How maturity influences annulus‐endplate integration in the ovine intervertebral disc: a micro‐ and ultra‐structural study

Abstract

The annulus-endplate anchorage system plays a vital role in structurally linking the compliant disc to its adjacent much more rigid vertebrae. Past literature has identified the endplate as a region of weakness, not just in the mature spine but also in the immature spine. The aim of this structural study was to investigate in detail the morphological changes associated with annulus-endplate integration through different stages of maturity. Ovine lumbar motion segments were collected from two immature age groups: (i) newborn and (ii) spring lamb (roughly 3 months old); these were compared with a third group of previously analysed mature ewe samples (3-5 years). Sections from the posterior region of each motion segment were obtained for microstructural analysis and imaged in their fully hydrated state via differential interference contrast (DIC) optical microscopy. Selected slices were further prepared and imaged via scanning electron microscopy (SEM) to analyse fibril-level modes of integration. Despite significant changes in endplate morphology, the annular fibre bundles in all three age groups displayed a similar branching mechanism, with the main bundle splitting into several sub-bundles on entering the cartilaginous endplate. This morphology, previously described in the mature ovine disc, is thought to strengthen significantly annulus-endplate integration. Its prevalence from an age as young as birth emphasizes the critical role that it plays in the anchorage system. The structure of the branched sub-bundles and their integration with the surrounding matrix were found to vary with age due to changes in the cartilaginous and vertebral components of the endplate. Microscopically, the sub-bundles in both immature age groups appeared to fade into the surrounding tissue due to their fibril-level integration with the cartilaginous endplate tissue, this mechanism being particularly complex in the spring lamb disc. However, in the fully mature disc, the sub-bundles remained as separate entities throughout the full depth of their anchorage into the cartilaginous endplate. Cell morphology was also found to vary with maturity within the cartilaginous matrix and it is proposed that this relates to endplate development and ossification.

Multiscale structural characterization of the vertebral endplate in animal models

“…This is important because it indicates that the analysed preparations were of a similar age. In animals, the GP undergoes mineralization as the aging process progresses (Casaroli et al, 2018; Rodrigues et al, 2017).…”

Cartilaginous endplates: A comprehensive review on a neglected structure in intervertebral disc research

“…The integration at the inner AF, however, is more complex as the collagen fibers of the AF lamellae are continuous with those of the CEP (Figure 1). 49,56 Additionally, SEM analysis of ovine discs has shown that the collagen fibers of the AF branch, which strengthens the annulus–EP anchorage by increasing the interface area over which shear forces are distributed 57 . These fibers also intertwine and sometimes merge with the fibrils of the CEP.…”

“… 49 , 56 Additionally, SEM analysis of ovine discs has shown that the collagen fibers of the AF branch, which strengthens the annulus–EP anchorage by increasing the interface area over which shear forces are distributed. 57 These fibers also intertwine and sometimes merge with the fibrils of the CEP. The strength of this connection is essential for the CEP ability to resist tensile loading.…”

Importance of Matrix Cues on Intervertebral Disc Development, Degeneration, and Regeneration

“…It is furthermore proposed that early fibre organisation informs the development of intracellular actin stress fibres within developing AF cells [86], which in turn may guide the production of aligned ECM and help redistribute mechanical loading as the IVD develops. Intriguingly, the orientation and integration of AF fibres into the BEPs, CEPs, and surrounding tissues appears to be driven by physical constraints caused by the emergence of the endplates and vertebral bodies [34,85,87], implying that physical matrix cues are central to AF development.…”