220 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS disjunctum regions, respectively. Degradation of desmosomes is believed to be primarily responsible for the desquamation of the outer corneocyte layers. Some of the important roles of the stratum corneum are to prevent the desiccation of underlying regions, provide moisture retention, and restrict the percutaneous absorption of extraneous substances. The water content of the stratum corneum is dependent upon the quantity of water diffusing into the corneum from underlying layers, its ability to retain water, and the amount of water lost due to evaporation. The barrier property of the stratum corneum (barrier to water loss) is believed to reside in the intercellular lipid lamellae (4) that form a highly ordered, impermeable membranous structure between the corneocytes. An increase in water loss may occur either as a result of decreased ambient humidity or by surfactant-induced cellular perturbations. Dry skin is attributed to defective desquamation, but the mechanisms responsible for surfactant-induced dry skin are not well understood. Although the intercellular lipids are considered responsible for the barrier action of skin, no consensus has emerged regarding the role that the lipids play in dry skin conditions (2). Damage to the ultrastructure of epidermis has been shown to cause decreased water retention properties of the stratum corneum and abnor- mal scaling (5-6). Our current understanding of the influence of surfactants on stratum corneum lipids is not clear, as conflicting evidence exists in the literature. For example, depletion of lipids in surfactant-exposed (7) or solvent-exposed skin (8) is believed to be responsible for increased water loss and for dry skin conditions. On the contrary, Fulmer and Kramer (9) reported that the effect of sodium dodecyl sulfate (SDS) on stratum corneum did not result in lipid depletion. However, significant differences in specific lipid classes were observed. Similarly, lack of damage to lipids was also reported by Fartasch et al. (10), who observed the presence of intact lipid lamellae in the outer layers of surfactant- treated stratum corneum. The present in vitro study attempts to correlate the ultrastructure of the stratum corneum with its water retention properties and the integrity of the barrier after exposure of human cadaver skin to three different surfactant systems. The observed ultrastructural preservation of lipids and corneocytes (using TEM) and surface features (using ESEM) was correlated with TEWL. MATERIALS AND METHODS TREATMENT, WASH, AND TEWL MEASUREMENTS A 4" x 8" piece of abdominal skin from a 31-year-old Caucasian male was mounted (after the removal of the adipose tissue) on an in vitro skin-washing device (11), and the ends of the skin were secured. The temperature of the reservoir was maintained at 37øC using a circulating water bath. The surface of the skin was divided into four equal sections for test treatments using water and three formulations (Table I). Baseline TEWL readings were taken at eight sites in each section. Skin sections were subjected to the following wash procedure: (a) The section was soaked for 15 s under running tap water at 40øC (b) the test bar was wetted under running tap water (40øC) and rotated ten times in the palm (c) the product was applied to the surface of the skin with fingers for 2.0 minutes

SURFACTANT-SKIN INTERACTIONS Table I Principal Constituents of Different Formulations Product Main constituents Formulation A: Isethionate bar Fatty acid isethionate, stearic acid, sodium tallowate, sodium isethionate, coconut fatty acid, sodium sterate, sodium alkyl benzene sulfonate, water, sodium cocoate, fragrance Sodium tallowate/cocoate, glycerin, almond oil, water Sodium cocoate, sodium tallowate, water, fragrance, sodium chloride Formulation B: Glycerin-oil bar Formulation C: Pure-soap bar using a back-and-forth motion, with fingers rewetted after one minute (d) post-wash, the skin surface was rinsed with running tap water (40øC) for 30 seconds (e) steps 2, 3, and'4 were repeated for a total of 15 washes (f) after the final wash, the surface of the skin was patted dry using a paper towel. The apparatus was allowed to equilibrate for one hour. Subsequently, TEWL measurements were taken from the sites used for base- line. The difference between the initial and final values, the delta TEWL value, is presented in Table II. ENVIRONMENTAL SCANNING ELECTRON MICROSCOPY Four 1-cm biopsies from each piece of treated skin were examined in the ElectroScan Model E-3 ESEM. Since no specimen preparation is required for observation using ESEM, the biopsies were placed on a 1-cm aluminum stub for examination in a moist environment. The microscope (equipped with a Peltier cooling stage) was operated at -8øC, and a chamber pressure was maintained at 7.3 Torr. All samples were viewed at an accelerating voltage of 10 kV. TRANSMISSION ELECTRON MICROSCOPY Small pieces of skin (-1 x 2 mm) were dissected from each piece of exposed tissue that corresponded to various TEWL values. Tissue pieces were fixed overnight at 4øC in Table II Transepidermal Water Loss of Tissue Examined by TEM or ESEM A TEWL in g/m 2 hr Specimen Formulation Formulation Formulation no. Water A B C Technique 1 0.37 1.61 2.12 11.53 TEM 2 0.64 2.02 2.30 16.65 3 1.03 2.56 2.76 32.67 4 1.59 2.86 3.98 34.52 5 0.49 1.17 2.54 6.39 ESEM 6 0.56 1.61 3.93 8.99 7 0.66 3.05 4.10 22.34 8 0.92 3.80 4.79 58.35