Newly identified intervertebral fat pad degenerates after intervertebral disc injury in a rat model of degeneration.

Chronic low back pain due to intervertebral disc (IVD) degeneration (IVDD) is a major global health burden. Interactions between IVD tissues and surrounding structures are important for spinal health and pathology, yet many studies focus on structures within the IVD and neglect a deeper investigation of adjacent tissues. This study describes a newly identified intervertebral fat pad (IVFP) in rat lumbar spines, its changes following IVD injury, and similar structures in mice and humans. IVFPs were analyzed histologically using naive and injured IVDs from a rat model of IVDD, in which 5-month-old rats underwent a triple-puncture annulus fibrosus (AF) injury of L3-4, L4-5, and L5-6 IVDs. Sagittal and coronal histologic samples were stained with safranin O/fast green and analyzed at 3, 7, 14, and 56 days post-injury. Naive and sham IVDs demonstrated the consistent presence of an IVFP between the anterior AF and anterior longitudinal ligament (ALL) in anterior and anterolateral regions of the IVD, without presence at posterior or postero-lateral IVD regions. The IVFP gradually disappeared in injured IVDs, and became largely absent by 56 days post-injury. Post-injury changes to the IVFP also included adipocyte shrinkage, fibrous tissue infiltration, and gradual IVFP disappearance, together suggesting progressive degeneration. IVFP-like structures were identified histologically in mouse and human IVDs, providing evidence of its presence across species. Fat pads studied in other musculoskeletal joints play roles in health and disease, suggesting a need for further study investigating the potential role of the IVFP in IVDD pathomechanisms and therapeutics.