Overview

Number23

Nucleus water content (young adult)≈70–90%

Diurnal height change≈1–2 cm

There are 23 discs, from C2–C3 to L5–S1 (none between occiput and C1 or C1 and C2). Each disc is a secondary cartilaginous joint (symphysis) with three components. The annulus fibrosus is made of 15–25 concentric lamellae of fibrocartilage; collagen fibres in each lamella run obliquely (≈60–65° to the vertical), alternating direction in successive layers — a cross-ply arrangement that resists twisting and contains the nucleus. Its outer lamellae (type I collagen) attach to the ring apophyses; inner lamellae (type II collagen) blend with the end plates. The posterolateral annulus is thinnest.

The nucleus pulposus, lying slightly posterior to the centre, is a gelatinous remnant of the notochord — ≈70–90% water in the young, held by proteoglycans (aggrecan) in a type II collagen network. Under load it behaves as a fluid, distributing pressure evenly onto the annulus and end plates. The vertebral (cartilaginous) end plates of hyaline cartilage cover the vertebral bodies and allow nutrients to diffuse in.

Adult discs are avascular apart from their outer annulus, which is innervated by the sinuvertebral (recurrent meningeal) nerves — so tears of the outer annulus can be painful. Discs lose water through the day (we are ≈1–2 cm shorter by evening) and with age, when the nucleus becomes fibrous and less able to distribute load. Discs are thickest in the lumbar region and wedge-shaped in the cervical and lumbar regions, creating the lordoses.

Location: Between the bodies of adjacent vertebrae from C2–C3 to L5–S1.

Parts & subdivisions

Nucleus pulposus
Gelatinous core (notochord remnant); hydrated proteoglycan gel.
Annulus fibrosus
Concentric fibrocartilage lamellae with alternating fibre angles.
Vertebral end plates
Hyaline cartilage plates for nutrient diffusion.

Anatomy

Innervation: Outer third of the annulus: sinuvertebral nerves posteriorly; branches of grey rami communicantes laterally and anteriorly.
Arterial supply: Avascular in adults except the outer annulus; nutrition by diffusion through the end plates.
Development: Nucleus pulposus from the notochord; annulus fibrosus from sclerotome mesenchyme between vertebral bodies.

Function

Absorbs and distributes compressive loads
Allows movement between vertebral bodies
Binds vertebrae together
Contributes ≈25% of presacral column length and forms the cervical and lumbar curvatures

Clinical significance

Herniated (prolapsed) disc

Nucleus pulposus extrudes through a posterolateral annular tear, where the posterior longitudinal ligament is narrow, compressing the nerve root. In the lumbar spine the traversing root is affected (L4–L5 → L5); in the cervical spine the root of the lower vertebra (C5–C6 → C6).

Schmorl nodes

Vertical herniation of nucleus through the end plate into the vertebral body — common incidental finding; prominent in Scheuermann disease.

Degenerative disc disease

Dehydration ("black disc" on T2 MRI), loss of height and annular fissures with age; contributes to facet arthropathy, osteophytes and stenosis.

Test yourself

The sternal angle (of Louis) is a key landmark because it lies at the level of:

The 1st costal cartilage and T2
The 4th costal cartilage and T6
The xiphisternal joint and T9
The 2nd costal cartilage and the T4–T5 intervertebral disc

Answer: The 2nd costal cartilage and the T4–T5 intervertebral disc

The manubriosternal joint lies opposite the 2nd costal cartilage (used to count ribs) and the T4–T5 disc. This plane marks the start and end of the aortic arch, the tracheal bifurcation, the pulmonary trunk bifurcation and the superior/inferior mediastinum boundary.