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Intervertebral discs
The intervertebral discs (or discs) are fibrocartilages lying between adjacent surfaces of the vertebrae. They form a fibrocartilaginous joint between the vertebral bodies, linking them together.
Collectively, the discs make up one third to one quarter of the total spinal column’s length, forming an interpose between adjacent vertebrae from the axis (C2) to the sacrum.
There are about 23 discs in the spine; 6 cervical, 12 thoracic, and 5 in the lumbar region. The intervertebral discs are approximately 7-10 mm thick and 4 cm in diameter (anterior – posterior plane) in the lumbar region of the spine.
It consists of a thick outer ring of fibrous cartilage called the anulus (derived from the Latin word “anus” meaning ring) or annulus (anulus fibrosus disci intervertebralis), which surrounds an inner gel-like centre or more gelatinous core known as the nucleus pulposus (meaning “pulpy interior”). The nucleus pulposus is sandwiched inferiorly and superiorly by cartilage endplates.
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Embryology
Development of the intervertebral disc begins in the 5th week. Mesenchymal cells from the sclerotomes, which are found around the notochord, migrate cranially, opposite the centre of the myotome, where they form the intervertebral (IV) disc. As development progresses, the notochord degenerates and disappears where it is surrounded by the developing vertebral bodies. Between the vertebrae, the notochord expands to form the gelatinous centre of the intervertebral disc – the nucleus pulposus. This nucleus is later surrounded by circularly arranged fibres that form the anulus fibrosus. The anulus fibrosus and nucleus pulposus together constitute the intervertebral disc.
Structure
Anulus fibrosus
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The anulus is made up of a series of 15-25 concentric rings, or lamellae, with the collagen fibres lying parallel within each lamella. The fibres are oriented at approximately 60 degrees to the vertical axis, alternating to the left and right of it in adjacent lamellae. Elastin fibres lie between the lamellae, possibly helping the disc to return to its original arrangement following bending (flexion or extension).
Cells of the anulus are elongated, thin, and aligned parallel to the collagen fibres. Towards the inner anulus the cells are oval. The anulus is relatively stiff, providing greater strength to the disc and withstands compressive force.
Nucleus pulposus
The central nucleus pulposus contains collagen fibres which are organized randomly, and elastin fibres which are arranged radially. These fibres are embedded in a highly hydrated aggrecan containing gel.
Endplate
The endplate is the third morphologically distinct part of the intervertebral disc. It is a thin horizontal layer, usually less than 1 mm thick. This structure interfaces the disc and the vertebral body. Collagen fibres within it run horizontally and parallel to the vertebral bodies, and become continuous with the disc.
Functions
- The intervertebral disc forms the fibrocartilaginous joint which allows slight movement of the vertebral column, and acts as a ligament to hold the vertebrae together.
- The discs act as fibrocartilaginous cushions, serving as the spine’s shock absorbing system. This cushions the effect of shock and stress produced when an individual walks, runs, bends, or twists.
- The intervertebral discs prevent friction between two moving vertebrae by preventing vertebral bodies from grinding against each other.
Nutrition
The intervertebral discs, like other cartilages, have no blood supply. They form the largest structures in the body without their own blood vessels. During embryonic development, and at birth, they possess some vascular supply which terminate in their endplates and anulus fibrosus. However, those blood vessels quickly deteriorate leaving them with no direct blood supply during postnatal life and in adulthood. The nutrients they require are absorbed from circulating blood by means of osmosis.
Innervation
The intervertebral disc is innervated through the sinovertebral nerves. The nerve fibres are mainly restricted to the outer lamellae in the endplate. Most of those sinovertebral nerves are meningeal branches of spinal nerves.
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Clinical points
Intervertebral disc anomalies can lead to symptoms of back pain, neck pain, and sciatica. There are many other causes of back pain and sciatica, for example spinal stenosis, osteoarthritis in the spine, and spondylolisthesis. Disorders affecting the spinal disc (intervertebral disc) include disc herniation, degenerative disc disease and infection like discitis.
Disc herniation
A spinal disc herniation can happen when the gel-like material of the nucleus pulposus is forced out of the surrounding anulus fibrosus, which can put pressure on the nearby nerve. This can give the symptoms of sciatica if it impinges on the roots of the sciatic nerve.
Degeneration
Before the age of 40, approximately 25% of people show evidence of disc degeneration at one or more levels. Beyond that age more than 60% of people show evidence of disc degeneration at one or more levels on magnetic resonance imaging.
One effect of ageing and disc degeneration is that the nucleus pulposus begins to dehydrate and the concentration of proteoglycans in the matrix decreases, thus limiting the ability of the disc to absorb shock. This general shrinking of disc size is partially responsible for the common decrease in height as humans age. The anulus fibrosus also becomes weaker with age and has an increased risk of tearing. In addition, the cartilage endplates begin thinning, fissures begin to form, and there is sclerosis of the subchondral bone (vertebral).
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