Intervertebral disc degeneration in warmblood horses: Histological and biochemical characterization

Gross morphology of healthy and degenerated intervertebral discs (IVDs) is largely similar in horses as in dogs and humans. For further comparison, the biochemical composition and the histological and biochemical changes with age and degeneration were analyzed in 41 warmblood horses. From 33 horses, 139 discs and 2 fetal vertebral columns were evaluated and scored histologically. From 13 horses, 73 IVDs were assessed for hydration, DNA, glycosaminoglycans, total collagen, hydroxyl-lysyl-pyridinoline, hydroxylysine, and advanced glycation end-product (AGE) content. From 7 horses, 20 discs were assessed for aggrecan, fibronectin, and collagen type 1 and 2 content. Histologically, tearing of the nucleus pulposus (NP) and cervical annulus fibrosus (AF), and total histological score (tearing and vascular proliferation of the AF, and chondroid metaplasia, chondrocyte-like cell proliferation, presence of notochordal cells, matrix staining, and tearing of the NP) correlated with gross degeneration. Notochordal cells were not seen in IVDs of horses. Age and gross degeneration were positively correlated with AGEs and a fibrotic phenotype, explaining gross degenerative changes. In contrast to dogs and humans, there was no consistent difference in glycosaminoglycan content and hydration between AF and NP, nor decrease of these variables with age or degeneration. Hydroxylysine decrease and collagen 1 and AGEs increase were most prominent in the NP, suggesting degeneration started in the AP. In caudal cervical NPs, AGE deposition was significantly increased in grossly normal IVDs and total collagen significantly increased with age, suggesting increased biomechanical stress and likelihood for spinal disease in this part of the vertebral column.

Automated summary

The gross morphology of intervertebral discs in horses is similar to dogs and humans, but there are differences in the biochemical composition and histological changes. Age and degeneration are associated with advanced glycation end-products (AGEs) and fibrosis. Unlike dogs and humans, there is no consistent difference in glycosaminoglycan content and hydration between the annulus fibrosus (AF) and nucleus pulposus (NP) in horses. Degeneration starts in the NP, indicated by decreased hydroxylysine and increased collagen 1 and AGEs. Increased AGE deposition and collagen content in the caudal cervical NP suggest a higher risk for spinal diseases in this area.