The biology behind the human intervertebral disc and its endplates.

The intervertebral discs (IVDs) are roughly cylindrical, fibrocartilaginous, articulating structures connecting the vertebral bodies, and allowing movement in the otherwise rigid anterior portion of the vertebral column. They also transfer loads and dissipate energy. Macroscopically the intervertebral disc can be divided into an outer annulus fibrosus surrounding a centrally located nucleus pulposus. The endplates surround the IVD from both the cranial and caudal ends, and separate them from the vertebral bodies and prevent the highly hydrated nucleus pulposus from bulging into the adjacent vertebrae. The IVD develop from the mesodermal notochord and receive nutrients mostly through the cartilaginous endplates. Physiologically they are innervated only in the outer annulus fibrosus by sensory and sympathetic perivascular nerve fibres, branches from the sinuvertebral nerve, the ventral rami of spinal nerves or from the grey rami communicantes. The IVD undergo changes with ageing and degeneration, the latter having two types i.e. "endplate-driven" involving endplate defects and inward collapse of the annulus fibrosus and "annulus-driven" involving a radial fissure and/or an IVD prolapse. This review summarises and updates the current state of knowledge on the embryology, structure, and biomechanics of the IVD and its endplates. To further translate this into a more clinical context this review also demonstrates the impact of ageing and degeneration on the above properties of both the IVD and its endplates.