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THE BALD AREA OF THE GIJINEAPIG* HERSCHEL S. ZACKHEIM, M.D. AND LENORE LANGS, B.S.

The purpose of this report is to draw attentioncell layer. Melanocytes, as revealed by the DOPA to the presence of a hairless area behind the eartechnic, are abundant in the basal layer if the of the guinea-pig. In conversations with others,region is pigmented (Fig. 3). Directly under the it is apparent that many investigators are notepidermis is a thin layer of fine collagen fibers. aware of it. While its occurrence has been men-The bulk of the cutis is composed of coarse tioned (1), we have not found any publishedcollagen fibers similar to the cutis elsewhere. The detailed description of it. cutis of the bald region is of good thickness, but The area varies from about 1.0 to 1.5 cm. inis somewhat thinner than that of the adjacent diameter depending on the size of the animal.skin or back. Representative sections of the cutis It is symmetrically located caudal to the ear, andof the bald spot varied in thickness from 0.5 to is separated from the posterior surface of the1.0 mm., while the cutis of the adjacent skin and ear by a rim of coarse hairs. It may be completelyback ranged from 0.7 to 1.2 mm. Scattered bald or contain some scattered fine hairs (Fig. 1).striated muscle fibers were frequently found in the This region tends to be slightly larger andmid or upper cutis of the bald spot (Figs. 2, 3). more completely bald in albinos than in coloredIn some sections these fibers seemed to branch animals, although good sized bald areas may beoff from the underlying panniculus carnosus, found in pigmented animals as well. although as a rule this connection was not In colored animals the bald region is usuallyobvious. It is also possible that they are connected more deeply pigmented than the epidermis of theto the striated muscle at the base of the ear. adjacent hairy skin and rest of the trunk, and inStriated muscle fibers in the cutis were not seen some cases the contrast may be quite striking.in skin adjacent to the bald area or from the The pigmentation of the bald area may be solidback. There is a network of fine elastic fibers in or mottled (Fig. 1), or the area may be sharplythe upper cutis which becomes fragmentary in divided into a pigmented and non-pigmentedthe mid and lower cutis. These fibers are finer portion if it is in a region where the colored andand less abundant than those of the hairy coat. white parts of the hairy coat are contiguous. There was no noticeable difference in the type or quantity of cellular elements in the hairless cutis MICROSCOPIC APPEA RANCE as compared to that elsewhere. Mast cells were Aside from the absence or sparsity of hairinfrequent. Underlying the cutis, and separated follicles and sebaceous glands, the skin of thefrom it by connective tissue is a thick layer of bald area is essentially similar to that of thestriated muscle, the panniculus carnosus. hairy coat (Fig. 2). One interesting finding is the DISCUSSION frequent occurrence of isolated striated muscle The presence of a skin area which is completely, fibers in the upper and mid cutis. Sweat glands or almost completely, devoid of both hair follicles are absent, just as they are absent elsewhere in and sebaceous glands, as well as sweat glands, guinea-pig skin (2). may be of value for a wide variety of investigative The stratum corneum is usually well developed studies. Even though the area is not large, it may and is roughly 50—75 in thickness. The epidermis be large enough for many purposes. In contrast is three to four cells thick and has a thin granularto the skin of the hairless mouse (3), the bald *Fromthe Detroit Institute of Cancer Researcharea of the guinea-pig may be completely lacking (William L. Simpson, M.D., Director) and the Department of Dermatology, Wayne State Uni-in sebaceous glands, as well as hair follicles, and versity College of Medicine (Hermann Pinkus,does not contain keratinizing sebaceous cysts. M.D., Chairman), Detroit, Michigan. This region also lends itself for studies on pig- This work was supported in part by an institu- tional grant from the United Foundation ofmentation inasmuch as the behavior of epidermal Greater Detroit through the Michigan Cancermelanocytes independent of any relationship to Foundation and in part by a grant from the Re-hair follicles may be observed. Also the heavier search Corporation of Detroit Receiving Hospital. Received for publication June 26, 1961. pigmentation of the epidermis in this area as 347 S.' t I:t ?'

1/ /4 Fja. 1. Bald area behind the ear of a large brown guinea-pig. This was 1.6 cm. in diameter. Note mottled pigmentation.

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Fic. 2. Section through bald area of an albino guinea-pig. No pilo-sebaceous structures or sweat glands are present. A thin strand of striated muscle (arrow) traverses the cutis. A thick striated muscle layer, the panniculus carnosus (p.c.), underlies the cutis and is separated from it by some loose connec- tive tissue. H & E, X 70. 348 THE GUINEA-PIG BALD AEEA 349

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— s- —. L C • lMt•—,_ -- e \ p _30 A, a 2 FIG.3. Closer view of pigmented epidermis of bald spot. There are many melanocytes in the bAsal cell layer. Arrow points to an isolated striated muscle fiber. DOPA preparation stained with hematoxylin, X 210. compared to the more lightly pigmented epi-coat. Because of these features, this arEa may dermis of the hairy coat may have investigativelend itself to a variety of investigations relating possibilities. to the physiology and inter-relationships of the epidermis, cutis and melanocyte system uncom- SUMMARY plicated by the presence of pilo-sebaceous struc- Attention is directed to the presence of atures or sweat glands. bald, or almost completely bald, area behind the REFERENCES car of the guinea-pig. This region is particularly1. HIERONYMI, E.: Die Haut des Meerschwein- well developed in albinos, and may be com- chens, in Pathologic der Laboratoriumstiere, p. 593. Berlin, Springer-Verlag, 1958. pletely devoid of hair follicles and sebaceous2. MONTAGNA, WILLIAM: Personal Communica- glands. It also contains no sweat glands. In tion. 3. LORENc, E. AND WINKELMANN, R. K.: Evalua- colored animals the epidermis of this region is tion of dermatologic therapy. A.M.A. Arch. more heavily pigmented than that of the hairy Dcrm., 83: 761, 1961.