Tendons: structure, function and challenges to clinical treatment
Wang, Calvin C.
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As dense connective tissues, tendons play a vital role in the transmission of contractile forces from muscle to bone. This link between muscles and bones provides the means in transferring tensile forces produced by muscles on to the connected bone. During movement, tendons slide over surrounding bony and articular surfaces and are thus commonly subjected to shear and compression forces in addition to tensile force. Fibrillar collagen, proteoglycan and various glycoproteins make up the composition of tendinous tissue and contribute to its ability to withstand these forces. Tendons contain a distinct population of cells, called tenocytes. Tenocytes undertake a flattened morphology within the tendon matrix and contain cytoplasmic projections which extend longitudinally and laterally towards other tenocytes. An intercellular network of cells thus maintains the extracellular environment of the tendon and allows a coordinated response to external mechanical stimuli. Defects to load-bearing connective tissue elements such as tendons whether due to trauma, overuse, age-related diseases or degenerative diseases, are often limited in their healing potential and thus contributes often to persistent, chronic clinical symptoms. Chronic disease, overuse or acute injuries damages the tendon. This damage compromises the transmission of tensile forces and because of the hypovascularity of some tendinous tissues and many other reasons, a healing response often is severely insufficient in regenerating tissue back to its original constitution. Even the best treatment options for such tendinopathies, supplemented with the body’s own healing response fail to produce quality outcomes. An understanding of the molecular, cellular and mechanical characteristics of tenocytes, tendon matrix and the tendon system as a whole will be vital for the development of effective therapies for all tendinopathies. It is the goal of this current work to outline the current molecular, cellular, mechanical and clinical understanding of tendons. A broad address to tendon biology should help illustrate the key dimensional aspects that must be considered when attempting the effective translation of research into useful clinical therapies.