Alterations in epidermal stem cells within the pilosebaceous unit in atrophic acne scars

Abstract Background Atrophic acne scarring is a common sequela of inflammatory acne, causing significant problems for affected patients. Although prolonged inflammation and subsequent aberrant tissue regeneration are considered the underlying pathogenesis, the role of epidermal stem cells, which are crucial to the regeneration of pilosebaceous units, remains unknown. Objectives To examine the changes occurring in epidermal stem cells in atrophic acne scars. Methods Changes in collagen, elastic fibre and human leukocyte antigen (HLA)‐DR expression were analysed in normal skin and inflammatory acne lesions at days 1, 3 and 7 after development. The expression of epidermal stem cell markers and proliferation markers was compared between normal skin and mature atrophic acne scar tissue. Results In acne lesions, inflammation had invaded into pilosebaceous units over time. Their normal structure had been destructed and replaced with a reduced amount of collagen and elastic fibre. Expression of stem cell markers including CD34, p63, leucine‐rich repeat‐containing G protein‐coupled receptor (LGR)6 and LGR5, which are expressed in the interfollicular epidermis, isthmus and bulge of hair follicles, significantly decreased in atrophic acne scar tissue compared to normal skin. Epidermal proliferation was significantly reduced in scar tissue. Conclusions These findings suggest that as inflammatory acne lesions progress, inflammation gradually infiltrates the pilosebaceous unit and affects the resident stem cells. This disruption impedes the normal regeneration of the interfollicular epidermis and adnexal structures, resulting in atrophic acne scars.