Local Increase of Pigmentation in the Skin of CBA Mice by the Application of Benzene *

The skin of a normal adult black-agouti (gray) mouse appears to be dull white in color. On the surface, however, small patches of black are to be seen at certain exposed sites, such as, in the male, on the tip of the penis and on the end of the scrotum, and, in the female, on the dorsal lip of the vagina. Other larger black areas may be made out in the skin after the hair has been removed. With advancing age these pigmented areas become progressively enlarged or extended and new ones appear, particularly on the dorsum and on the ventral aspect of the hind legs. Examination of the inner (corial) surface of the removed skin also discloses black patches which are particularly noticeable in the groins. With advancing age these deeper black patches in the skin also become more extended until, in very old gray mice (especially CBA mice which have been selected toward longevity over a period of years) the skin on the entire dorsum is more or less heavily mottled with pigmentation. The histological details of these pigmentary changes during the aging process are being further investigated. Steiner-Wourlisch5 has made a careful study of the melanotic pigment in the skin of several animals, induding the gray mouse (black-agouti). She described the general distribution of melanoblastic cells of the skin aided by the Dopa reaction. It was found that the gray mouse, in the stock examined, possessed melanoblastic cells not alone in the epidermis, but also in the corium where they were very plentiful. The melanoblastic cells occurred in the upper half of the corium, yet they may be separated from the basal layer of epithelium by a zone more or less free of pigment. At times, isolated pigmented cells were close to the basement membrane of the epidermis. Dendrites of these cells ran more or less parallel to the surface of the epidermis and in some places showed a concen-