Forty-five 5-week-old female Swiss mice were subjected to a single interscapular painting of 0.6 per cent methylcholanthrene in benzene. Microscopic changes of hair follicles in the painted area were followed to 15 days after painting. Normal skins from the same region of fifteen animals were used as controls.

Cyclic changes were observed in the thickness of the subcutis, indicating substantial alterations in blood supply and fat and striking individual variations of the hair follicular cycles in the same region (interscapular) of the skin in animals of even the same strain, age, and sex.

The response of hair follicles to the carcinogen is a highly localized phenomenon. Altered hair follicles, as well as normal follicles, were both present in the painted area. The changed follicles were first transformed into clublike solid clusters of cells connected to or isolated from the epidermis and then into cystic formations. There is a close association in the response of hair follicles and sebaceous glands. In no instance were these altered follicles observed to have attached sebaceous glands. Hair follicles in the period of late catagen or telogen phase (waiting to grow) seem to be more susceptible to the carcinogen than follicles in other phases. Follicles in the mid-anagen phase (vigorously growing period) are more resistant.

Cytological evidence indicates that the stage is set in both epidermis and follicles in the first 15 days after a single application in much the same way in both tissues. Degenerated, binucleated, and giant cells, cells with accentric nuclei and keratinated cytoplasmic clumps, and increases of cellular, nuclear, and nucleolar sizes were observed.

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Aided by grants from the Charles F. Kettering Foundation and from the American Cancer Society.

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