Early-stage bilayer tissue-engineered skin substitute formed by adult skin progenitor cells produces an improved skin structure in vivo

BACKGROUND: In recent years, significant progress has been made in developing highly complex tissue-engineered skin substitutes (TESSs) for wound healing. However, the lack of skin appendages, such as hair follicles and sweat glands, and the time required, are two major limitations that hinder its broad application in the clinic. Therefore, it is necessary to develop a competent TESS in a short time to meet the needs for clinical applications. METHODS: Adult scalp dermal progenitor cells and epidermal stem cells together with type I collagen as a scaffold material were used to reconstitute bilayer TESSs in vitro. TESSs at 4 different culture times (5, 9, 14, and 21 days) were collected and then grafted onto full-thickness wounds created in the dorsal skin of athymic nude/nude mice. The skin specimens formed from grafted TESSs were collected 4 and 8 weeks later and then evaluated for their structure, cell organization, differentiation status, vascularization, and formation of appendages by histological analysis, immunohistochemistry, and immunofluorescent staining. RESULTS: Early-stage bilayer TESSs after transplantation had a better efficiency of grafting. A normal structure of stratified epidermis containing multiple differentiated layers of keratinocytes was formed in all grafts from both early-stage and late-stage TESSs, but higher levels of the proliferation marker Ki-67 and the epidermal progenitor marker p63 were found in the epidermis formed from early-stage TESSs. Interestingly, the transplantation of early-stage TESSs produced a thicker dermis that contained more vimentin- and CD31-positive cells, and importantly, hair follicle formation was only observed in the skin grafted from early-stage TESSs. Finally, early-stage TESSs expressed high levels of p63 but had low expression levels of genes involved in the activation of the apoptotic pathway compared to the late-stage TESSs in vitro. CONCLUSIONS: Early-stage bilayer TESSs reconstituted from skin progenitor cells contained more competent cells with less activation of the apoptotic pathway and produced a better skin structure, including hair follicles associated with sebaceous glands, after transplantation, which should potentially provide better wound healing when applied in the clinic in the future.