Abstract
While much is known about the ligament, the precise morphology and function of the thin layer of connective tissue lining its surface, termed the epiligament, have not been fully studied yet. Herein, we aimed at reviewing the recent findings on the structural and functional significance of the epiligament in both animal models and human tissue. The epiligament is made up of various connective tissue cells such as fibroblasts, fibrocytes, mast cells, and adipocytes and contains a number of neurovascular bundles. Arrangement of collagen fibers in the epiligament is rather chaotic, in multiple directions, which allows for greater mobility and resistance to stress. Differences in the collagen content and types of enzymes of the group of matrix metalloproteinases between the epiligament and the underlying ligament tissue have been reported and are reviewed herein. While the ligament tissue mainly contains collagen type I, the epiligament is also rich in collagen types III and V. As suggested by a number of studies, the epiligament plays a key role in ligament repair as a donor of cells and matrix metalloproteinases, particularly matrix metalloproteinase-2 and 9, which are essential for scar tissue remodeling. In conclusion, future studies will likely reveal additional functional aspects of the epiligament, which may allow scientists to devise more suitable treatment strategies for damaged ligaments in a world where injuries resulting from sports activities or daily routine have long merited their due attention.
Highlights
BackgroundLigaments are solid bands made up of fascicles of collagen fibers which attach to the ends of two bones forming a joint and reinforce it
While much is known about the ligament, the precise morphology and function of the thin layer of connective tissue lining its surface, termed the epiligament, have not been fully studied yet
We aimed at reviewing the recent findings on the structural and functional significance of the epiligament in both animal models and human tissue
Summary
Ligaments are solid bands made up of fascicles of collagen fibers which attach to the ends of two bones forming a joint and reinforce it. The second layer contains spindle-shaped and elongated fibroblasts, surrounded by collagen and a small number of blood vessels [4,13]. Fibroblasts in the EL are not static cells; instead, they produce varying quantities of collagen types I, III, and V, fibronectin, and enzymes of the group of matrix metalloproteinases (MMPs), especially MMP-2 and 9 [8,9,10,19,20,21] These substances are all upregulated to promote adequate ligament repair after injury. MMPs are implicated in a number of physiological and pathological processes throughout the body, including wound healing, ligament repair, myocardial remodeling induced by hypertension, tumor invasion, and metastasis and many others [36,37,38,39,40] They are secreted by various cells - fibroblasts, osteoblasts, endothelial cells, macrophages, neutrophils, lymphocytes [10]. These findings are inconclusive; they show the role of the MMPs expressed in the EL in the regeneration of the ligament and support their significance for the healing potential of individual ligaments
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