Human epidermal basal cell responses to ultraviolet-B differ according to their location in the undulating epidermis

Background: Exposure of skin to excessive ultraviolet-B (UVB) radiation causes epidermal hyperproliferation that leads to epidermal hyperplasia, however, it is not yet clear exactly how these responses progress. Objectives: We attempted to clarify the response patterns involved with epidermal hyperproliferation following UVB radiation. Methods: UVB was irradiated at 2 minimal erythema doses (MED) to human back skin and epidermal morphologic changes were evaluated using in vivo confocal laser microscopy. Skin biopsy specimens were collected from exposed and from non-exposed regions, and were subjected to histochemical and immunohistochemical analysis. Results: The in vivo confocal laser microscopic analysis showed that UVB-induced epidermal hyperplasia was prominent at the epidermal rete ridges. Further, 3 days after UVB exposure, numerous Ki67-positive epidermal cells were observed in the epidermal rete ridges, but not in the epidermis at the top of the dermal papilla. These results suggest that cells highly responsive to UVB exist in the epidermal rete ridges and that their hyperproliferation leads to elongation of the epidermal rete ridges. In contrast, the number of keratin 10-positive basal cells, known as transitional cells, was increased throughout the epidermis, suggesting that an upward migration of keratinocytes from the epidermal basal layer occurred regardless of their location. However, diffusion of melanin to the suprabasal layers was markedly observed in epidermal regions above the dermal papillae, suggesting the occurrence of strong upper cell movement at this position. Conclusion: Based on our results, we conclude that differences in keratinocyte responses to UVB radiation exist in cells located in the undulating epidermal basal layer. (c) 2004 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved.