Variation in Epidermal Morphology in Human Skin at Different Body Sites as Measured by Reflectance Confocal Microscopy

Abstract

Two methods of estimating stratum corneum thickness using reflectance confocal microscopy were examined, and epidermal thickness measurements at multiple body sites were compared. Measurements of stratum corneum thickness were made using the derivative method, which is based on the rate of change of image intensity as a proxy for keratin concentration, and simple visual analysis of confocal images. To compare epidermal thickness we collected 1491 z-axis stacks of confocal images from 10 body sites in 39 subjects. An artefact associated with the imaging process interfered with the derivative method for stratum corneum thickness, and simple visual analysis is to be preferred. Although some epidermal properties varied by site, the most striking finding was the degree of within-site variation, which accounted for between 50% and 74% of the total variation observed. The majority of this variation was not due to measurement error, and represents genuine topographical irregularity. This finescale variation limits the ease of use of reflectance confocal microscopy for quantitative studies of the epidermis and stratum corneum.There is a scale to skin structure and hence skin biology that is not served well by techniques such as conventional transmission microscopy and subcellular analyses on the one hand, and macroscopic examination with the naked eye on the other. For instance, we have shown that differences in epidermal or stratum corneum (SC) thickness at different body sites may be important determinants of site variation in sensitivity to ultraviolet radiation (UVR) (1, 2). However, study of these factors is difficult (3); repeated biopsy at various ultraviolet doses and time-points is ethically problematic, and biopsy means that a single area of skin cannot be observed longitudinally, nor can the three-dimensional structure of skin in vivo be appreciated. The fact that the epidermis and the components of the epidermis are not flat regular two-dimensional sheets, but irregular undulating three-dimensional structures, is a quality that has relevance to many aspects of skin biology, including percutaneous absorption, UVR-induced carcinogenesis and ageing. The present study examined the use of reflectance confocal microscopy (RCM) to study epidermal morphology, comparing different methods of measuring SC thickness.RCM has been described as a new gold standard for the measurement of epidermal thickness (3, 4). It provides a non-invasive imaging tool capable of obtaining en face images of living tissue in vivo (3). It has none of the problems associated with biopsy-obtained samples that become more or less distorted (swelling or shrinkage) during different steps of the preparation procedure (5-8).The interpretation of RCM images is based largely on pattern recognition and histological knowledge of skin anatomy (4, 9, 10). For SC thickness one manufacturer of reflectance confocal microscopes suitable for use on human skin, Lucid (Rochester, NY, USA), have proposed a method that we refer to as the derivative method (DM) (pe...

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