Mesenchymal-epithelial interactions in the skin: aiming for site-specific tissue regeneration.

Since trunk skin (or non-palmoplantar skin) is less durable under mechanical stress than sole skin (palm, plantar or palmoplantar skin), conventional trunk-derived skin grafts (including the trunk dermis) commonly result in erosion and ulceration when transplanted on to plantar wounds caused by various injuries including, diabetes mellitus or collagen diseases (including systemic sclerosis, polyarthritis nodosa and rheumatoid arthritis). However, trunk-derived epidermis can adopt a plantar phenotype, characterized by keratin 9 expression, hypopigmentation and thick suprabasal layers, through factors derived from plantar dermal fibroblasts in the wounds. Thus, intractable plantar wounds with exposed bones can be treated with the combination of bone marrow exposure, occlusive dressing and epidermal grafting. The higher expression of dickkopf 1 (DKK1), an inhibitor of canonical Wnt/beta-catenin signals, in the plantar dermis partly explains these phenomena. Thus, mesenchymal-epithelial interactions play important roles not only in embryogenesis (the embryonic development) but also in maintaining the homeostasis of adult tissue. The topographical (site-specific) interactions of growth factors and substances, including DKKs, fibroblast growth factors (FGFs) and transforming growth factor-beta (TGF-beta) family proteins including bone morphogenetic proteins (BMPs), may explain the site-specific differences in the skin in addition to the expression patterns of HOX genes and sonic hedgehogs (Shhs). We review the importance of dermal-epidermal interactions in tissue homeostasis and regeneration, especially in palms and soles.