Morphologic features of the cervical intervertebral disks and adjacent vertebral bodies of horses.

The intervertebral disks and adjacent vertebral bodies of the 2nd to 7th cervical vertebrae from 17 horses (3 months to 19 years of age) were examined grossly and by light microscopy. In each specimen, the disk tissue was fibrocartilagenous centrally and became more fibrous peripherally, but without a distinct demarcation between the nucleus pulposus and anulus fibrosus. Demarcation between the anulus fibrosus and the dorsal longitudinal ligament was not apparent. With increasing age of the horses, cellularity of the central fibrocartilagenous portion of the disk decreased. The caudal end plates of the vertebrae had an increase in width and bone density with increasing age. In specimens from horses of all ages, the cranial end plate of the vertebrae had only a thin rim of dense bone. The cranial physis of each vertebral body was closed at 3 years of age, whereas the caudal physis of vertebral bodies was discontinuous in 6- to 9-year-old horses and was completely absent in horses greater than or equal to 12 years. Disruption of fibers of the anulus fibrosus by small amounts of fibro-cartilage was observed in disks from 5 horses. This change was similar to a Hansen type II disk prolapse.