Characterization of the intrinsic properties of the anterior cruciate and medial collateral ligament cells: An in vitro cell culture study

Abstract

The poor healing abilities of the anterior cruciate ligament (ACL) in contrast to those of the medial collateral ligament (MCL) are well known. Different intrinsic properties of the constituent cells of these ligaments have been proposed to be one of the factors in the differential repair mechanisms. To examine this hypothesis, we have established primary cell lines of ACL and MCL from the tissue explants of approximately similar dimensions and have studied their behavior in vitro. The outgrowth of cells from ACL explants was slower than from MCL explants, as shown by the size of the surrounding clusters of cells. Both ACL and MCL cultures exhibited typical fibroblastic morphology. No significant differences were observed in either attachment or growth of cells from the attached explants derived from various segments of ACL and MCL. Growth curves of ACL and MCL cultures at both passage numbers 2 and 6 showed a slower rate of proliferation of ACL cells than MCL cells (p < 0.005). DNA synthesis measured in terms of [³H]thymidine incorporation (CPM/10³ cells) of both log phase (ACL = 607.5 ± 5.4 vs. MCL = 1356.4 ± 11.3) and confluent (ACL = 83.0 ± 3.6 vs. MCL = 189.8 ± 5.4) cultures, supports the conclusion that differential proliferation rates of these cells exist in culture. FITC‐phalloidin staining (for actin) of later passage cultures (P₃‐P₅) showed a spread‐out appearance of ACL cells and an elongated appearance of MCL cells. Relatively more stress fibers were seen within ACL cells. SDS‐PAGE and Western blot analysis of cellular proteins revealed higher actin (43 kDa) content in ACL cells than in MCL cells. In vitro wound closure assay was performed by creating a uniform wound of 0.6 mm width in the confluent layer of ACL and MCL cultures. By 48 h postwounding, cell‐free zones created in ACL cultures were occupied partially by single cells in a nonconfluent fashion. In contrast, the wounded zone in the MCL cultures was almost completely covered by the cells. Results presented in this report demonstrate a lower proliferation and migration potential of ACL cells in comparison with MCL cells. These differences in intrinsic properties of ACL and MCL cells that were observed in vitro might contribute to the differential healing potentials of these ligaments in vivo.