Abstract
PurposeDifferences in the tissue-specific collagen maturation process between tendon and ligament are still unknown. Collagen cross-link formation is crucial for the collagen maturation process. The aim of this study is to examine collagen maturation processes of anterior cruciate ligament (ACL), medial collateral ligament (MCL), and patellar tendon (PT) in vitro, in order to determine the optimal cell source for tissue engineering of ligament.MethodsCells derived from the ACL, MCL, and PT of New Zealand white rabbits were isolated. Each cell type was cultured for up to 4 weeks after reaching confluence. Cell–matrix layers were evaluated and compared for their morphology, collagen cross-links, and gene expression levels of lysine hydroxylase 1 and 2, lysyl oxidase (LOX), tenomodulin, collagen1A1 (Col1A1), and collagen3A1 (Col3A1).ResultsTransmission electron microscopy photomicrographs verified that collagen fibrils were secreted from all three types of fibroblasts. A higher ratio of dihydroxylysinonorleucine/hydroxylysinonorleucine was evident in the ligament compared to the tendon, which was consistent with lysine hydroxylase 2/lysine hydroxylase 1 gene expression. The gene expression of LOX, which regulates the total amount of enzymatic cross-linking, and the gene expression levels of Col1A1 and Col3A1 were higher in the ACL matrix than in the MCL and PT matrices.ConclusionACL, MCL, and PT cells have distinct collagen maturation processes at the cellular level. In addition, the collagen maturation of ACL cells is not necessarily inferior to that of MCL and PT cells in that all three cell types have a good ability to synthesize collagen and induce collagen maturation. This bioactivity of ACL cells in terms of ligament-specific mature collagen induction can be applied to tissue-engineered ACL reconstruction or remnant preserving procedure with ACL reconstruction.
Highlights
The anterior cruciate ligament (ACL) is one of the most frequently injured structures in the knee
There were no significant differences in the DNA content between the ACL, medial collateral ligament (MCL), and patellar tendon (PT) matrices when the cells reached confluence (Fig. 1)
The DHLNL/HLNL ratio, which is an indicator of tissuespecific collagen maturation, in the matrices formed by ACL- and MCL-derived cells was significantly higher than in the matrix formed by PT-derived cells (Fig. 2a)
Summary
The anterior cruciate ligament (ACL) is one of the most frequently injured structures in the knee. Some studies have reported that ACL-derived fibroblasts are inferior to medial collateral ligament (MCL)- and patellar tendon (PT)-derived fibroblasts in terms of their proliferation [4, 6, 20, 29, 41], migration [9, 16, 23, 29, 41], and collagen synthesis [7, 20, 41]. ACL cells are thought to have relatively poor healing capabilities. This poor healing capacity leads to the need for ACL reconstruction in many cases, and more than 100,000 ACL reconstructions are performed every year in the USA [21]. Autologous tendon grafts (the bone-patellar tendon-bone graft, the hamstring tendons, and the quadriceps tendon) are most commonly used in ACL reconstruction because tendons and ligaments are grossly similar, both being dense, white, and composed mainly of type I collagen
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