Integrated Characterization of Proximal and Distal Nerve Stumps in a Model of Chronic Motor Nerve Transection

Prolonged peripheral nerve discontinuity subjects the proximal stump to chronic axotomy and the distal stump to chronic denervation, resulting in pathological changes that may compromise regeneration following delayed repair. Although these changes have been investigated in both nerve stumps, much of the existing evidence derives from mixed sensorimotor nerve models, leaving the histopathological changes of chronic motor nerve injury incompletely understood. Addressing this gap requires a well-defined, reproducible model that maintains prolonged motor nerve discontinuity and enables concurrent characterization of both nerve stumps. This study aimed to establish a chronic motor nerve transection model and characterize the distinct structural and microenvironmental changes occurring in both nerve stumps. The buccal branch of the facial nerve was transected in Lewis rats, and the nerve ends were secured to adjacent muscle to prevent spontaneous reconnection. At 10 weeks post-injury (p.i.), proximal and distal stumps were compared with uninjured nerves using histological, immunofluorescence, and morphometric analyses. Persistent functional impairment was observed throughout the study period. The proximal stump exhibited traumatic neuroma formation, altered axonal morphology, and loss of myelin integrity, but largely preserved its underlying nerve architecture and microenvironment. In contrast, the distal stump exhibited reduced neurofilament expression, clusters of small-caliber myelinated axons, and increased myelinated axon density compared to both uninjured nerves and the proximal chronically injured segment. These changes were accompanied by endoneurial tube atrophy, increased collagen I deposition, vascular remodeling, and immune-cell accumulation, indicating extensive structural and microenvironmental remodeling. Collectively, these findings demonstrate that chronic motor nerve transection produces two distinct pathological compartments: a proximal stump characterized by persistent but disorganized regeneration-associated features and a distal stump characterized by pronounced degenerative and microenvironmental changes. This study establishes a reproducible model of chronic peripheral nerve injury and provides a foundation for investigating how these compartment-specific pathologies contribute to regenerative failure and for developing targeted therapeutic strategies to improve regeneration following delayed nerve repair.