During the past 20 years, it has become generally accepted that the modulation of fibroblastic cells towards the myofibroblastic phenotype, with acquisition of specialized contractile features, is essential for connective-tissue remodelling during normal and pathological wound healing. Yet the myofibroblast still remains one of the most enigmatic of cells, not least owing to its transient appearance in association with connective-tissue injury and to the difficulties in establishing its role in the production of tissue contracture. It is clear that our understanding of the myofibroblast its origins, functions and molecular regulation will have a profound influence on the future effectiveness not only of tissue engineering but also of regenerative medicine generally.

To evaluate whether ␣-smooth muscle actin (␣-SMA) plays a role in fibroblast contractility, we first compared the contractile activity of rat subcutaneous fibroblasts (SCFs), expressing low levels of ␣-SMA, with that of lung fibroblasts (LFs), expressing high levels of ␣-SMA, with the use of silicone substrates of different stiffness degrees. On medium stiffness substrates the percentage of cells producing wrinkles was similar to that of ␣-SMA-positive cells in each fibroblast population. On high stiffness substrates, wrinkle production was limited to a subpopulation of LFs very positive for ␣-SMA. In a second approach, we measured the isotonic contraction of SCF-and LF-populated attached collagen lattices. SCFs exhibited 41% diameter reduction compared with 63% by LFs. TGF␤1 increased ␣-SMA expression and lattice contraction by SCFs to the levels of LFs; TGF␤-antagonizing agents reduced ␣-SMA expression and lattice contraction by LFs to the level of SCFs. Finally, 3T3 fibroblasts transiently or permanently transfected with ␣-SMA cDNA exhibited a significantly higher lattice contraction compared with wild-type 3T3 fibroblasts or to fibroblasts transfected with ␣-cardiac and ␤-or ␥-cytoplasmic actin. This took place in the absence of any change in smooth muscle or nonmuscle myosin heavy-chain expression. Our results indicate that an increased ␣-SMA expression is sufficient to enhance fibroblast contractile activity. INTRODUCTIONEarly during healing of an open wound, resident dermal fibroblasts proliferate from the wound margin and migrate into the provisional matrix composed of a fibrin clot. About 1 week after wounding, the provisional matrix is replaced by neo-formed connective tissue, known as granulation tissue, essentially composed of small vessels, extracellular matrix, and fibroblastic cells that become activated and modulate into myofibroblasts. The main feature of myofibroblasts is represented by an important contractile apparatus similar to that of smooth muscle (Gabbiani et al., 1971), and in particular by the neo-expression of ␣-smooth muscle actin (␣-SMA), the actin isoform typical of vascular smooth muscle cells (Skalli et al., 1986). Myofibroblasts are recognized to play a central role in closing the wound tissue, through their capacity to produce a strong contractile force (for review see Grinnell, 1994;Rønnov-Jessen et al., 1996;Powell et al., 1999;Serini and Gabbiani, 1999), possibly generated within stress fibers, similar to those present in cultured fibroblasts (Skalli et al., 1986;Serini and Gabbiani, 1999). Because wound contraction takes place when de novo expressed ␣-SMA is incorporated in stress fibers (Darby et al., 1990), it has been suggested that this actin isoform plays an important role in granulation tissue contraction (for review see Serini and Gabbiani, 1999). Although stress fibers have been proposed to function as contractile organelles (Burridge, 1981;Katoh et al., 1998) and their presence has been correlated with the production of isometric tension (Harris et al., 1981), at p...

No abstract

The generation of tension in granulation tissue undergoing contraction is believed to be a cell-mediated event. In this study we used attached collagen lattices as a model system for studying the cellular mechanisms of tension generation by fibroblasts in an extracellular matrix. Fibroblasts in attached collagen lattices developed stress fibers, surface associated fibronectin fibrils, and a fibronexus-like transmembrane association interconnecting the two structural components. Release of the attached collagen lattice from its points of attachment resulted in a rapid, symmetrical contraction of the collagen lattice. Rapid contraction occurred within the first 10 minutes after release of the lattice from the substratum, with greater than 70% of the contraction occurring within the first 2 minutes. Rapid contraction resulted in a shortening of the elongate fibroblasts and compaction of the stress fibers with their subsequent disappearance from the cell. Cytochalasin D treatment prior to release disrupted the actin cytoskeleton and completely inhibited rapid contraction. The removal of serum prior to release inhibited rapid contraction, while the re-addition of serum restored rapid contraction. These results demonstrate that fibroblasts can develop tension in an attached collagen lattice and that upon release of tension the fibroblasts undergo contraction resulting in a rapid contraction of the collagen lattice. Fibroblast contraction is dependent upon an organized actin cytoskeleton and is promoted by the presence of serum.