Histological Structure and Development
When studying a histological slide of an intervertebral disc, it is important to consider that the section often includes adjacent parts of the vertebral bodies. This is typically embryonic material where ossification is not yet complete.
Vertebrae develop via endochondral (indirect) ossification, similar to spongy and long bones. First, a cartilaginous model forms from hyaline cartilage, which is then destroyed and replaced by bone tissue.
A histological slide of the disc reveals:
- Elements of the disc itself: fibrocartilage, dense connective tissue, and the nucleus pulposus.
- Elements of the vertebrae: hyaline cartilage and areas of indirect ossification — zones of contact between degenerating cartilage and forming bone trabeculae.
Anatomy and Components of the Disc
The intervertebral disc (discus intervertebralis) is part of the intervertebral symphysis (symphysis intervertebralis) — a cartilaginous joint between adjacent vertebral bodies. Unlike synchondroses, which consist of continuous cartilage tissue, symphyses possess a narrow cleft-like space within the cartilage. Historically referred to as "hemijoints", they are not diarthroses, yet they allow greater mobility than a synchondrosis.
The intervertebral disc comprises:
- Anulus fibrosus — concentric layers of fibrocartilage.
- Nucleus pulposus — a remnant of the notochord.
Hyaline cartilage endplates lie adjacent to the vertebral bodies.
Regional Features and Topography
The characteristics of the discs vary depending on the region of the vertebral column:
- Cervical region: Discs are thickest relative to the vertebral body size.
- Thoracic region: Discs are thinner; the intervertebral disc in the thoracic region serves as an example of a permanent synchondrosis. An important landmark is that the posterior projection of the lower border of the trachea (its bifurcation) corresponds to the intervertebral disc between the $T_{IV}$ and $T_V$ vertebrae. At the level of the bifurcation, the depth from the skin surface is 6–12 cm.
- Lumbar region: Discs are the most massive.
- Sacral region: Intervertebral discs act as temporary synchondroses. Over time, they gradually calcify and transform into synostoses.
Clinical Significance: Degeneration and Compression
Intervertebral disc degeneration can lead to changes in the intervertebral foramina of the cervical spine ($C_{II}$–$C_{VII}$).
- Intervertebral foramen stenosis: Narrowing of the lumen leads to compression of nerve roots and blood vessels. Normally, an intervertebral foramen contains nerve roots (radices nervorum), an artery, and a vein, surrounded by adipose tissue.
- Deformation of the uncinate process (processus uncinatus): Occurs secondary to degenerative processes in the intervertebral disc, further exacerbating stenosis and neural compression.
- C8 radiculopathy: Results in pain within the C8 dermatome, potential paresthesias, hypalgesia in the C8 innervation zone, weakness or paresis of the hypothenar muscles, possible hypothenar muscle atrophy, and diminished triceps and Tromner reflexes.
- Intradural extramedullary tumors: Most commonly originate from the meninges of adjacent dorsal roots. They can grow in an "hourglass" shape, extending into the vertebral canal, growing through the intervertebral foramen, and emerging externally. Early signs include radicular pain and paresthesias. As the tumor grows, it causes compression of the dorsal roots, followed by spinal cord compression: primarily affecting the posterior columns, secondarily involving the pyramidal tract descending in the lateral funiculus. Rapidly progressive lower extremity spastic paresis, paresthesias with a sensation of cold, and ascending loss of epicritic and proprioceptive sensitivity may occur.