Are sweat glands an alternate penetration pathway? Understanding the morphological complexity of the axillary sweat gland apparatus

To build an effective barrier against the penetration of extrinsic agents is one of the skin's main functions. The barrier properties of the stratum corneum and the epidermis have been subject to extensive studies in the past while the role of skin appendages as possible pathways of penetration are only rarely described. In order to study the possible penetration barriers in these complex appendages, a careful investigation of their morphology and ultrastructure has to be done. Studying the morphology of axillary skin appendages requires clear-cut criteria for the differentiation between eccrine, apocrine and apoeccrine glands. Therefore we studied the distribution of proteins described to be specific for either eccrine or apocrine glands (CD15, CD44, S-100 and milk fat globulin) on axillary skin samples from healthy young adults by immunofluorescence. Additionally, we examined the distribution of cytoskeletal proteins such as cytokeratins (1/10/11, 14, 18) and F-actin. For a more detailed understanding of the possible versatile barrier elements of the axillary sweat glands, we studied the distribution of tight-junction-associated proteins (occludin, claudin 1, claudin 4). The coils and the dermal duct may provide an active barrier built of tight junctions as occludin and claudin 4 are co-localized. However, the intra-epidermal duct did not show any co-localization of the investigated proteins. By combining morphological features as revealed by F-actin staining and the distribution of the above-mentioned proteins, immunocytochemical typing of eccrine and apocrine glands becomes possible. With this tool, we could also confirm the existence of apoeccrine glands and locate them in their 'natural environment'