α-Smooth muscle actin is expressed by fibroblastic cells of the ovine anterior cruciate ligament and its free tendon graft during remodeling

Abstract

Contractile fibroblastic cells expressing the α-smooth muscle actin isoform, so-called myofibroblasts, have been identified to play a possible role during the healing of the medial collateral ligament by means of restoring the tissues in situ strain via extracellular matrix contraction. Recently, these cells have also been identified to be a normal part of the human anterior cruciate ligament. It has been hypothesized that myofibroblasts play a role in the wrinkling of the extracellular matrix. The goal of the present study was to identify myofibroblasts in the intact ovine anterior cruciate ligament and a free autologous tendon graft during remodeling after anterior cruciate ligament reconstruction. In 36 mature merino sheep the anterior cruciate ligament was replaced with an ipsilateral Achilles tendon split graft. Midsubstance tissue samples were immunostained for α-smooth muscle actin at 6, 9, 12, 24, 52, and 104 weeks. Myofibroblasts were identified in the intact ovine anterior cruciate ligament as well as in the Achilles tendon graft prior to implantation. During remodeling the first myofibroblasts were found at six weeks within newly formed fiber bundles. At 24, 52, and 104 weeks myofibroblast distribution and cell density were similar to those of the intact ovine anterior cruciate ligament. These findings indicate that α-smooth muscle actin containing fibroblastic cells are a regular part of the intact as well as the remodeled anterior cruciate ligament. There is evidence that myofibroblasts may be involved in maintaining tissue homeostasis in the mature ligament e.g., by means of crimp formation. The presence of these cells during the early remodeling may further indicate that α-smooth muscle actin containing fibroblastic cells are involved in the earliest stages of fiber bundle formation. The role and function of this special cell type for the anterior cruciate ligament needs to be further clarified.