No abstract

Sebum is a complex mixture of lipids, which is secreted by mammalian sebaceous glands, and forms a fluid film over the skin surface. After sebum is secreted, it becomes mixed with lipid from the keratinizing epithelium and forms the skin surface lipid film (SSLF). Until now, direct fine structural observation of the SSLF has been lacking. In the present work, we viewed the detailed structures of the human SSLF by ruthenium tetroxide staining. The results showed that the SSLF formed an amorphous sheet of variable thickness on the skin surface instead of forming lipid droplets, as had been the usual assumption. In general, its thickness was < 0.5 microm or even negligible in sebum-poor extremities. However, in the sebum-rich face, its thickness was > 4 microm in focal areas. Consistent with the thickness of SSLF, the sebum quantity showed great regional variation. It varied from 1 microg/cm2 (leg) to 189 +/- 42.7 microg/cm2 (mean +/- SD: face). The SSLF was composed of numerous fine granules of about 4-5 nm in a random orientation. Within the SSLF, variable amounts of deranged lipid lamellae derived from corneocytes were mixed with sebum. As well as on the skin surface, a similar amount of sebum was also found between the desquamating corneocytes in the uppermost several layers of the stratum corneum (SC). We also observed the presence of intercellular lipid lamellae in the outer layers of the SC: their lipid envelope remained intact even in desquamated corneocytes. Our results provide some new insights concerning the structure of the SSLF and its relationship with the SC.

Previous studies have shown that (1) epidermal TNF alpha mRNA levels are increased following acute disruption of the cutaneous permeability barrier; (2) this increase is maximal at 1 h and decreases to control levels by 8 h; and (3) in essential fatty acid-deficient (EFAD) mice, a chronic model of barrier perturbation, TNF alpha mRNA levels are also elevated several-fold over controls. In the present study we determined, using immunocytochemical procedures, epidermal TNF alpha protein levels following either acute of chronic barrier disruption and the localization of any increase. Frozen, paraffin and Antibed sections of skin were incubated with polyclonal anti-mouse TNF alpha antisera and detection was accomplished by either immunoperoxidase or fluorescence procedures. We found that (1) TNF alpha-immunoreactive protein was present in normal mouse epidermis, and was primarily localized to the upper nucleated layers where it displayed a diffuse cytosolic pattern; (2) acute disruption of the barrier with acetone or tape-stripping resulted in TNF alpha staining that was more intense throughout all of the nucleated epidermal cell layers in comparison with normal epidermis; (3) the increase in TNF alpha staining occurred as early as 2 h after barrier disruption; and (4) increased TNF alpha staining was also observed in the stratum corneum of EFAD mice. These results indicate that epidermal TNF alpha protein levels increase after both acute and chronic barrier disruption, and are consistent with the hypothesis that TNF alpha may signal and/or coordinate portions of the cutaneous response to barrier disruption.

Prior studies in hairless mice have demonstrated that acute barrier disruption by acetone treatment increases the molecular weight (MW) cutoff of polyethylene glycol (PEG) penetration through the skin. The objective of the present study was to further investigate the dependence of permeability on MW with different forms of barrier disruption. A series of PEGs ranging in MW from near 300 to over 1000 Da were used to study the effects of tape stripping and sodium dodecyl sulfate (SDS) treatment on the MW permeability profiles of mouse skin in vitro. The 12-h percutaneous penetration of all the PEG 300, 600, and 1000 oligomers generally increased as a function of transepidermal water loss (TEWL) of the skin, either tape-stripped or SDS-treated. In addition, the total penetration of PEG oligomers across control skin, and skin tape-stripped and SDS-treated to different degrees of barrier disruption progressively decreased with increasing MW. There were no significant differences in the percutaneous penetration of the PEG oligomers between skin tape-stripped and SDS-treated to the same degree of barrier disruption. The penetration enhancement relative to control skin was more prominent with larger molecules. The MW cutoff for skin penetration increased with the degree of barrier disruption irrespective of the treatment applied, and was 986 Da (tape stripping) and 766 Da (SDS treatment) at TEWL levels in the range 10-20 g/m(2) per h in comparison with 414 Da for control skin. In accordance with previous findings in acetone-treated mouse skin, the results strongly suggest that, irrespective of the form of barrier disruption applied, not only higher amounts but also more varieties of chemicals (larger molecules) may penetrate skin with a compromised barrier than normal skin.