Achilles tendons of MRL/MpJ mice show scar-mediated healing after tenotomy

Tendon healing in Murphy Ross Large (MRL/MpJ) mice was examined using an Achilles tendon tenotomy model, a full transection model, compared to a patella focal injury model in which tendons in MRL/MpJ mice showed regenerative healing. After complete transection, the Achilles tendons of MRL/MpJ mice healed via scar formation, regardless of sex, as did C57BL/6J mice. Tensile testing found that mechanical properties of injured tendons of both MRL/MpJ female and male mice exhibited comparable levels to those of C57BL/6J female and male mice, respectively after 4 weeks of healing, which is during the remodeling phase. After 10 weeks of healing, injured tendons of MRL/MpJ mice possessed smaller heterotopic ossification volumes than those of C57BL/6J mice. Proteomics analysis revealed similar alterations to signaling pathways in injured tendons of MRL/MpJ and C57BL/6J male mice after 4 weeks of healing. However, among the altered pathways, actin cytoskeleton and integrin signaling pathways were two of the top pathways that were more prominently activated in C57BL/6J males than in MRL/MpJ males. These findings indicate that MRL/MpJ mice possess limited capacity to regenerate injured tendons after complete rupture, and that tenotomized Achilles tendons of MRL/MpJ male mice have lesser induction of heterotopic ossification and lower activation of the signaling pathways also induced with injury in C57BL/6J male mice.