br Introduction The grafting of cultured keratinocytes to

Introduction The grafting of cultured keratinocytes to promote regeneration represents one of the oldest clinical examples of stem cell therapy (Green, 2008). The skin constitutes an essential barrier between the living tissues of the body and the external environment, and skin tissues have evolved to maintain that barrier: water is retained and noxious substances and invasive organisms are excluded, and new skin normally can be regenerated rapidly in the event of a break in this barrier. However, large interruptions in the skin are life threatening: burns can result in deep, extensive wounds that are slow to close without medical intervention. The gold-standard treatment for large wounds is autologous split-skin grafts, but this is not possible for extensive full- or partial-thickness burns covering over 50% of the body surface area. In addition to acute skin injuries, chronic wounds are now a growing medical challenge as nonhealing wounds become more common in aging populations of the developed world, and increase further with rising rates of diabetes and resulting circulatory deficiencies. Large wounds are usually grafted with cadaveric skin (if available) to form a temporary barrier until the allogeneic glucagon receptor are immunologically rejected. Alternatively, cultured epithelial autografts can be used for covering such wounds. The patient’s own epidermal cells are isolated, expanded in the laboratory, and used to replace the damaged skin (Green et al., 1979; Compton et al., 1989) without any tissue rejection. The major disadvantage of this approach is that it takes at least 3 weeks to grow enough cells for successful grafting, due to the low number of keratinocyte stem cells recovered from skin biopsies. Much work has also been directed toward developing bioengineered skin substitutes using cultured cells (keratinocytes and/or fibroblasts) with a suitable matrix (Pham et al., 2007), but the difficulty of achieving permanent wound coverage for patients with large or intransigent wounds persists (Turk et al., 2014; Kamel et al., 2013). Bioengineered products have been hampered by immune rejection, vascularization problems, difficulty of handling, and failure to integrate due to scarring and fibrosis. Furthermore, no currently available bioengineered skin replacement can fully replace the anatomical and functional properties of the native skin, and appendage development is absent in the healed area of full-thickness culture-grafted wounds. Thus, alternative sources of cells for engineering skin substitutes are urgently required to address this area of clinical need. One possibility is to use fetal skin as a potential cell source for tissue-engineered skin. Several types of fetal cells have been shown to have higher proliferative capacities and to be less immunogenic than their adult counterparts, suggesting potential allogeneic applications (Guillot et al., 2007; Davies et al., 2009; Montjovent et al., 2009; Götherström et al., 2004; Zhang et al., 2012). Lying between embryonic and adult cells in the developmental continuum, fetal cells offer several advantages as cell sources for therapeutic applications. Fetal cells are likely to harbor fewer of the mutations that accumulate over the lifetime of an organism, and may also possess greater proliferative potential and plasticity than adult stem cells. Although all stem cells are self-renewing and multipotent by definition, it is believed that stem cells from younger donors should have greater potential (Van Zant and Liang, 2003; Roobrouck et al., 2008). In addition, fetal cells may possess immunomodulatory properties associated with the fetal/maternal interface (Gaunt and Ramin, 2001; Kanellopoulos-Langevin et al., 2003). The use of early or midtrimester fetal tissue for skin tissue engineering was first suggested by Hohlfeld et al. (2005), who developed dermal-mimetic constructs using fetal dermal fibroblasts. Although their technique was reported to promote healing of severe burns, engraftment was only temporary and did not provide permanent cover.