An Ex Vivo Model of Laser-Induced Microdamage to Study Tendon Repair

While it is generally accepted that tendon healing following widespread extracellular matrix trauma is limited, tenocytes are thought to have the capacity to repair small amounts of microdamage generated through activities of daily living to maintain the mechanobiological function of the tendon. Despite this, few studies have directly studied the mechanisms governing this process. To address this, we developed a tunable ex vivo model of extracellular matrix microdamage in live tendon explants that enables us to track both clearance of denatured collagen and closure of a microsized defect in the tendon matrix. The purpose of this study was to controllably induce varying levels of localized microdamage to the tendon explants and identify (1) if thresholds for repair exist and (2) whether repair mechanisms are dependent on initial damage size. We found that within 3 weeks, all tendon explants were able to clear damaged matrix to some extent regardless of the damage size. Interestingly, larger injuries resulted in a more robust rate of damaged matrix clearance in the later weeks, while smaller injuries exhibited a more consistent rate that led to full clearance in two explants. While all groups demonstrated some closure of the hole over the course of culture, only a small subset of explants with smaller initial hole sizes demonstrated nearly complete closure. Greater than 50% clearance of denatured collagen was typically associated with an accompanying closure of the defect, suggesting a strong relationship between clearance and closure. Overall, our work characterizes a novel laser-induced microdamage model, which will be a powerful asset for investigating mechanisms of damage accumulation and/or repair important to maintaining the mechanobiological function of the tendon, as well as identifying local tendon-specific factors that can be leveraged for therapeutics.Impact StatementIn this study, we developed a novel laser-based ablation system that produces tunable and reproducible micron-sized damage in live tendon explants. To our knowledge, this is the first direct demonstration that intrinsic cell-mediated responses can be sufficient to initiate tendon ECM repair in response to a localized microdamage injury. Moreover, this model provides a platform for direct visualization and analysis of local tissue repair, offering new opportunities to uncover mechanisms underlying microdamage accumulation and healing that maintain the mechanobiological function of the tendon, as well as to identify local tendon-specific factors that can be leveraged for therapeutics.