Human skin surface lipid film: An ultrastructural study and interaction with corneocytes and intercellular lipid lamellae of the stratum corneum

Sebum is a complex mixture of lipids, which is secreted by mammalian sebace ous glands, and forms a fluid film over the skin surface. After sebum is se creted, it becomes mixed with lipid from the keratinizing epithelium and fo rms the skin surface lipid film (SSLF). Until now, direct fine structural o bservation 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 thickne ss on the skin surface instead of forming lipid droplets, as had been the u sual assumption. In general, its thickness was 0.5 mu m or even negligible in sebum-poor extremities, However, in the sebum-rich face, its thickness w as > 4 mu m in focal areas. Consistent with the thickness of SSLF, the sebu m quantity showed great regional variation. It varied from 1 mu g/cm(2) (le g) to 189 +/- 42.7 mu g/cm(2) (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 provi de some new insights concerning the structure of the SSLF and its relations hip with the SC.