j. $oc. Cosmet. Chem., 34, 317-326 (September/October 1983) The use of the domestic pig as an animal model of human dry skin and for comparison of dry and normal skin properties DONALD L. BISSETT and JAMES F. McBRIDE, The Procter & Gamble Company, Miami Valley Laboratories, Cincinnati, OH 45247. Received February 1, 1983. Presented at the Society of Cosmetic Chemists Annual Scientific Seminar, Cincinnati, Ohio, May 1983. Synopsis At low relative humidity, the domestic pig develops dry, scaly skin on its dorsum. This dry skin is visually like that seen on humans, and responds to treatment with known skin conditioners. This animal model is valuable as a tool for screening skin conditioning materials and comparing the properties of dry and normal skins. Comparison of dry and normal dorsal pig skins revealed no differences in stratum corneum thickness, turnover rate, water-binding capacity, or "natural moisturizing factor" (NMF) content. Dry skin did have an increased nonpolar lipid content. This lipid was confined to the upper stratum corneum and was sebaceous in origin. Dry skin in the pig appears to be a phenomenon of the upper stratum corneum. INTRODUCTION The importance of water in skin conditioning was first reported by Blank in 1952 (1). He observed that applied oils did not soften dehydrated stratum corneum (callus), but that water was readily absorbed by the tissue and softened it, indicating that the water content of stratum corneum was of great importance in maintaining flexibility and, presumably, good skin condition. It is generally accepted that dry skin has a reduced water content. However, there is no generally accepted model of the properties of dry skin. Most determinations have been done using abnormal tissue, such as callus (1), or diseased skin, such as that present in ichthyosis, psoriasis, and essential-fatty-acid deficiency (2-6). These tissues were used because of the lack of dry skin for in vitro research and because of their visual similarity to dry skin. The results from use of these abnormal tissues predict that dry skin is thickened, is hyperproliferative, is depleted in "natural moisturizing factor" (NMF) (7), is deficient in water-binding capacity, has an abnormal lipid composition, and has an abnormal permeability to water. The lack of an animal model for dry skin has been a barrier to progress in this area. We report here the development of an animal model of dry skin and some of the properties of that dry skin. 317

318 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS MATERIALS AND METHODS White domestic pigs (40-50 pounds) were obtained from a local farmer and maintained on Purina Pig Chow © . To generate dry skin, they were housed in an insulated room equipped with a custom-built dehumidifier (Bry-Air Inc., Sunbury, OH) capable of maintaining a fresh-air environment of 10% relative humidity at 70øF. Hair was removed from the dorsal surface with electric clippers. Visual grading of skin condition was done by two trained graders using a 0 to 5 scale (see Table I) at the start of a study and Table I Grading Scale for Skin Condition Grade Description 0 1.0 2.0 3.0 4.0 5.0 Skin smooth and lustrous, no detectable dryness or ashing Skin lustrous, slight ashing visible Skin dry, without luster moderate ashing covering general surface area Skin slightly rough overall with slight scaling high amount of ashing covering total area Skin has moderate to high roughness moderate scales with some small cracks high ashing overall Skin highly rough with high scaling, large cracks and high overall ashing prior to the start of each week's treatment regimen. Treatment with skin conditioning materials was done by applying one ml of material per 150 cm 2 once daily (five days per week for four weeks). For generation of visually normal skin from dry skin, petrolatum was applied. Additional normal skin was obtained by housing a pig in a shaded pen outside in the summer. These two types of normal skin had the same properties, as determined by the methods described below. Animals were sacrificed by injection of sodium pentobarbital in an ear vein. Human skin conditioning work was done using winter (in Cincinnati, Ohio) elbow dry skin. 0.3 ml of product was applied twice daily to the elbow. Visual grading of skin was done by two trained graders using a 0 to 5 scale (Table I) at the start of the study and after one and two weeks of treatment. Determination of stratum corneum turnover in vivo was done using dansyl chloride (8), which was obtained from Sigma Chemical Co., St. Louis, MO. For microscopy, whole skin punches (8 mm diameter) were frozen in liquid nitrogen-cooled Freon ©, mounted in Freeze-ease ©, and cryosectioned (from dermis toward epidermis) at 8 •tm on an Ames cryostat. Sections were unstained or were stained with Sudan black (9) or hematoxylin and eosin (10) and examined with a Zeiss photomicroscope. For measurement of stratum corneum thickness, tissues were coated with collodion prior to sectioning the presence of collodion on the surface of unstained sections served as a marker for tissue intactness. Mitotic indexes were determined as described by McOsker and Beck (11). To verify that dansyl chloride had penetrated into the entire stratum corneum in the turnover determination, tissue treated in vivo for 24 hours with dansyl chloride was sectioned as above and examined with a Zeiss photomicroscope with fluorescent optics using an ultraviolet exciter the entire stratum corneum but not the viable epidermis was stained. Stratum corneum was isolated as described by Van Duzee (12). Transepidermal water loss (TEWL) was done in vitro using a Meeco electrolytic moisture analyzer (13) with