4: ACTA2+ Cells Activation and Dermal Changes During Skin Adaptation to Mechanical Forces

PURPOSE: Tissue expansion (TE) is based on the skin’s exceptional ability to regenerate under mechanical stress and is widely used to repair skin defects. However, knowledge about molecular mechanisms involved in maintaining skin integrity and homeostasis is limited. The present study aims 1) to elucidate the role of myofibroblasts in the mechanism of adaptation to mechanical stress exerted by an inflated tissue expander, and 2) to describe morphological changes in collagen structure that could lead to the re-establishment of dermal tension. METHODS: TE was performed on a porcine model. Each expander was placed subcutaneously over the ribs and two weeks later inflated with 30 cc of saline to induce subtle tension. After 1 day (acute stretch) and 7 days (sub-acute stretch) of expansion, the full-thickness skin biopsies were collected from the apex of the expander and control unexpanded skin (contralateral sites). Skin samples were fixed in formalin and embedded in paraffin for histological evaluation (Russell-Movat Pentachrome staining) or fixed in 4% PFA and embedded in OCT for immunohistochemistry staining (IF) of α-smooth muscle actin (α-SMA), a marker of myofibroblast. Area of fluorescent signal from α-SMA was calculated using ImageJ while collagen morphology was evaluated optically after staining. RESULTS: We compared the presence of α-SMA fluorescence between control biopsies and expanded biopsies after 1-day and 7-days of stretch. The immunofluorescence staining revealed 2.32 times more α-SMA fluorescent staining in skin expanded for 1 day (p = 0.0065) and 2.19 times more α-SMA fluorescence in skin expanded for 7 days (p = 0.0047). The increase in number of α-SMA positive cells were mostly observed in the outermost 400µm of papillary dermis. Histological staining showed minimal collagen morphological changes in both the papillary and reticular dermis after 1-day of expansion. However, shortening of fibril length, increased density, and increased disorder were observed in the papillary dermis collagen after 7-days of subtle expansion.