Abstract. Epithelial-mesenchymal interactions control epidermal growth and differentiation, but little is known about the mechanisms of this interaction. We have examined the effects of human dermal microvascular endothelial cells (DMEC) and fibroblasts on keratinocytes in conventional (feeder layer) and organotypic cocultures (lifted collagen gels) and demonstrated the induction of paracrine growth factor gene expression. Clonal keratinocyte growth was similarly stimulated in cocultures with irradiated DMEC and fibroblasts as feeder cells. This effect is most probably caused by induction of growth factor expression in cocultured dermal cells. Keratinocytes stimulated mRNA levels for KGF and IL-6 in both mesenchymal cell types and GM-CSF in fibroblasts. The feeder effect could not be replaced by conditioned media or addition of isolated growth factors.In organotypic cocultures with keratinocytes growing on collagen gels (repopulated with dermal cells), a virtually normal epidermis was formed within 7 to 10 d. Keratinocyte proliferation was drastically stimulated by dermal cells (histone 3 mRNA expression and BrdU labeling) which continued to proliferate as well in the gel. Expression of all typical differentiation markers was provoked in the reconstituted epithelium, though with different localization as compared to normal epidermis. Keratins K1 and K10 appeared coexpressed but delayed, reflecting conditions in epidermal hyperplasia. Keratin localization and proliferation were normalized under in vivo conditions, i.e., in surface transplants on nude mice. From these data it is concluded that epidermal homeostasis is in part controlled by complex reciprocally induced paracrine acting factors in concert with cell-cell interactions and extracellular matrix influences.T HERE is accumulating evidence that epithelial-mesenchymal interactions regulate tissue homeostasis in surface epithelia such as epidermis (6,7,9,13,19,20,(30)(31)(32)47). In vivo, these processes are difficult to study due to many variables involved such as the different dermal cell types and superimposed influences of systemic factors of the blood circulation. Thus, in vitro model systems have been developed to mimic epidermal-dermal interactions and to study regulation of epidermal cell proliferation and differentiation (for review see reference 19). It had been shown that coculture on postmitotic fibroblastic mouse 3T3 cells (42, 54) as well as human dermal fibroblasts (30) are required to support human keratinocyte growth at clonal densities in serum containing medium. Under conventional (submerged) culture conditions keratinocyte proliferation is the predominant phenomenon whereas terminal differentiation and tissue organization are reduced or aberrant cornDr. Smola's present address is Department of Dermatology, University of Cologne, Joseph-Stelzmann-Strasse 9, 5000 Cologne, Germany.Address correspondence to Dr. N. E. Fusenig, Division of Differentiation and Carcinogenesis, DKFZ, Im Neuenheimer Feld 280, 6900 Heidelberg, Germany. pared to the ...

The development of epidermis and epidermal appendages from dissociated cells of neonate mouse skin was examined by transplantation of cell suspensions to subdermally prepared, protected graft beds. Using Ficol gradients and culture procedures, we prepared subfractions of primary cell suspensions consisting of essentially pure epidermal cells or fibroblasts. Reformation of an epithelium structurally similar to the epidermis was observed from transplanted epidermal-cell suspensions, but formation of hair follicles and development of normal epidermal microarchitecture was observed only when epidermal cells were transplanted together with cells of dermal origin. This pattern was observed following transplantation of either fresh-cell isolates or cells cultured up to 7 days prior to transplantation.