The myofibroblast: one function, multiple origins

Abstract

The crucial role played by the myofibroblast in wound healing and pathological organ remodeling is well established; the general mechanisms of extracellular matrix synthesis and of tension production by this cell have been amply clarified. This review discusses the pattern of myofibroblast accumulation and fibrosis evolution during lung and liver fibrosis as well as during atheromatous plaque formation. Special attention is paid to the specific features characterizing each of these processes, including the spectrum of different myofibroblast precursors and the distinct pathways involved in the formation of differentiated myofibroblasts in each lesion. Thus, whereas in lung fibrosis it seems that most myofibroblasts derive from resident fibroblasts, hepatic stellate cells are the main contributor for liver fibrosis and media smooth muscle cells are the main contributor for the atheromatous plaque. A better knowledge of the molecular mechanisms conducive to the appearance of differentiated myofibroblasts in each pathological situation will be useful for the understanding of fibrosis development in different organs and for the planning of strategies aiming at their prevention and therapy.

Figure 1

ECM compliance controls development of the myofibroblast phenotype in three-dimensional collagen gels. Differentiated rat lung myofibroblasts were grown in mechanically restrained collagen gels produced very soft with a concentration of 0.5 mg/ml (A, B) and comparably stiff with 3.5 mg/ml (C, D). Cells were immunostained after 36 hours for α-SMA (A, C: red; B, D: blue), β-cytoplasmic actin (A, C: green; B, D: red), focal adhesion protein vinculin (B, D: green), and nuclei (A, C: blue) and observed by confocal microscopy. Note that cells in soft gels attain a dendritic morphology with cortical actin distribution and point-like small adhesion sites; α-SMA is not organized in stress fibers (A, inset). In stiff collagen, myofibroblasts develop α-SMA-positive stress fibers (C, inset) that terminate in supermature FAs. Bars: 25 μm and 10 μm (insets).

Figure 2

One cell, multiple origins. Differentiated myofibroblasts are characterized by increased production of ECM proteins and by the development of α-SMA-positive stress fibers that are connected with the ECM at sites of supermature FAs and between cells via adherens junctions. The main myofibroblast progenitor after injury of different tissues seems to be the locally residing fibroblast, which transiently differentiates into a protomyofibroblast, characterized by α-SMA-negative stress fibers. In the liver, myofibroblasts are additionally recruited from HSCs that follow an activation process and from epithelial cells that undergo EMT. In the lung, endothelial-to-mesenchymal transition may provide another mechanism to generate myofibroblasts. During atheromatous plaque formation, de-differentiating SMCs (ie, that lose late SMC markers) from the media are suggested to be the major source of myofibroblastic cells. The relative contribution of BM-derived circulating fibrocytes to the formation of differentiated myofibroblasts in different fibrotic lesions is unclear at present; it is conceivable that fibrocyte transdifferentiation terminates at the protomyofibroblast stage.

Figure 3

Cytoskeletal protein expression in a coronary artery atheromatous plaque classified as erosion. Coronary arteries were immunostained for α-SMA (A), smooth muscle myosin heavy chains (B), and smoothelin (C). Insets highlight regions of intimal thickening at higher magnification. Intimal SMCs express high amounts of α-SMA, very low amounts of smooth muscle myosin heavy chains, and no smoothelin, indicating their modulation toward the myofibroblastic phenotype. Note that the media express the three proteins similarly in all cases. Bars: 250 μm and 25 μm (insets).