Endogeneously regulated site-specific differentiation of human palmar skin keratinocytes in organotypic cocultures and in nude mouse transplants.

Palmar and plantar epidermis is characterized by specific features such as the development of a striking lucidum, a very thick stratum corneum, prominent rete ridges and the unique expression of keratin K9. Using organotypic cocultures of keratinocytes and fibroblasts, we investigated to which extent the specific phenotype of palmar keratinocytes is maintained in vitro and under systemic host influences after transplantation onto nude mice. In vitro, palmar keratinocytes developed a thick epithelium with a prominent, although parakeratotic stratum corneum showing no significant differences in proliferation and differentiation in coculture with either palmar or nonpalmoplantar fibroblasts. All differentiation markers including keratohyaline and membrane coating granules as well as keratin K9 were also found, but at reduced levels and with slightly altered localization. In transplants, substantial normalization towards the palmar phenotype occurred. In 3-week-old grafts, a homeostatic state was reached, as illustrated by a constant thickness of the stratum Malpighii, presence of keratin K10 throughout the entire suprabasal compartment, increased numbers of K9- and filaggrin-positive cells, and reduction of keratins K16 and K17. At the ultrastructural level, numerous membrane coating granules and an enlargement of keratohyaline granules were seen accordingly, and immunofluorescence showed intense continuous lining of the dermo-epidermal junction by laminin, type IV collagen and integrin alpha 6. The high percentage of bromodesoxyuridine-positive cells, mainly in the basal compartment, underlined the hyproproliferative state, comparable to palmoplantar epidermis. In conclusion, (i) palmar keratinocytes can preserve the potential to express their specific phenotype upon transfer to culture conditions, and (ii) this intrinsic property is not significantly modulated by the type of cocultured fibroblasts. This suggests that fibroblasts act primarily by sustaining keratinocyte proliferation which is permissive for the fully differentiated phenotype.