Covering by a flap induces apoptosis of granulation tissue myofibroblasts and vascular cells

It has recently been shown that during the healing of an open wound, apoptosis mediates the decrease in cellularity during the transition between granulation tissue and scar. Because reduced contraction and a decrease in the number of fibroblastic cells have been described in wounds covered with a successful skin graft, we hypothesized that apoptosis could be responsible for these phenomena. Using in situ labeling of fragmented DNA, immunohistochemistry for alpha-smooth muscle actin, and electron microscopy, we have studied in rats the evolution of 10-day-old wound tissue covered with a total skin flap (containing epidermis, dermis, and the cutaneous muscle). In 10-day-old wound tissue, few apoptotic vascular cells and rare apoptotic myofibroblasts were present; the number of apoptotic cells increased slightly 72 hours later. In wounds covered with total skin flaps, the number of apoptotic vascular and myofibroblastic cells increased drastically 6 hours after flap application with a maximum at 24 and 48 hours, respectively. A decrease of apoptotic cell number was noted at 72 hours; at this time, the size of the granulation tissue was greatly reduced and showed extracellular matrix remodeling. Total flaps were more efficient in the induction of granulation tissue cell apoptosis compared with dermo-epidermal flaps. Moreover, the control application of full-thickness skin autografts, which were not viable 7 days later, did not induce apoptosis 24 hours after implantation. Our results indicate that covering granulation tissue with a skin flap results in a massive apoptotic process, possibly by means of a (some) locally released substance(s).