Disks can be divided into two histochemical types: 1) chondrodystrophoid and 2) nonchondrodystrophoid or fibroid. The word "chondrodystrophoid" literally means faulty development or nutrition of cartilage. In humans, chondrodystrophoism is recognized physically (phenotypically) as dwarfism, where individuals are smaller than normal and whose parts (especially limbs) are disproportionate. Certain breeds of dogs, such as dachshunds, show their chondrodystrophism by having disproportionately short and angulated limbs. However, phenotypic characteristics alone can not be used to identify chondrodystrophoid dogs. Other breeds, such as miniature poodles and beagles, have been histochemically identified to have chondrodystrophoid disks and yet do not appear outwardly to be chondrodystrophoid.
When comparing the disks of nine month old dogs, chondrodystrophoid disks characteristically have a larger ratio of transitional versus peripheral zone in the annulus fibrosus. Also the cells of the transitional zone lack clear orientation as is typical in nonchondrodystrophoid disks. The nucleus pulposus in chondrodystrophoid is almost completely composed of dense fibrocartilage which appears to have completed the chondrofication process. There are only isolated "islands" of notochordal cell remnants seen. In contrast the intracellular matrix of the nonchondrodystrophoid disk is loose and fibrillar and contains notochordal cells only.
The amount of pressure that builds up inside the disk when forces are applied depends on two factors: 1) the water binding properties of the nucleus (more water equals more elasticity) and 2) the degree of resistance and elasticity of the annulus and surrounding structures. These factors are highly dependent on the histochemical makeup of the disk and the changes it undergoes during aging.
...Biochemical differences between chondrodystrophoid and nonchondrodystrophoid disks are apparent shortly after birth and explain the differences in the types of degeneration that occur. The degeneration that occurs in chondrodystrophoid disks is called chondroid metaplasia because the nucleus pulposus is gradually replaced with cartilage. Degeneration takes place rapidly and begins as early as 6 months of age starting at the periphery of the nucleus pulposus and progressing centrally. A dramatic and rapid increase in collagen content, as much as 30-40% by dry weight, is seen between 6 and 12 months of age
...In comparison, nonchondrodystrophoid disks degenerate by fibroid metaplasia with the process becoming clinically significant at 8 to 10 years of age.
...However, because of the unique metabolic differences of their disks, the chondrodystrophoid breeds are far more likely to develop significant intervertebral disk disease in their lifetimes. Analysis of the frequency of occurrence of disk disease within particular breeds shows that standard and miniature dachshunds are at the highest risk of all dog breeds followed by Pekingese. Approximately one in every four dachshunds will have some degree of disk related problems in their lifetimes. The age of incidence for chondrodystrophoid breeds is highest between three and seven years, as opposed to eight to ten years for nonchondrodystrophoid breeds.