The aged epidermal permeability barrier. Structural, functional, and lipid biochemical abnormalities in humans and a senescent murine model

Abstract

Aged epidermis displays altered drug permeability, increased susceptibility to irritant contact dermatitis, and often severe xerosis, suggesting compromise of the aged epidermal barrier. To delineate the functional, structural, and lipid biochemical basis of epidermal aging, we compared barrier function in young (20-30 yr) vs aged (> 80 yr) human subjects, and in a murine model. Baseline transepidermal water loss in both aged humans and senescent mice was subnormal. However, the aged barrier was perturbed more readily with either acetone or tape stripping (18 +/- 2 strippings vs 31 +/- 5 strippings in aged vs young human subjects, respectively). Moreover, after either acetone treatment or tape stripping, the barrier recovered more slowly in aged than in young human subjects (50 and 80% recovery at 24 and 72 h, respectively, in young subjects vs 15% recovery at 24 h in aged subjects), followed by a further delay over the next 6 d. Similar differences in barrier recovery were seen in senescent vs young mice. Although the total lipid content was decreased in the stratum corneum of aged mice (approximately 30%), the distribution of ceramides (including ceramide 1), cholesterol, and free fatty acids was unchanged. Moreover, a normal complement of esterified, very long-chain fatty acids was present. Finally, stratum corneum lamellar bilayers displayed normal substructure and dimensions, but were focally decreased in number, with decreased secretion of lamellar body contents. Thus, assessment of barrier function in aged epidermis under basal conditions is misleading, since both barrier integrity and barrier repair are markedly abnormal. These functional changes can be attributed to a global deficiency in all key stratum corneum lipids, resulting in decreased lamellar bilayers in the stratum corneum interstices. This constellation of findings may explain the increased susceptibility of intrinsically aged skin to exogenous and environmental insults.