Histological comparison of intervertebral disc degeneration in nonchondrodystrophic and chondrodystrophic dogs in different stages of degeneration

Abstract

In dogs and humans intervertebral disc (IVD) degeneration is a common cause of painful diseases like type I and II IVD herniation. However, IVD degeneration does not always lead to clinical signs of disease. Hans J. Hansen (1952) distinguished different types of IVD herniations in the two different dog types: chondrodystrophic (CD) and non-chondrodystrophic (NCD). CD dogs have short legs due to genetically disturbed endochondral ossification processes in the growth plates of tubular bones. These dogs show accelerated degeneration of IVDs. In certain circumstances under compression, they can suddenly result explosive IVD herniations. Hansen termed these herniations type I herniations, causing severe trauma and hemorrhage to the spinal cord. NCD dogs have tubular bones of normal length and commonly show, if at all, slower developing herniations (Hansen type II herniations) at average 7-8 years of age. Hansen adjudged type I herniations to chondroid degeneration of the nucleus pulposus (NP) in the IVD, whereas type II herniations were the result of fibroid degeneration of the NP. However, Hansen's research was not based on objective microscopic and macroscopic grading schemes. Further histopathological investigation of the degenerative process of CD and NCD dogs is thus required. In this project the histopathological process of IVD degeneration in CD and NCD dogs were compared objectively by using macroscopic and microscopic grading schemes. IVDs were sampled from cadavers of both dog types. Degeneration of the IVDs were graded according to Thompson's macroscopic grading scheme. Histological survey was done on blinded slides using the Boos scoring system for histopathological evaluation that was modified and validated for canines by De Nies et al (2010). Macroscopic and histological scoring of the IVDs were combined and statistically analyzed to elucidate differences between the CD and NCD dogs. One quantitative difference was found between CD and NCD dogs in Thompson grade I: the NP of CD dogs contained significantly more chondrocytes than the NP of NCD dogs. In all Thompson grades in both dogs types only chondrocytes and no fibrocyte-like cells were found in the degenerating NPs. Therefore the term 'fibroid degeneration' seems to be redundant. In conclusion, CD and NCD dogs are similar regarding the fundamental histological steps of IVD degeneration. However, the process of IVD degeneration appears to be highly accelerated in CD dogs.