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

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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 μm or even negligible in sebum-poor extremities. However, in the sebum-rich face, its thickness was > 4 μm in focal areas. Consistent with the thickness of SSLF, the sebum quantity showed great regional variation. It varied from 1 μg/cm2 (leg) to 189 ± 42.7 μg/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.