The ultraviolet microscopic transmission characteristics of human stratum corneum

Synopsis This study has evaluated the ultraviolet light transmission characteristics of human stratum corneum at the single cell level using a low light level video microscope to measure the mean percentage transmission of light at different wavelengths and the variation in transmission across the stratum corneum. Stratum corneum was isolated by an enzymic technique and examined on a low light level UV video microscope. Quantitative evaluation of the transmitted monochromatic light for the underside of the layer was measured directly using a Kontron UNIPS image processor or indirectly with a Quantimet 920 image processor after video recording. Transmission distribution histograms were obtained from samples of stratum corneum taken from human breast, scalp, abdomen and leg. Mean transmission values were also derived and compared with diffuse transmission values obtained using the same tissue mounted on an integrating sphere. The UV microscopic transmission characteristics of enzyme separated stratum corneum clearly demonstrated that this structure was not an ideal diffuser. Uniform light intensity on the surface of the stratum corneum led to areas of transmitted intensity in close proximity that differed by factors ofthree to six fold, e.g. between regions of high (>70%) and low (<20%) brightness. However, the average transmission was found to be compatible with published data obtained by diffuse transmission spectrophotometry, taking into account the enhanced transmission arising from stratum corneum immersion in phosphate buffered saline. This was confirmed by the elevated values obtained by diffuse transmission spectrophotometry in this study for samples of stratum corneum prepared for UV microscopy being higher than these found in published data. It is obvious from these findings that viable cells in the epidermis are not exposed to a uniform incident light intensity even when this is true for the surface of skin. Studies of skin response to ultraviolet light at the single cell level must take account of the possibility of preferential exposure of specific sites in any subsequent explanation of cell sensitivity. This is in addition to the already well established cell cycle dependent ultraviolet sensitivity.