Tendon-Targeted Knockout of Collagen XII Disrupts Murine Achilles Tendon Structure, Function, and Gene Expression at Later Stages of Postnatal Development

knockout mice, we evaluated Achilles tendons at postnatal days 10, 30, and 60 with histological, ultrastructural, biomechanical, and compositional approaches. Mechanical testing revealed progressive deficits in knockout tendons, including reduced maximum load, stiffness, and modulus at P30 and P60, as well as altered viscoelastic properties across all timepoints. Structural analyses demonstrated increased tendon length across all timepoints and greater cell density, rounder nuclei, impaired collagen fiber realignment, and smaller fibril diameters with disrupted maturation, with more notable changes at later postnatal developmental timepoints. Gene expression profiling revealed early downregulation of matrix and signaling genes at P10, followed by upregulation of matrix remodeling and matricellular genes at P30 and P60, consistent with a persistent, maladaptive remodeling response. Together, these findings establish collagen XII as a critical regulator of hierarchical tendon development, integrating fibrillogenesis, cell morphology, mechanical function, and gene expression to ensure proper tendon development.