EFFECTS OF SEASON ON STRATUM CORNEUM 195 STRUCTURAL PROTEINS Figure 6 shows results for the key structural proteins keratin-1,10,11. These keratins are formed during terminal differentiation of keratinocytes to form the SC (59–61). Keratin-1,10,11 levels were signifi cantly higher in summer compared to winter. Involucrin (not shown) was not signifi cantly different between the seasons. The increased level of structural proteins is consistent with improved barrier function in summer. A previous report that keratin 10 is lower in dry skin compared to normal skin (62) is consistent with our fi ndings of lower keratin-1,10,11 in winter. CYTOKINES Infl ammatory cytokines are elevated in irritated skin, and the ratio of IL-1ra:IL-1α is especially sensitive to irritation (18,19,22). Figure 7 shows that the ratio of IL-1ra:IL-1α was signifi cantly higher in winter compared to summer. The reduced IL-1ra:IL-1α ratio is consistent with healthier and less infl amed skin in sum- mer. It is interesting that erythema grades were not lower in summer even though IL- 1ra:IL-1α ratio was reduced, indicating some increased level of underlying infl ammation in dry skin that does not exhibit as increased redness. This may represent an example of what Kligman called invisible dermatoses (63). SC LIPIDS The multilamellar intracellular lipids of the SC are the main barrier to water loss to the environment allowing us to live in a terrestrial environment (64–67). SC lipid profi les have been found to be altered in dry skin (68), surfactant-damaged skin (2,68,69), and in patients with AD (70–74). Ceramides are long-chain fatty acids amide linked to one of four sphingoid bases. Three types of fatty acids can also be involved in SC lipid profi les, nonhydroxy fatty acids [N], α-hydroxy fatty acids [A], and esterifi ed ω-hydroxy fatty acids [EO]. This leads to the 12 Figure 6. Structural proteins, keratin-1,10,11 normalized to BCA protein analysis.

JOURNAL OF COSMETIC SCIENCE 196 classes of ceramides as shown in Table II (72,75,76). Ceramides, especially long-chain ce- ramides such as those in the ceramide EOS class (formerly ceramide 1), have been found to be especially important for the proper formation of the SC barrier lipid structure (77,78). Results of the lipid analysis sorted by largest difference between summer and winter are given in Table III. The ceramide nomenclature is given in Table II: C30_C18_2 is a ceramide EO with 30 carbon chain esterifi ed to a C18 fatty acid with two double bonds, N30_0_P18 Figure 7. IL-1ra:IL-1α ratio. The ratio was signifi cantly higher in winter. Table II Fatty Acids and Sphingoid Bases That Can Combine to Produce 12 Possible Ceramide Classes: 11 Are Known in Skin.* *Modifi ed from Reference (75) with permission of Allured publishing.