An In Vitro Study of Cellular and Molecular Mechanisms of Ligament Scarring

When ligaments such as the medial collateral ligament (MCL) are injured, they generally heal but form scar tissue, which is composed of a disorganized collagen matrix that is over-produced by fibroblasts. Scar tissue has inferior structural and mechanical properties, which can lead to joint instability. Excessive fibroblast contraction is thought to contribute to tissue scarring. Previous studies have shown that both TGF-â1 and TGF-â3 increase fibroblast contraction and collagen synthesis. However, TGF-â1 enhances scar tissue formation whereas TGF-â3 actually reduces it. In addition, both TGF-â isoforms have been found to increase the expression of á-SMA, which correlates with increased fibroblast contractility. An increase in tension at the wound site has also been found to increase á-SMA protein levels. Therefore these factors are all important in the wound healing process. The overall objective of this thesis research was to investigate cellular and molecular mechanisms that affect scar tissue formation in healing ligaments. Contraction forces, collagen synthesis, and á-SMA protein expression of healing and normal MCL fibroblasts in response to treatment with TGF-â1, TGF-â3, and collagen gel tension were investigated. A novel culture force monitor (CFM) system was used to quantify forces of fibroblast contraction. It was found that healing MCL fibroblasts produced greater contractile forces and higher levels of collagen synthesis than normal MCL fibroblasts. In addition, treatment with TGF-â1 or TGF-â3 increased contraction forces in healing fibroblasts compared to untreated controls, with TGF-â1 consistently producing greater contraction forces than treatment with TGF-â3. TGF-â1 and TGF-â3 also induced higher levels of á-SMA protein expression compared to untreated fibroblasts. Consistent with the contraction forces, fibroblasts treated with TGF-â1 expressed higher levels of á-SMA protein than those treated with TGF-â3. Further, it was found that when tension in gels embedded with normal MCL fibroblasts was released, expression of á-SMA protein also decreased. Thus, this study showed that healing and normal fibroblasts have differential contractile and collagen synthesis abilities. The results of this study showed that the presence of TGF-â1, TGF-â3, and tension in the matrix should be regulated to improve ligament healing. Decreasing the ratio of TGF-â1 to TGF-â3 in an injured ligament may decrease fibroblast contraction and thus reduce scar formation in healing MCLs. Finally, reducing tension levels in healing ligaments and hence down-regulating á-SMA protein expression may also decrease ligament scarring