Micromorphometry and cellular characteristics of the canine cervical intervertebral discs.

BACKGROUND: Dogs have a high prevalence of disc-associated cervical spinal cord disorders. Despite this, there are no descriptions of the micromorphometry or cellular characteristics of canine cervical intervertebral discs. HYPOTHESIS/OBJECTIVES: To compare micromorphometric and cellular characteristics at 4 disc regions (outer annulus [OA], inner annulus [IA], transitional zone [TZ], nucleus pulposus [NP]). We hypothesized that measurements would differ between chondrodystrophic (CH) and nonchondrodystrophic (NCH) dogs. ANIMALS: Six CH dogs and 6 NCH dogs, under 3 years old. METHODS: Histologic sections of all cervical discs from each dog were examined. Micromorphometric variables included relative ratio of ventral to dorsal annulus fibrosus (AF), number and mean width of AF fibers, and relative percentage of NP. Cellular variables included cell density, morphology, and grouping. RESULTS: The NP from CH dogs was smaller, more rounded, and more dorsally located compared with that from NCH dogs. The NP occupied a greater proportion of the disc in NCH dogs compared with CH dogs (27.7 versus 15.8%; P<.001). The ratio of ventral to dorsal AF was approximately 3:1 in the CH group and 2:1 in the NCH group. Cellular variables were region dependent. Cell density was 2.4-fold higher in the OA compared with the NP. Approximately 50 of NP cells and 4.5% of OA cells occurred in groups. CONCLUSIONS AND CLINICAL IMPORTANCE: Micromorphometric variables differed by CH status. Cellular variables differed by disc region. Our findings potentially can explain the high incidence of intervertebral disc extrusions in CH dogs compared with NCH dogs.