Diffusible factors released by fibroblasts support epidermal morphogenesis and deposition of basement membrane components

Epithelial-mesenchymal interactions play an important role in controlling epidermal morphogenesis and homeostasis but little is known about the mechanisms of these interactions. To examine whether diffusible factors produced by fibroblasts and/or keratinocytes support epidermal morphogenesis and basement membrane formation, organotypic keratinocyte monocultures were established in media collected either from organotypic fibroblast or keratinocyte-monocultures or from keratinocyte-fibroblast cocultures, and the expression of keratin 10, 16, and 17 and basement membrane components (types IV and VII collagen, laminin 5, nidogen, BP 180, LAD-1) were examined. We found that diffusible factors released by keratinocytes were not sufficient to support the establishment of normalized epidermal phenotype and deposition of basement membrane components in contrast to fibroblast- or keratinocyte/fibroblast-derived factors. Keratinocytes appear to affect the spectrum of secreted soluble factors, as keratinocyte/fibroblast-derived factors were more effective to accomplish continuous linear deposition of laminin 5 and of nidogen. The finding that released amounts of keratinocyte growth factor and granulocyte macrophage colony stimulating factor were not sufficient to fully support epidermal morphogenesis and deposition of basement membrane components is suggestive for the involvement of other released diffusible factors. Generation of organotypic keratinocyte monocultures in the presence of fibroblast- or keratinocyte/fibroblast-derived soluble factors resulted in enhanced expression of keratins K16 and K17 and the absence of type IV collagen. This observation indicates that next to paracrine acting factors, epidermal homeostasis is controlled by mutual keratinocyte-fibroblast interaction.