Elastic tissues of the intervertebral disc.

Elastic fibres have been generally considered to play no significant role in the mechanical functioning of the intervertebral disc since earlier studies reported that the elastic fibre network was sparse and irregular. However, a recent study has reported that the network is highly organized and that the distribution and orientation of elastic fibres varies from region to region. In the annulus, elastic fibres appear densely distributed in the region between the lamellae and also in 'bridges' across the lamellae. They are also organized in the nucleus where long straight fibres are radially oriented and anchor perpendicularly or obliquely into the cartilaginous endplate. Immunohistochemistry using specific antibodies indicates that elastin is present in the network, as is fibrillin. Biochemical studies show, however, that the amino acid composition of the residue remaining after alkaline (NaOH) extraction or CNBr digestion contains a higher concentration of polar amino acids than ligamentum nuchal elastin. The composition of the residue suggests that disc elastin may cross-link strongly with some other matrix components. With such coupling, it is thought that elastic fibres could play a significant mechanical role even though overall elastin is less than 5% of the total dry weight of the disc.