The Role of Fibroblasts in Tissue Engineering and Regeneration

T. Wong; J.A. McGrath; H. Navsaria

Disclosures

The British Journal of Dermatology. 2007;156(6):1149-1155. 

In This Article

Epithelial-mesenchymal Interactions and Tissue Engineering

In vitro studies have shown that fibroblasts secrete soluble factors that diffuse to the overlying epidermis and influence keratinocytes in a paracrine manner.[17,18] Keratinocytes in monoculture produce only a thin epidermal layer and without mesenchymal support undergo apoptosis after about 2 weeks in culture. However, a study investigating the direct effect of fibroblasts on the overlying epidermis involved seeding keratinocytes on to fibroblast-embedded collagen gels, which were then submerged for several days to reach confluency, followed by culture at the air-liquid interface for 2 weeks.[18] They found that dermal fibroblasts promoted the development of identifiable keratinocyte layers into basal, prickle, granular and cornified layers in addition to promoting keratinocyte proliferation.[18] It is now well documented that culturing fibroblasts at the air-liquid interface promotes optimal differentiation approaching that of skin in vivo.

Fibroblasts release cytokines and growth factors that have autocrine and paracrine effects. Autocrine activity includes the transforming growth factor (TGF)-β-induced synthesis and secretion of connective tissue growth factor which promotes collagen synthesis as well as fibroblast proliferation.[19] Paracrine activity affects keratinocyte growth and differentiation, specifically through fibroblast secretion of keratinocyte growth factor (KGF), granulocyte-macrophage colony-stimulating factor, interleukin (IL)-6 and fibroblast growth factor (FGF)-10.[20,21,22] In response, keratinocytes synthesize IL-1 and parathyroid hormone-related peptide which, in turn, stimulate fibroblasts to produce KGF and thus a double paracrine loop exists.[23,24] Furthermore, keratinocytes cultured alone express IL-1 relatively weakly but when cocultured with fibroblasts show increased expression of IL-1 and c-Jun.[17] Therefore, fibroblasts incorporated into a dermal substrate play a role in producing ECM and stimulatory growth factors, which provides the optimum environment to support epidermis formation and to facilitate wound healing.

A study investigating the effect of fibroblasts on epidermal regeneration using collagen gels showed that the fibroblast density is an important factor to consider for the development of normal epidermal morphology and keratinocyte differentiation and that the optimum density still needs to be established.[18] Fibroblasts also contribute to basement membrane formation partly by producing collagen types IV and VII, laminin 5 and nidogen, but also through the secretion of cytokines that stimulate keratinocytes to produce basement membrane components.[25] TGF-β secreted by fibroblasts has been shown to induce the synthesis of collagen types IV and VII by keratinocytes.[26,27]

Neovascularization and lymphangiogenesis are also important processes for the maintenance of normal skin homeostasis and wound healing, for which fibroblasts have an important paracrine role. Members of the vascular endothelial growth factor (VEGF) family include VEGF-A, -B, -C and -D, which are produced by normal human fibroblasts and are important in regulating vascular and lymphatic endothelial cell proliferation through specific receptors.[28] VEGF-A is well known to be involved in the activation of resident endothelial cells and endothelial progenitor cells capable of vasculogenesis; VEGF-B is less mitogenic for endothelial cells while VEGF-C and -D have the same receptor specificity, binding to VEGF receptor 2 (VEGF-R2) to mediate angiogenesis and binding to VEGF-R3 to influence lymphangiogenesis.[28,29] A recent study involved the culture of dermal fibroblasts overexpressing VEGF-C with human dermal microvascular endothelial cells. As a result, the endothelial cells were activated and increased expression of matrix metalloproteinase-1, which enabled them to digest surrounding collagen, invade the gel and form capillary-like branching tubules, representative of neoangiogenesis.[29] Furthermore, there is additional evidence that TGF-β1 can stimulate neosynthesis of VEGF-B, -C and -D in vitro.[28]

Dermal fibroblasts are heterogeneous and studies have shown that the origin of the dermal fibroblasts is important in determining the phenotypic expression of the overlying keratinocytes. For instance, nonpalmoplantar keratinocytes do not normally express keratin 9 but when cocultured with palmoplantar fibroblasts, locally derived fibroblast factors are produced and the keratinocytes subsequently express keratin 9 and develop a thicker epidermis.[30] In addition, nonpalmoplantar epidermis grafted onto palmoplantar dermis becomes hypopigmented in contrast to nonpalmoplantar skin grafting. cDNA microarray analyses subsequently revealed that dickkopf 1 (DKK1, an inhibitor of Wnt signalling) had increased expression, and Wnt signalling is necessary for melanocyte growth, differentiation and pigmentation.[30]

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