On how nucleus-endplate integration is achieved at the fibrillar level in the ovine lumbar disc

The intervertebral disc nucleus has traditionally been viewed as a largely unstructured amorphous gel having little obvious integration with the cartilaginous endplates (CEPs). However, recent work by the present authors has provided clear evidence of structural cohesion across the nucleus-endplate junction via a distinctive microanatomical feature termed insertion nodes. The aim of this study was to explore the nature of these insertion nodes at the fibrillar level. Specially prepared vertebra-nucleus-vertebra composite samples from ovine lumbar motion segments were extended axially and chemically fixed in this stretched state, and then decalcified. Sections taken from the samples were prepared for examination by scanning electron microscopy. A close morphological correlation was obtained between previously published optical microscopic images of the nodes and those seen using low magnification SEM. Progressively high magnifications provided insight into the fibrillar-level modes of structural integration across the nucleus-endplate junction. The closely packed fibrils of the CEP were largely parallel to the vertebral endplate and formed a dense, multi-layer substrate within which the nodal fibrils appeared to be anchored. Our idealised structural model proposes a mechanism by which this integration is achieved. The nodal fibrils, in curving into the CEP, are locked in place within its close-packed layers of transversely aligned fibrils, and probably at multiple levels. Secondly, there appears to be a subtle interweaving of the strongly aligned nodal fibrils with the multi-directional endplate fibrils. It is suggested that this structural integration provides the nucleus with a form of tethered mobility that supports physiological functions quite distinct from the primary strength requirements of the disc.