CORNEOCYTE SIZE AND PERMEABILITY 21 30 20 10 r = 0.92 e 0.001 6 l/T 7 5 10 Transepidermal water loss (TEWL) (g. m -2. h -• ) Figure 4. In vivo relationship between transepidermal water loss (TEWL) and percutaneous absorption of benzoic acid according to anatomic site, age, and sex, in man. 1. Arm (upper-outer, male, 20-30 yr). 2. Arm (upper-outer, male, 45- 55 yr). 3. Arm (upper-outer, male, 65-80 yr). 4. Arm (upper-outer, female, 20-30 yr). 5. Abdomen (male, 20-30 yr). 6. Postauricular (male, 20-30 yr). 7. Forehead (male, 20-30 yr). 8. Forehead (female, 20-30 yr). 9. Forearm (ventral-elbow, male, 20-30 yr). 10. Forearm (ventral- mid, male, 20-30 yr). 11. Forearm (ventral-wrist, male, 20-30 yr). and the relatively slight difference (a factor of 3) in the penetration of benzoic acid between these two areas? The permeability of the postauricular area towards both water and benzoic acid is close to that of the forehead. From the morphologic standpoint, the closeness of the capil- laries to the surface of the skin may promote the resorption of substances and give the postauricular area its high permeability (23). For common sites, the rank order in corneocyte size established by Piewig et al. (17) and Marks et al. (11) is confirmed in our work. Thus, in areas such as the abdomen, the upper-outer arm, the ventral-mid and ventral-elbow forearm and the abdomen, there is no significant difference in corneocyte size. It is now accepted that an inverse correlation between epidermal cell proliferation and corneocyte size exists (24). Moreover, it has been suggested that there might be differences in the epidermal turnover according to anatomical location (25-28). Note (Table I, Figure 2) that anatomic sites in which corneocyte size does not vary are all areas which are more or less protected by clothing. On the contrary, in regions such as the wrist, the postauricular area, or the forehead, more exposed to environmental factors (including sunlight), corneocytes are smaller. Taking into account the fact that in all the sites tested tb.e total thickness of the stratum corneum (19) and the individual thickness of the corneocytes (29) do not change, the relationship between the flat area of the horny cells and the barrier function of the stratum corneum must be addressed. In permeability phenomena, the current trend is to assign priority to intercellular rather

22 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS than transcellular penetration. Thus, if we consider anatomic sites to contain equal volumes of stratum corneum but corneocytes of unequal volume, then the intercorneal space will be greater in stratum corneum with smaller corneocytes. This could explain the high permeability of sites such as the wrist, the postauricular region, or the fore- head. Thus, it seems likely that corneocyte size may constitute an important factor in the differences in permeability observed, but it can only account for part of the explana- tion. If any of the functions of skin permeability depend more on the physicochemical properties of the intercellular cement than on the volume of the intercellular spaces, this will be evident when sites of equal thickness of stratum corneum and equal volume of corneocytes are compared. Thus, although corneocyte size in areas such as the forearm (ventral-elbow and ventral-mid), arm (upper-outer), and abdomen does not change, there are great differences in permeability to water (TEWL) and benzoic acid. This indirectly confirms the importance of the chemical composition of the intercellular cement in the barrier properties of the horny layer (30,31). As shown in Figure 3 (a,b), when the surface of the corneocytes reaches a critically high area, the volume of the intercorneal spaces is no longer rate-limiting for both TEWL and penetration. These results might appear to contradict those of Marks et al. (11), linking TEWL and corneocyte size. Indeed these authors described a hyperbolic rela- tionship between these parameters. However, they studied anatomic areas of different corneal thickness, thus introducing an important additional source of variation. More- over, the palm must be considered as completely distinct from other skin sites in terms of TEWL (32). If we take into account only the results obtained by Marks et al. on the forearm, the wrist, and the forehead (Figure 3c), there is no major discrepancy between their results and ours. Influence of aging. Our results show no difference in benzoic acid absorption between 20-30 and 45-55 age groups. In subjects aged 65-80, on the other hand, absorption of this molecule is greatly reduced (factor 4). These findings agree with those of Mal- kinson and Fergusson who failed to show any difference in percutaneous absorption of hydrocortisone in adults of ages 41 to 58 (33). The reduced absorption of benzoic acid in the elderly is similar to that obtained with testosterone (34,35). It is a reasonable assumption that this age-related change in the barrier properties could be a consequence of alterations in keratinization and epidermal cell production and manifests itself in altered structure and function of the stratum corneum. Thus, in the 65-80 age group, the surface area of corneocytes was 20 to 25% greater than that measured in volunteers of ages 45-55 and 20-30. Concomitant to this age-associated increase in corneocyte surface area (8- 10), there is a decrease of roughly 30-50% in the epidermal turnover rate between the third and eighth decades of life (36-38). Total thickness of the stratum corneum (39) and individual thickness of corneocytes (11,40) do not change with advancing age. Then the volume of the intercellular spaces must decrease as the surface area of the corneocytes increases. These intercellular spaces probably act as a "reservoir" for topically applied molecules (16). Moreover, it has been clearly demonstrated that, for a given compound, the smaller the capacity of this reser- voir, the less that molecule is absorbed (6, 15,16,41,42). Since the general morpholog- ical organization of the stratum corneum does not appear to be affected by aging (39,43), it would be tempting to conclude that the large differences observed in percu- taneous absorption of benzoic acid with age are solely related to a decrease in the volume