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

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

The cartilaginous endplates (CEP) are key components of the intervertebral disc (IVD) necessary for sustaining the nutrition of the disc while distributing mechanical loads and preventing the disc from bulging into the adjacent vertebral body. The size, shape, and composition of the CEP are essential in maintaining its function, and degeneration of the CEP is considered a contributor to early IVD degeneration. In addition, the CEP is implicated in Modic changes, which are often associated with low back pain. This review aims to tackle the current knowledge of the CEP regarding its structure, composition, permeability, and mechanical role in a healthy disc, how they change with degeneration, and how they connect to IVD degeneration and low back pain. Additionally, the authors suggest a standardized naming convention regarding the CEP and bony endplate and suggest avoiding the term vertebral endplate. Currently, there is limited data on the CEP itself as reported data is often a combination of CEP and bony endplate, or the CEP is considered as articular cartilage. However, it is clear the CEP is a unique tissue type that differs from articular cartilage, bony endplate, and other IVD tissues. Thus, future research should investigate the CEP separately to fully understand its role in healthy and degenerated IVDs. Further, most IVD regeneration therapies in development failed to address, or even considered the CEP, despite its key role in nutrition and mechanical stability within the IVD. Thus, the CEP should be considered and potentially targeted for future sustainable treatments.

Species variation in the cartilaginous endplate of the lumbar intervertebral disc

Backgrounds

Cartilaginous endplate (CEP) plays an essential role in intervertebral disc (IVD) health and disease. The aim was to compare the CEP structure of lumbar IVD and to reveal the detailed pattern of integration between the CEP and bony endplate (BEP) from different species.

Methods

A total of 34 IVDs (5 human, 5 goat, 8 pig, 8 rabbit, and 8 rat IVDs) were collected, fixed and midsagittally cut; in each IVD, one-half was used for histological staining to observe the CEP morphology, and the other half was used for scanning electron microscopy (SEM) analysis to measure the diameters and distributions of collagen fibers in the central and peripheral CEP areas and to observe the pattern of CEP-BEP integration from different species.

Results

The human, pig, goat, and rabbit IVDs had the typical BEP-CEP structure, but the rat CEP was directly connected with the growth plate. Human CEP was the thickest (896.95 ± 87.71 μm) among these species, followed by pig, goat, rat, and rabbit CEPs. Additionally, the mean cellular density of the rabbit CEP was the highest, which was 930 ± 202 per mm2, followed by the rat, goat, pig, and human CEPs. In all the species, the collagen fiber diameter in the peripheral area was much bigger than that in the central area. The collagen fiber diameters of CEP from the human, pig, goat, and rat were distributed between 35 nm and 65 nm. The BEP and CEP were connected by the collagen from the CEP, aggregating into bundles or cross links with each other to form a network, and anchored to BEP.

Conclusions

Significant differences in the thickness, cellular density, and collagen characterization of CEPs from different species were demonstrated; the integration of BEP-CEP in humans, pigs, goats, and rabbits was mainly achieved by the collagen bundles anchoring system, while the typical BEP-CEP interface did not exist in rats.

Elastic Fibers in the Intervertebral Disc: From Form to Function and toward Regeneration

Despite extensive efforts over the past 40 years, there is still a significant gap in knowledge of the characteristics of elastic fibers in the intervertebral disc (IVD). More studies are required to clarify the potential contribution of elastic fibers to the IVD (healthy and diseased) function and recommend critical areas for future investigations. On the other hand, current IVD in-vitro models are not true reflections of the complex biological IVD tissue and the role of elastic fibers has often been ignored in developing relevant tissue-engineered scaffolds and realistic computational models. This has affected the progress of IVD studies (tissue engineering solutions, biomechanics, fundamental biology) and translation into clinical practice. Motivated by the current gap, the current review paper presents a comprehensive study (from the early 1980s to 2022) that explores the current understanding of structural (multi-scale hierarchy), biological (development and aging, elastin content, and cell-fiber interaction), and biomechanical properties of the IVD elastic fibers, and provides new insights into future investigations in this domain.

Biomechanical analysis of the cervical spine segment as a method for studying the functional and dynamic anatomy of the human neck

BACKGROUND

Traditionally, dynamic and functional anatomy, in particular the dynamic anatomy of the neck, is studied on cadaveric material. However, the development of in vivo visualization technologies and in silico modeling has made it possible to expand these possibilities. Despite significant progress in the study of dynamic and functional anatomy of the neck by means of in silico methods, the issues of validating the developed models and taking into account the pronounced nonlinearity of soft tissues as well as local anisotropy remain open. The aim of this study was to develop a virtual dynamic anatomical model of the human neck and reproduce the dynamic processes in the cervical spine from this model using the finite element method.

MATERIALS AND METHODS

Reverse engineering was used to generate a dynamic anatomical model of the neck from CT data (both male, 24 and 22 years old). Two segments of the cervical spine (C3-C5, C2-T1) were isolated from the resulting model for finite element analysis. Finite element mesh generation and contact interactions were performed using the HyperMesh software (Altair Engineering Inc, Troy, Michigan, USA). The anisotropic hyperelastic Holzapfel-Gasser-Ogden model was used to describe the material behavior of the fibrous rings of the disc. Material modeling and finite element analysis were performed using Abaqus CAE 6.14 software (Simulia, Johnston, Rhode Island, USA).

RESULTS

A technique for creating a virtual dynamic anatomical model of the neck was elaborated and implemented. The model includes 79 major anatomical structures of the neck segmented from radiological data. A finite element analysis of the cervical spine was performed. The results of finite element analysis of the C3-C5 segment under axial load were compared with in vitro data. The proposed model shows nonlinear deformation of the disc under static loading; the model predicted displacement values agree well with the experimental ones. The displacement of the С3-С5 central vertebra with an axial load of 800 N reaches a value of 0.65 mm. For the segment C2-T1, data on intradiscal pressure, stress plots and displacements during flexion were obtained. The maximum stress value of 10.036 MPa is observed in the C3-C4 disc.

CONCLUSION

Simulation results using the proposed methodology are in good agreement with experimental data. The generated biomechanical models allow describing dynamic phenomena in the cervical spine and obtaining a wide range of quantitative properties of anatomical objects, which are otherwise inaccessible to classical methods for studying dynamic and functional anatomy.

A comprehensive tool box for large animal studies of intervertebral disc degeneration

Preclinical studies involving large animal models aim to recapitulate the clinical situation as much as possible and bridge the gap from benchtop to bedside. To date, studies investigating intervertebral disc (IVD) degeneration and regeneration in large animal models have utilized a wide spectrum of methodologies for outcome evaluation. This paper aims to consolidate available knowledge, expertise, and experience in large animal preclinical models of IVD degeneration to create a comprehensive tool box of anatomical and functional outcomes. Herein, we present a Large Animal IVD Scoring Algorithm based on three scales: macroscopic (gross morphology, imaging, and biomechanics), microscopic (histological, biochemical, and biomolecular analyses), and clinical (neurologic state, mobility, and pain). The proposed algorithm encompasses a stepwise evaluation on all three scales, including spinal pain assessment, and relevant structural and functional components of IVD health and disease. This comprehensive tool box was designed for four commonly used preclinical large animal models (dog, pig, goat, and sheep) in order to facilitate standardization and applicability. Furthermore, it is intended to facilitate comparison across studies while discerning relevant differences between species within the context of outcomes with the goal to enhance veterinary clinical relevance as well. Current major challenges in pre-clinical large animal models for IVD regeneration are highlighted and insights into future directions that may improve the understanding of the underlying pathologies are discussed. As such, the IVD research community can deepen its exploration of the molecular, cellular, structural, and biomechanical changes that occur with IVD degeneration and regeneration, paving the path for clinically relevant therapeutic strategies.

The ultrastructural organization of elastic fibers at the interface of the nucleus and annulus of the intervertebral disk

There has been no study to describe the ultrastructural organization of elastic fibers at the interface of the nucleus pulposus and annulus fibrosus of the intervertebral disk (IVD), a region called the transition zone (TZ). A previously developed digestion technique was optimized to eliminate cells and non-elastin ECM components except for the elastic fibers from the anterolateral (AL) and posterolateral (PL) regions of the TZ in ovine IVDs. Not previously reported, the current study identified a complex elastic fiber network across the TZ for both AL and PL regions. In the AL region, this network consisted of major thick elastic fibers (≈ 1 µm) that were interconnected with delicate (< 200 nm) elastic fibers. While the same ultrastructural organization was observed in the PL region, interestingly the size of the elastic fibers was smaller (< 100 nm) compared to those that were located in the AL region. Quantitative analysis of the elastic fibers revealed significant differences in the size (p < 0.001) and the orientation of elastic fibers (p = 0.001) between the AL and PL regions, with a higher orientation and larger size of elastic fibers observed in the AL region. The gradual elimination of cells and non-elastin extracellular matrix components identified that elastic fibers in the TZ region in combination with the extracellular matrix created a honeycomb structure that was more compact at the AF interface compared to that located close to the NP. Three different symmetrically organized angles of rotation (0⁰ and ±90⁰) were detected for the honeycomb structure at both interfaces, and the structure was significantly orientated at the TZ-AF compared to the TZ-NP interface (p = 0.003).

New evidence for structural integration across the cartilage-vertebral endplate junction and its relation to herniation

BACKGROUND CONTEXT

The cartilaginous and bony material that can be present in herniated tissue suggests that failure can involve both cartilaginous and vertebral-endplates. How structural integration is achieved across the junction between these two distinct tissue regions via its fibril and mineral components is clearly relevant to the modes of endplate failure that occur.

PURPOSE

To understand how structural integration is achieved across the cartilaginous-vertebral endplate junction.

STUDY DESIGN

A micro- and fibril-level structural analysis of the cartilage-vertebral endplate region was carried out using healthy, mature ovine motion segments.

METHODS

Oblique vertebra-annulus-vertebra samples were prepared such that alternate layers of lamellar fibers extended from vertebra to vertebra. The endplate region of each sample was then decalcified in a targeted manner before being loaded in tension along the fiber direction to achieve incomplete rupture within the region of the endplate. The failure regions were then analyzed with differential interference contrast microscopy and scanning electron microscopy.

RESULTS

Microstructural analysis revealed that failure within the endplate region was not confined to the cement line. Instead, rupture continued into the underlying vertebral endplate with bony material still attached to the now unanchored annular bundles. Ultrastructural analysis of the partially ruptured regions of the cement line revealed clear evidence of blending/interweaving relationships between the fibrils of the annular bundles, the calcified cartilage and the bone with no one pattern of association appearing dominant. These findings suggest that fibril-based structural cohesion exists across the cement line at the site of annular insertion, with strengthening via a mechanism somewhat analogous to steel-reinforced concrete. The fibrils are brought into a close intermingling association with interfibril forces mediated via the mineral component.

CONCLUSIONS

This study provides clear evidence of structural connectivity across the cartilaginous-vertebral endplate junction by the intermingling of their fibrillar components and mediated by the mineral phase. This is consistent with the clinical observation that in some disc herniations bony material can be still attached to the extruded soft tissue.

Three-dimensional microstructural reconstruction of the ovine intervertebral disc using ultrahigh field MRI

BACKGROUND

The intervertebral disc (IVD) is a complex organ that acts as a flexible coupling between two adjacent vertebral bodies and must therefore accommodate compression, bending, and torsion. It consists of three main components, which are elegantly structured to allow this: the annulus fibrosus (AF), the nucleus pulposus (NP), and the end-plates (EP).

PURPOSE

Thus far, it has not been possible to examine the microarchitecture of the disc directly in three dimensions in its unaltered state and thus knowledge of the overall architecture of the disc has been inferred from a range of imaging sources, or by using destructive methods.

STUDY DESIGN

A nondestructive ultrahigh field Magnetic Resonance Imaging (MRI) of 11.7 T was used together with image analysis to visualize the ovine IVDs.

METHODS

Three-dimensional image stacks from eight IVDs harvested from sheep, half of which were 4 to 5 years old and the others approximately 2 years old were reconstructed and examined, and their microstructure were imaged. The overall structure of the disc, including the average of 14 AF lamellae (9-28), NP, and EP was then visualized with particular attention given to integrating elements as radial translamellar cross-links, AF-NP transition zone EP-AF integration and EP-NP insertion nodes (ie the connecting junctions between the EP and NP). Moreover, collagen fiber orientation was determined at different depths and locations throughout the annulus.

RESULTS

It was found that there was a clearer demarcation in the AF-NP transition zone of the younger discs compared with the older ones. This difference was reflected in the visibility of AF-NP and EP-AF integration. It was also possible to view the fiber architecture of the AF-NP integration in greater depth than was possible previously with histological techniques. These fibers were mainly observed in the younger discs and their length was measured to be of 2.6 ± 0.2 mm.

CONCLUSIONS

The present results provide a substantial advance in visualization of the three-dimensional architecture of an intact IVD and the integration of its components.

Does an Annular Puncture Influence the Herniation Path?: An In Vitro Mechanical and Structural Investigation

STUDY DESIGN

A study of mechanically induced herniation in punctured ovine discs followed by structural analysis.

OBJECTIVE

To investigate whether an annular puncture influences the path that herniation takes by providing direct passage for nucleus through the annulus and therefore whether it increases the risk of acute herniation from overload at the site of damage independent of any longer-term degeneration.

SUMMARY OF BACKGROUND DATA

Ten years after treatment with discography both degenerative changes and frequency of herniation have been shown to increase compared to untreated discs. Although the effect of an annular puncture over time has been widely investigated the question of whether it increases the risk of acute herniation has not been resolved.

METHODS

The posterolateral annuli of healthy ovine lumbar discs were punctured with either a 25-gauge (n = 8) or a larger 18-gauge (n = 8) needle and then compressed in a flexed posture of 10° until initial indications of failure. The entire volume of the disc was visually assessed for structural damage by obtaining progressive, full transverse cross-sections of its entire height thus exposing all regions of the disc.

RESULTS

There was no association between the 25-gauge puncture and disc disruption and herniation. In contrast, nuclear material was observed to migrate through the 18-gauge needle puncture. Disruption of the lateral inner annulus was observed in 12 out of the 16 discs tested.

CONCLUSION

The risk of acute herniation through the puncture site is dependent on the needle diameter used. Under the conditions employed the lateral inner annulus remains the site most vulnerable to disruption independent of the presence of a posterolateral puncture.

LEVEL OF EVIDENCE

N /A.

Biomechanics of the human intervertebral disc: A review of testing techniques and results

Many experimental testing techniques have been adopted in order to provide an understanding of the biomechanics of the human intervertebral disc (IVD). The aim of this review article is to amalgamate results from these studies to provide readers with an overview of the studies conducted and their contribution to our current understanding of the biomechanics and function of the IVD. The overview is presented in a way that should prove useful to experimentalists and computational modellers. Mechanical properties of whole IVDs can be assessed conveniently by testing 'motion segments' comprising two vertebrae and the intervening IVD and ligaments. Neural arches should be removed if load-sharing between them and the disc is of no interest, and specimens containing more than two vertebrae are required to study 'adjacent level' effects. Mechanisms of injury (including endplate fracture and disc herniation) have been studied by applying complex loading at physiologically-relevant loading rates, whereas mechanical evaluations of surgical prostheses require slower application of standardised loading protocols. Results can be strongly influenced by the testing environment, preconditioning, loading rate, specimen age and degeneration, and spinal level. Component tissues of the disc (anulus fibrosus, nucleus pulposus, and cartilage endplates) have been studied to determine their material properties, but only the anulus has been thoroughly evaluated. Animal discs can be used as a model of human discs where uniform non-degenerate specimens are required, although differences in scale, age, and anatomy can lead to problems in interpretation.

Numerical Prediction of the Mechanical Failure of the Intervertebral Disc under Complex Loading Conditions

Finite element modeling has been widely used to simulate the mechanical behavior of the intervertebral disc. Previous models have been generally limited to the prediction of the disc behavior under simple loading conditions, thus neglecting its response to complex loads, which may induce its failure. The aim of this study was to generate a finite element model of the ovine lumbar intervertebral disc, in which the annulus was characterized by an anisotropic hyperelastic formulation, and to use it to define which mechanical condition was unsafe for the disc. Based on published in vitro results, numerical analyses under combined flexion, lateral bending, and axial rotation with a magnitude double that of the physiological ones were performed. The simulations showed that flexion was the most unsafe load and an axial tensile stress greater than 10 MPa can cause disc failure. The numerical model here presented can be used to predict the failure of the disc under all loading conditions, which may support indications about the degree of safety of specific motions and daily activities, such as weight lifting.

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

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.

A multiscale structural investigation of the annulus-endplate anchorage system and its mechanisms of failure

BACKGROUND CONTEXT

The annulus-endplate anchorage system performs a critical role in the disc, creating a strong structural link between the compliant annulus and the rigid vertebrae. Endplate failure is thought to be associated with disc herniation, a recent study indicating that this failure mode occurs more frequently than annular rupture.

PURPOSE

The aim was to investigate the structural principles governing annulus-endplate anchorage and the basis of its strength and mechanisms of failure.

STUDY DESIGN

Loading experiments were performed on ovine lumbar motion segments designed to induce annulus-endplate failure, followed by macro- to micro- to fibril-level structural analyses.

METHODS

The study was funded by a doctoral scholarship from our institution. Samples were loaded to failure in three modes: torsion using intact motion segments, in-plane tension of the anterior annulus-endplate along one of the oblique fiber angles, and axial tension of the anterior annulus-endplate. The anterior region was chosen for its ease of access. Decalcification was used to investigate the mechanical influence of the mineralized component. Structural analysis was conducted on both the intact and failed samples using differential interference contrast optical microscopy and scanning electron microscopy.

RESULTS

Two main modes of anchorage failure were observed--failure at the tidemark or at the cement line. Samples subjected to axial tension contained more tidemark failures compared with those subjected to torsion and in-plane tension. Samples decalcified before testing frequently contained damage at the cement line, this being more extensive than in fresh samples. Analysis of the intact samples at their anchorage sites revealed that annular subbundle fibrils penetrate beyond the cement line to a limited depth and appear to merge with those in the vertebral and cartilaginous endplates.

CONCLUSIONS

Annulus-endplate anchorage is more vulnerable to failure in axial tension compared with both torsion and in-plane tension and is probably due to acute fiber bending at the soft-hard interface of the tidemark. This finding is consistent with evidence showing that flexion, which induces a similar pattern of axial tension, increases the risk of herniation involving endplate failure. The study also highlights the important strengthening role of calcification at this junction and provides new evidence of a fibril-based form of structural integration across the cement line.

How healthy discs herniate: a biomechanical and microstructural study investigating the combined effects of compression rate and flexion

STUDY DESIGN

Microstructural investigation of compression-induced disruption of the flexed lumbar disc.

OBJECTIVE

To provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and an elevated rate of compression.

SUMMARY OF BACKGROUND DATA

At the level of the motion segment failure of the disc in compression has been extensively studied. However, at the microstructural level the exact mechanisms of disc failure are still poorly understood, especially in relation to loading posture and rate.

METHODS

Seventy-two healthy mature ovine lumbar motion segments were compressed to failure in either a neutral posture or in high physiological flexion (10°) at a displacement rate of either 2 mm/min (low) or 40 mm/min (high). Testing at the high rate was terminated at stages ranging from initial wall tearing through to facet fracture so as to capture the evolution of failure up to full herniation. The damaged discs were then analyzed microstructurally.

RESULTS

Approximately, 50% of the motion segments compressed in flexion at the high rate experienced annulus or annulus-endplate junction failure, the remainder failed via endplate fracture with no detectable wall damage. The average load to induce disc failure in flexion was 18% lower (P < 0.05) than that required to induce endplate fracture. Microstructural analysis indicated that wall rupture occurred first in the posterior mid-then-outer annulus.

CONCLUSION

Disc wall failure in healthy motion segments requires both flexion and an elevated rate of compression. Damage is initiated in the mid-then-outer annular fibers, this a likely consequence of the higher strain burden in these same fibers arising from endplate curvature. Given the similarity in geometry between ovine and human endplates, it is proposed that comparable mechanisms of damage initiation and herniation occur in human lumbar discs.

LEVEL OF EVIDENCE

N/A.

Why do some intervertebral discs degenerate, when others (in the same spine) do not?

This review suggests why some discs degenerate rather than age normally. Intervertebral discs are avascular pads of fibrocartilage that allow movement between vertebral bodies. Human discs have a low cell density and a limited ability to adapt to mechanical demands. With increasing age, the matrix becomes yellowed, fibrous, and brittle, but if disc structure remains intact, there is little impairment in function, and minimal ingrowth of blood vessels or nerves. Approximately half of old lumbar discs degenerate in the sense of becoming physically disrupted. The posterior annulus and lower lumbar discs are most affected, presumably because they are most heavily loaded. Age and genetic inheritance can weaken discs to such an extent that they are physically disrupted during everyday activities. Damage to the endplate or annulus typically decompresses the nucleus, concentrates stress within the annulus, and allows ingrowth of nerves and blood vessels. Matrix disruption progresses by mechanical and biological means. The site of initial damage leads to two disc degeneration "phenotypes": endplate-driven degeneration is common in the upper lumbar and thoracic spine, and annulus-driven degeneration is common at L4-S1. Discogenic back pain can be initiated by tissue disruption, and amplified by inflammation and infection. Healing is possible in the outer annulus only, where cell density is highest. We conclude that some discs degenerate because they are disrupted by excessive mechanical loading. This can occur without trauma if tissues are weakened by age and genetic inheritance. Moderate mechanical loading, in contrast, strengthens all spinal tissues, including discs.

Influence of maturity on nucleus-endplate integration in the ovine lumbar spine

PURPOSE

Recent investigations using an ovine spine model have established that the disc nucleus contains a highly convoluted fibre network with endplate-to-endplate connectivity, this connectivity being achieved via distinctive nodal attachment points. The purpose of this study was to investigate how this nodal anchoring system might be influenced by maturation.

METHODS

Lumbar motion segments were dissected from newborn, 3, 12 months and fully mature ovine animals, subjected to a novel annular ring-severing procedure to remove the strain-limiting influence of the annulus, then either mechanically tested to destruction or examined microstructurally and ultrastructurally. The morphology of the nodes and their linear density within the relatively thin section planes were analysed to provide a basis for comparison between the four age groups.

RESULTS

Mechanical testing following ring severing revealed that the remaining nuclear material in all samples, irrespective of maturity, had the ability to transmit a substantial load from endplate to endplate. Imaging of the ring-severed samples from all age groups in their stretched, but unruptured state revealed the presence of axially aligned fibrosity in the nucleus region consistent with endplate-to-endplate connectivity. Endplate insertion nodes were observed in all age groups. Ultrastructural examination revealed that the fibrillar architecture of these nodes in the newborn discs was similar to that observed in the nodes of mature discs. However, there was a rapid increase in their linear density between birth and 3 months, after which this remained constant.

CONCLUSIONS

The nodal attachment points identified previously in mature ovine discs are also present in newborn, and 3- and 12-month-old animals with an initial rapid increase in their linear density between birth and 3 months, after which it remained constant. The size and morphology of the attachment points were similar for all ages. Our study suggests that the increase in nodal density in the ovine disc endplate is part of an adaptive response to the loading environment that the disc is exposed to from birth to maturity.