Tendon Mechanobiology and Further Research Opportunities

Tendons are fibrous connective tissues that function to transfer muscle-generated force to the skeleton, facilitating the movement of the limbs. Tendon injury is common in horses, and the risk of injury increases with ageing. Healing is often insufficient, with the formation of functionally inferior fibrotic scar tissue leading to chronic tendinopathy, and current treatment remains ineffective and largely palliative. The most commonly affected tendon in the horse is the forelimb superficial digital flexor tendon (SDFT). This tendon is prone to injury because it has an additional function as an energy store, stretching and recoiling with each stride to store and return energy, which reduces the energetic cost of locomotion by up to 36%. Failure to develop successful treatments for tendon injury is largely due to poor understanding of normal tendon mechanobiology, and how the dysregulation that occurs with ageing and injury leads to accumulation of damage and a failed healing response. Determining the mechanisms by which healthy tendon remains fatigue resistant and identifying the causes of decreased fatigue resistant with ageing will allow the development of targeted, effective treatments for tendinopathy.

The healthy tendon is comprised of highly aligned subunits, known as fascicles. Fascicles are rich in type I collagen, which provides high strength, but low extensibility. To provide the large extension required in energy-storing tendons we have discovered that the tendon stretches and recoils through sliding between the fascicles, enabling tendons to stretch further than their constituent fascicles. The fascicles are bound together by a softer, elastic matrix called the interfascicular matrix, and our findings indicate that maintenance of IFM sliding behaviour is critical for healthy function in energy-storing tendons, motivating our further work towards understanding IFM composition and organisation.  [...]