Neonatal enthesis healing involves non-inflammatory formation of acellular scar through ECM secretion by resident cells

Abstract

Wound healing is a well-orchestrated process that typically recruits the immune and vascular systems to restore the structure and function of the injured tissue. Injuries to the enthesis, a hypocellular and avascular tissue, often result in fibrotic scar formation and loss of mechanical properties, thereby severely affecting musculoskeletal function and life quality. This raises questions about the healing capabilities of the enthesis. Here, we established an injury model to the Achilles entheses of neonatal mice to study the possibility that at an early age, the enthesis can heal more effectively. Histology and immunohistochemistry analyses revealed an atypical process that did not involve inflammation or angiogenesis. Instead, neonatal enthesis healing was mediated by secretion of collagen types I and II by resident cells, which formed a permanent hypocellular and avascular scar. Transmission electron microscopy showed that the cellular response to injury, including ER stress, autophagy and cell death, varied between the tendon and cartilage ends of the enthesis. Single-molecule in situ hybridization, immunostaining, and TUNEL assays verified these differences. Finally, gait analysis showed that these processes effectively restored function of the injured leg. Collectively, these findings reveal a novel healing mechanism in neonatal entheses, whereby local ECM secretion by resident cells forms an acellular ECM deposit in the absence of inflammation markers, allowing gait restoration. These insights into the healing mechanism of a complex transitional tissue may lead to new therapeutic strategies for adult enthesis injuries.