louis time) because the mitotic frequency of mouse epidermis is highest at this time (4). Only three paintings were made. A second group of mice were similarly treated with benzene only. A third group was examined untreated. Mice of the first 2 groups were sacrificed for examination 7 to 30 days after the first painting by a single blow on the head at 10 A.M. The painted area of skin was in each case removed and spread over a piece of filter paper. Both skin and paper were immersed in a jar of 1 per cent acetic acid (5) and were maintained at 5 ~ C. for 3 to 5 hours in order to macerate the connective tissue fibers which bind the epidermis to the dermis. For normal skin * A part of a dissertation submitted to the Board of Graduate Studies of Washington University in partial fulfillment of the requirements for the degree of Master of Science in Anatomy. about 3 hours were required for maceration whereas for the hyperplastic skin 4 to 5 hours were necessary. Maceration in excess of 5 hours damages the basal layer and greatly impairs stainability of the cells. After 3 to 5 hours in cold …