Region and strain-dependent diffusivities of glucose and lactate in healthy human cartilage endplate

Regional structure-function relationships of lumbar cartilage endplates

Cartilage endplates (CEPs) act as protective mechanical barriers for intervertebral discs (IVDs), yet their heterogeneous structure-function relationships are poorly understood. This study addressed this gap by characterizing and correlating the regional biphasic mechanical properties and biochemical composition of human lumbar CEPs. Samples from central, lateral, anterior, and posterior portions of the disc (n = 8/region) were mechanically tested under confined compression to quantify swelling pressure, equilibrium aggregate modulus, and hydraulic permeability. These properties were correlated with CEP porosity and glycosaminoglycan (s-GAG) content, which were obtained by biochemical assays of the same specimens. Both swelling pressure (142.79 ± 85.89 kPa) and aggregate modulus (1864.10 ± 1240.99 kPa) were found to be regionally dependent (p = 0.0001 and p = 0.0067, respectively) in the CEP and trended lowest in the central location. No significant regional dependence was observed for CEP permeability (1.35 ± 0.97 * 10-16 m4/Ns). Porosity measurements correlated significantly with swelling pressure (r = -0.40, p = 0.0227), aggregate modulus (r = -0.49, p = 0.0046), and permeability (r = 0.36, p = 0.0421), and appeared to be the primary indicator of CEP biphasic mechanical properties. Second harmonic generation microscopy also revealed regional patterns of collagen fiber anchoring, with fibers inserting the CEP perpendicularly in the central region and at off-axial directions in peripheral regions. These results suggest that CEP tissue has regionally dependent mechanical properties which are likely due to the regional variation in porosity and matrix structure. This work advances our understanding of healthy baseline endplate biomechanics and lays a groundwork for further understanding the role of CEPs in IVD degeneration.

Non-enzymatic glycation reduces glucose transport in the human cartilage endplate independently of matrix porosity or proteoglycan content

Background

Intervertebral disc degeneration is associated with low back pain, which is a leading cause of disability. While the precise causes of disc degeneration are unknown, inadequate nutrient and metabolite transport through the cartilage endplate (CEP) may be one important factor. Prior work shows that CEP transport properties depend on the porosity of the CEP matrix, but little is known about the role of CEP characteristics that could influence transport properties independently from porosity. Here, we show that CEP transport properties depend on the extent of non-enzymatic glycation of the CEP matrix.

Methods and Results

Using in vitro ribosylation to induce non-enzymatic glycation and promote the formation of advanced glycation end products, we found that ribosylation reduced glucose partition coefficients in human cadaveric lumbar CEP tissues by 10.7%, on average, compared with donor- and site-matched CEP tissues that did not undergo ribosylation (p = 0.04). These reductions in glucose uptake were observed in the absence of differences in CEP porosity (p = 0.89) or in the amounts of sulfated glycosaminoglycans (sGAGs, p = 0.47) or collagen (p = 0.61). To investigate whether ribosylation altered electrostatic interactions between fixed charges on the sGAG molecules and the mobile free ions, we measured the charge density in the CEP matrix using equilibrium partitioning of a cationic contrast agent using micro-computed tomography. After contrast enhancement, mean X-ray attenuation was 11.9% lower in the CEP tissues that had undergone ribosylation (p = 0.02), implying the CEP matrix was less negatively charged.

Conclusions

Taken together, these findings indicate that non-enzymatic glycation negatively impacts glucose transport in the CEP independent of matrix porosity or sGAG content and that the effects may be mediated by alterations to matrix charge density.

Indentation mechanics and native collagen content in the cartilaginous endplate: A comparison between porcine cervical and human lumbar spines

This study characterized the regional indentation mechanics and native collagen content in cartilaginous endplates (CEPs) from the porcine cervical spine, young human lumbar spine, and aged human lumbar spine. Seventeen endplates were included in this study: six porcine cervical, nine young human lumbar, and two aged human lumbar. Width and depth measurements were obtained using a digital caliper and used to size-normalize and identify the central, anterior, posterior, and lateral regions. Regional microindentation tests were performed using a serial robot, where surface locations were loaded/unloaded at 0.1 mm/s and held at a constant 10 N force for 30 s. Loading stiffness and creep displacement were obtained from force-displacement data. Immunofluorescence staining for type I and type II collagen was subsequently performed on sagittal sections of all endplate regions. 255 images were obtained from which fluorescence intensity, sub-surface void area, and cartilage thickness were measured. CEPs from the young human lumbar spine were, on average, 27% more compliant, 0.891 mm thicker, had a lower fluorescence intensity for native collagen proteins within the cartilage (-58%) and subchondral bone (-24%), and had a sub-surface void area that was 19.7 times greater than porcine cervical CEPs. Compared to aged human lumbar CEPs, young human lumbar CEPs were 57% stiffer, 0.568 mm thicker, had a higher fluorescence intensity for native collagen proteins within the cartilage (+30%) and subchondral bone (+46%), and had a sub-surface void area that was 10.6 times smaller. Although not a perfect mechanical and structural surrogate, porcine cervical CEPs provided initial conditions that may be more representative of the young and healthy human lumbar spine compared to aged human cadaveric specimens. The indentation properties presented may have further applications to finite element models of the human lumbar spine.

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.

Does vertebral osteoporosis delay or accelerate lumbar disc degeneration? A systematic review

The effect of vertebral osteoporosis on disc degeneration is still debated. The purpose of this study was to provide a systematic review of studies in this area to further reveal the relationship between the two. Relevant studies were searched in electronic databases, and studies were screened according to inclusion and exclusion criteria, and finally, basic information of the included studies was extracted and summarized. This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. A total of 34 publications spanning 24 years were included in our study. There were 19 clinical studies, including 12 prospective studies and 7 retrospective studies. Of these, 7 considered vertebral osteoporosis to be positively correlated with disc degeneration, 8 considered them to be negatively correlated, and 4 considered them to be uncorrelated. Two cadaveric studies were included, one considered the two to be negatively correlated and one considered them not to be correlated. Seven animal studies were included, of which five considered a positive correlation between vertebral osteoporosis and disc degeneration and two considered a negative correlation between the two. There were also 6 studies that used anti-osteoporosis drugs for intervention, all of them were animal studies. Five of them concluded that vertebral osteoporosis was positively associated with disc degeneration, and the remaining one concluded that there was no correlation between the two. Our systematic review shows that the majority of studies currently consider an association between vertebral osteoporosis and disc degeneration, but there is still a huge disagreement whether this association is positive or negative. Differences in observation time and follow-up time may be one of the reasons for the disagreement. A large number of clinical and basic studies are still needed in the future to further explore the relationship between the two.

Diffusivity of Human Cartilage Endplates in Healthy and Degenerated Intervertebral Disks

The cartilage endplates (CEPs) on the superior and inferior surfaces of the intervertebral disk (IVD), are the primary nutrient transport pathways between the disk and the vertebral body. Passive diffusion is responsible for transporting small nutrient and metabolite molecules through the avascular CEPs. The baseline solute diffusivities in healthy CEPs have been previously studied, however alterations in CEP diffusion associated with IVD degeneration remain unclear. This study aimed to quantitatively compare the solute diffusion in healthy and degenerated human CEPs using a fluorescence recovery after photobleaching (FRAP) approach. Seven healthy CEPs and 22 degenerated CEPs were collected from five fresh-frozen human cadaveric spines and 17 patients undergoing spine fusion surgery, respectively. The sodium fluorescein diffusivities in CEP radial and vertical directions were measured using the FRAP method. The CEP calcification level was evaluated by measuring the average X-ray attenuation. No difference was found in solute diffusivities between radial and axial directions in healthy and degenerated CEPs. Compared to healthy CEPs, the average solute diffusivity was 44% lower in degenerated CEPs (Healthy: 29.07 μm2/s (CI: 23.96-33.62 μm2/s); degenerated: 16.32 μm2/s (CI: 13.84-18.84 μm2/s), p < 0.001). The average solute diffusivity had an inverse relationship with the degree of CEP calcification as determined by the normalized X-ray attenuation values (ß = -22.19, R2 = 0.633; p < 0.001). This study suggests that solute diffusion through the disk and vertebral body interface is significantly hindered by CEP calcification, providing clues to help further understand the mechanism of IVD degeneration.

Controversies in spine research: Organ culture versus in vivo models for studies of the intervertebral disc

Intervertebral disc degeneration is a common cause of low back pain, the leading cause of disability worldwide. Appropriate preclinical models for intervertebral disc research are essential to achieving a better understanding of underlying pathophysiology and for the development, evaluation, and translation of more effective treatments. To this end, in vivo animal and ex vivo organ culture models are both widely used by spine researchers; however, the relative strengths and weaknesses of these two approaches are a source of ongoing controversy. In this article, members from the Spine and Preclinical Models Sections of the Orthopedic Research Society, including experts in both basic and translational spine research, present contrasting arguments in support of in vivo animal models versus ex vivo organ culture models for studies of the disc, supported by a comprehensive review of the relevant literature. The objective is to provide a deeper understanding of the respective advantages and limitations of these approaches, and advance the field toward a consensus with respect to appropriate model selection and implementation. We conclude that complementary use of several model types and leveraging the unique advantages of each is likely to result in the highest impact research in most instances.

Application of single and cooperative different delivery systems for the treatment of intervertebral disc degeneration

Intervertebral disc (IVD) degeneration (IDD) is the most universal pathogenesis of low back pain (LBP), a prevalent and costly medical problem across the world. Persistent low back pain can seriously affect a patient's quality of life and even lead to disability. Furthermore, the corresponding medical expenses create a serious economic burden to both individuals and society. Intervertebral disc degeneration is commonly thought to be related to age, injury, obesity, genetic susceptibility, and other risk factors. Nonetheless, its specific pathological process has not been completely elucidated; the current mainstream view considers that this condition arises from the interaction of multiple mechanisms. With the development of medical concepts and technology, clinicians and scientists tend to intervene in the early or middle stages of intervertebral disc degeneration to avoid further aggravation. However, with the aid of modern delivery systems, it is now possible to intervene in the process of intervertebral disc at the cellular and molecular levels. This review aims to provide an overview of the main mechanisms associated with intervertebral disc degeneration and the delivery systems that can help us to improve the efficacy of intervertebral disc degeneration treatment.

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.

Mechanically induced histochemical and structural damage in the annulus fibrosus and cartilaginous endplate: a multi-colour immunofluorescence analysis

The annulus fibrosus (AF) and endplate (EP) are collagenous spine tissues that are frequently injured due to gradual mechanical overload. Macroscopic injuries to these tissues are typically a by-product of microdamage accumulation. Many existing histochemistry and biochemistry techniques are used to examine microdamage in the AF and EP; however, there are several limitations when used in isolation. Immunofluorescence may be sensitive to histochemical and structural damage and permits the simultaneous evaluation of multiple proteins-collagen I (COL I) and collagen II (COL II). This investigation characterized the histochemical and structural damage in initially healthy porcine spinal joints that were either unloaded (control) or loaded via biofidelic compression loading. The mean fluorescence area and mean fluorescence intensity of COL II significantly decreased (- 54.9 and - 44.8%, respectively) in the loaded AF (p ≤ 0.002), with no changes in COL I (p ≥ 0.471). In contrast, the EP displayed similar decreases in COL I and COL II fluorescence area (- 35.6 and - 37.7%, respectively) under loading conditions (p ≤ 0.027). A significant reduction (-31.1%) in mean fluorescence intensity was only observed for COL II (p = 0.043). The normalized area of pores was not altered on the endplate surface (p = 0.338), but a significant increase (+ 7.0%) in the void area was observed on the EP-subchondral bone interface (p = 0.002). Colocalization of COL I and COL II was minimal in all tissues (R < 0.34). In conclusion, the immunofluorescence analysis captured histochemical and structural damage in collagenous spine tissues, namely, the AF and EP.

In vivo fluid transport in human intervertebral discs varies by spinal level and disc region

Background

The lumbar discs are large, dense tissues that are primarily avascular, and cells residing in the central region of the disc are up to 6-8 mm from the nearest blood vessel in adults. To maintain homeostasis, disc cells rely on nutrient transport between the discs and adjacent vertebrae. Thus, diminished transport has been proposed as a factor in age-related disc degeneration.

Methods

In this study, we used magnetic resonance imaging (MRI) to quantify diurnal changes in T2 relaxation time, an MRI biomarker related to disc hydration, to generate 3D models of disc fluid distribution and determine how diurnal changes in fluid varied by spinal level. We recruited 10 participants (five males/five females; age: 21-30 years; BMI: 19.1-29.0 kg/m2) and evaluated the T2 relaxation time of each disc at 8:00 AM and 7:00 PM, as well as degeneration grade (Pfirrmann). We also measured disc height, volume, and perimeter in a subset of individuals as a preliminary comparison of geometry and transport properties.

Results

We found that the baseline (AM) T2 relaxation time and the diurnal change in T2 relaxation time were greatest in the cranial lumbar discs, decreasing along the lumbar spine from cranial to caudal. In cranial discs, T2 relaxation times decreased in each disc region (nucleus pulposus [NP], inner annulus fibrosus [IAF], and outer annulus fibrosus [OAF]), whereas in caudal discs, T2 relaxation times decreased in the NP but increased in the AF.

Conclusions

Fluid transport varied by spinal level, where transport was greatest in the most cranial lumbar discs and decreased from cranial to caudal along the lumbar spine. Future work should evaluate what level-dependent factors affect transport.

Consolidating and re-evaluating the human disc nutrient microenvironment

Background

Despite exciting advances in regenerative medicine, cell‐based strategies for treating degenerative disc disease remain in their infancy. To maximize the potential for successful clinical translation, a more thorough understanding of the in vivo microenvironment is needed to better determine and predict how cell therapies will respond when administered in vivo.

Aims

This work aims to reflect on the in vivo nutrient microenvironment of the degenerating IVD through consolidating what has already been measured together with investigative in silico models.

Materials and Methods

This work uses in silico modeling, underpinned by more recent experimentally determined parameters of degeneration and nutrient transport from the literature, to re‐evaluate the current knowledge in terms of grade‐specific stages of degeneration.

Results

Through modeling only the metabolically active cell population, this work predicts slightly higher glucose concentrations compared to previous in silico models, while the predicted results show good agreement with previous intradiscal pH and oxygen measurements. Increasing calcification with degeneration limits nutrient transport into the IVD and initiates a build‐up of acidity; however, its effect is compensated somewhat by a reduction in diffusional distance due to decreasing disc height.

Discussion

This work advances in silico modeling through a strong foundation of experimentally determined grade‐specific input parameters. Taken together, pre‐existing measurements and predicted results suggest that metabolite concentrations may not be as critically low as commonly believed, with calcification not appearing to have a detrimental effect at stages of degeneration when cell therapies are an appropriate intervention.

Conclusion

Overall, our initiative is to provoke greater deliberation and consideration of the nutrient microenvironment when performing in vitro cell culture and cell therapy development. This work highlights urgency for robust experimental glucose measurements in healthy and degenerating IVDs, not only to validate in silico models but to significantly advance the field in fully elucidating the nutrient microenvironment and refining in vitro techniques to accelerate clinical translation.

Patient-specific apparent diffusion maps used to model nutrient availability in degenerated intervertebral discs

INTRODUCTION

In this study, magnetic resonance imaging data was used to (1) model IVD-specific gradients of glucose, oxygen, lactate, and pH; and (2) investigate possible effects of covariate factors (i.e., disc geometry, and mean apparent diffusion coefficient values) on the IVD's microenvironment. Mathematical modeling of the patient's specific IVD microenvironment could be important when selecting patients for stem cell therapy due to the increased nutrient demand created by that treatment.

MATERIALS AND METHODS

Disc geometry and water diffusion coefficients were extracted from MRIs of 37 patients using sagittal T1-weighted images, T2-weighted images, and ADC Maps. A 2-D steady state finite element mathematical model was developed in COMSOL Multiphysics® 5.4 to compute concentration maps of glucose, oxygen, lactate and pH.

RESULTS

Concentration of nutrients (i.e., glucose, and oxygen) dropped with increasing distance from the cartilaginous endplates (CEP), whereas acidity levels increased. Most discs experienced poor nutrient levels along with high acidity values in the inner annulus fibrosus (AF). The disc's physiological microenvironment became more deficient as degeneration progressed. For example, minimum glucose concentration in grade 4 dropped by 31.1% compared to grade 3 (p < 0.0001). The model further suggested a strong effect of the following parameters: disc size, AF and CEP diffusivities, metabolic reactions, and cell density on solute concentrations in the disc (p < 0.05).

CONCLUSION

The significance of this work implies that the individual morphology and physiological conditions of each disc, even among discs of the same Pfirrmann grade, should be evaluated when modeling IVD solute concentrations.

Evaluation of human cartilage endplate composition using MRI: Spatial variation, association with adjacent disc degeneration, and in vivo repeatability

Cartilage endplate (CEP) biochemical composition may influence disc degeneration and regeneration. However, evaluating CEP composition in patients remains a challenge. We used T2* mapping from ultrashort echo-time (UTE) magnetic resonance imaging (MRI), which is sensitive to CEP hydration, to investigate spatial variations in CEP T2* values and to determine how CEP T2* values correlate with adjacent disc degeneration. Thirteen human cadavers (56.4 ± 12.7 years) and seven volunteers (36.9 ± 10.9 years) underwent 3T MRI, including UTE and T1ρ mapping sequences. Spatial mappings of T2* values in L4-S1 CEPs were generated from UTE images and compared between subregions. In the abutting discs, mean T1ρ values in the nucleus pulposus were compared between CEPs with high vs low T2* values. To assess in vivo repeatability, precision errors in mean T2* values, and intraclass correlation coefficients (ICC) were measured from repeat scans. Results showed that CEP T2* values were highest centrally and lowest posteriorly. In the youngest individuals (<50 years), who had mild-to-moderately degenerated Pfirrmann grade II-III discs, low CEP T2* values associated with severer disc degeneration: T1ρ values were 26.7% lower in subjects with low CEP T2* values (P = .025). In older individuals, CEP T2* values did not associate with disc degeneration (P = .39-.62). Precision errors in T2* ranged from 1.7 to 2.6 ms, and reliability was good-to-excellent (ICC = 0.89-0.94). These findings suggest that deficits in CEP composition, as indicated by low T2* values, associate with severer disc degeneration during the mild-to-moderate stages. Measuring CEP T2* values with UTE MRI may clarify the role of CEP composition in patients with mild-to-moderate disc degeneration.

Regeneration in Spinal Disease: Therapeutic Role of Hypoxia-Inducible Factor-1 Alpha in Regeneration of Degenerative Intervertebral Disc

The intervertebral disc (IVD) is a complex joint structure comprising three primary components—namely, nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP). The IVD retrieves oxygen from the surrounding vertebral body through CEP by diffusion and likely generates ATP via anaerobic glycolysis. IVD degeneration is characterized by a cascade of cellular, compositional, structural changes. With advanced age, pronounced changes occur in the composition of the disc extracellular matrix (ECM). NP and AF cells in the IVD possess poor regenerative capacity compared with that of other tissues. Hypoxia-inducible factor (HIF) is a master transcription factor that initiates a coordinated cellular cascade in response to a low oxygen tension environment, including the regulation of numerous enzymes in response to hypoxia. HIF-1α is essential for NP development and homeostasis and is involved in various processes of IVD degeneration process, promotes ECM in NP, maintains the metabolic activities of NP, and regulates dystrophic mineralization of NP, as well as angiogenesis, autophagy, and apoptosis during IVD degeneration. HIF-1α may, therefore, represent a diagnostic tool for early IVD degeneration and a therapeutic target for inhibiting IVD degeneration

A noninvasive fluorescence imaging-based platform measures 3D anisotropic extracellular diffusion

Diffusion is a major molecular transport mechanism in biological systems. Quantifying direction-dependent (i.e., anisotropic) diffusion is vitally important to depicting how the three-dimensional (3D) tissue structure and composition affect the biochemical environment, and thus define tissue functions. However, a tool for noninvasively measuring the 3D anisotropic extracellular diffusion of biorelevant molecules is not yet available. Here, we present light-sheet imaging-based Fourier transform fluorescence recovery after photobleaching (LiFT-FRAP), which noninvasively determines 3D diffusion tensors of various biomolecules with diffusivities up to 51 µm2 s-1, reaching the physiological diffusivity range in most biological systems. Using cornea as an example, LiFT-FRAP reveals fundamental limitations of current invasive two-dimensional diffusion measurements, which have drawn controversial conclusions on extracellular diffusion in healthy and clinically treated tissues. Moreover, LiFT-FRAP demonstrates that tissue structural or compositional changes caused by diseases or scaffold fabrication yield direction-dependent diffusion changes. These results demonstrate LiFT-FRAP as a powerful platform technology for studying disease mechanisms, advancing clinical outcomes, and improving tissue engineering.

Intervertebral disc degeneration and regeneration: a motion segment perspective

Back and neck pain have become primary reasons for disability and healthcare spending globally. While the causes of back pain are multifactorial, intervertebral disc degeneration is frequently cited as a primary source of pain. The annulus fibrosus (AF) and nucleus pulposus (NP) subcomponents of the disc are common targets for regenerative therapeutics. However, disc degeneration is also associated with degenerative changes to adjacent spinal tissues, and successful regenerative therapies will likely need to consider and address the pathology of adjacent spinal structures beyond solely the disc subcomponents. This review summarises the current state of knowledge in the field regarding associations between back pain, disc degeneration, and degeneration of the cartilaginous and bony endplates, the AF-vertebral body interface, the facet joints and spinal muscles, in addition to a discussion of regenerative strategies for treating pain and degeneration from a whole motion segment perspective.

Characterization of bovine and canine animal model cartilage endplates and comparison to human cartilage endplate structure, matrix composition, and cell phenotype

There is a need to further explore mechanisms of cartilage endplate (CEP) degeneration, due to its role in the onset and progression of intervertebral disc degeneration and low back pain. Therefore, the goal of this study was to evaluate structure, matrix composition, and cell phenotype between the human and bovine or canine, both clinically relevant animal models currently used to study the intervertebral disc, CEP. This information may be used in addition to other relevant studies, to help determine optimal animal models for use in studying the role of the CEP in intervertebral disc degeneration and back pain. Endplate structure, matrix composition, cell morphology, and gene expression were evaluated using a picrosirius red/alcian blue and hematoxylin and eosin stain, a dimethylmethylene blue assay, and quantitative reverse transcription polymerase chain reaction. The bovine and canine CEPs were thinner with more rounded cells and thicker bony endplates. The canine CEP contained significantly more sulfated glycosaminoglycans. The bovine CEP demonstrated higher expression of ACAN, COL1, and COL2 and lower expression of T, FBLN1, and collagen X (COLX) compared to the human CEP. The canine CEP had higher COL2 and lower COL1, KRT19, MKX, FBLN1, COLX expression compared to human. These similarities and differences between human and bovine or canine CEP are important to consider when evaluating which animal model is most optimal to use in future studies, interpreting research findings using these animal models and assessing translatability to the human condition.

Lumbar Vertebral Endplate Defects on Magnetic Resonance Imaging in Degenerative Spondylolisthesis: Novel Classification, Characteristics, and Correlative Factor Analysis

BACKGROUND

Endplate defects are commonly seen in patients with lumbar degenerative disease. However, little is known about the presence of endplate defects in patients with degenerative spondylolisthesis (DS). In the present study, we have introduced a classification system for endplate defects in patients with DS using routine magnetic resonance imaging findings and explored the correlative factors.

METHODS

Endplate defects were classified into 3 major categories (rim defects, focal defects, and erosive defects) and 5 subtypes (anterior type, posterior type, arc type at the anterior rim, notch type, and Schmorl's nodes). The incidence rates of the endplate defects were compared between the slippage and nonslippage levels. The correlations between the endplate defects and age, sex, disc degeneration, Modic changes (MCs), body mass index, slippage segment, and slippage degree were analyzed.

RESULTS

Endplate defects were present in 47.43% of the endplates in DS. The most common endplate defects were rim defects. The occurrence of endplate defects, especially anterior defects, was more common at the slippage levels. Endplate defects were associated with age and closely related to MCs and the severity of disc degeneration. The slippage degree, slippage segment, body mass index, and sex differences were not associated with endplate defects in our study. The results obtained using this novel classification system were stable and consistent.

CONCLUSIONS

The results from the present study have shown that the novel radiological classification system of endplate defects is reliable. Endplate defects were associated with slippage but not with the slippage degree or slippage segment differences in DS. The correlation between endplate defects and age and between MCs and disc degeneration were important features on the magnetic resonance imaging scans of patients with DS.

Nutrient supply and nucleus pulposus cell function: effects of the transport properties of the cartilage endplate and potential implications for intradiscal biologic therapy

OBJECTIVE

Intradiscal biologic therapy is a promising strategy for managing intervertebral disc degeneration. However, these therapies require a rich nutrient supply, which may be limited by the transport properties of the cartilage endplate (CEP). This study investigated how fluctuations in CEP transport properties impact nutrient diffusion and disc cell survival and function.

DESIGN

Human CEP tissues harvested from six fresh cadaveric lumbar spines (38-66 years old) were placed at the open sides of diffusion chambers. Bovine nucleus pulposus (NP) cells cultured inside the chambers were nourished exclusively by nutrients diffusing through the CEP tissues. After 72 h in culture, depth-dependent NP cell viability and gene expression were measured, and related to CEP transport properties and biochemical composition determined using fluorescence recovery after photobleaching and Fourier transform infrared (FTIR) spectroscopy.

RESULTS

Solute diffusivity varied nearly 4-fold amongst the CEPs studied, and chambers with the least permeable CEPs appeared to have lower aggrecan, collagen-2, and matrix metalloproteinase-2 gene expression, as well as a significantly shorter viable distance from the CEP/nutrient interface. Increasing chamber cell density shortened the viable distance; however, this effect was lost for low-diffusivity CEPs, which suggests that these CEPs may not provide enough nutrient diffusion to satisfy cell demands. Solute diffusivity in the CEP was associated with biochemical composition: low-diffusivity CEPs had greater amounts of collagen and aggrecan, more mineral, and lower cross-link maturity.

CONCLUSIONS

CEP transport properties dramatically affect NP cell survival/function. Degeneration-related CEP matrix changes could hinder the success of biologic therapies that require increased nutrient supply.

Effect of Sustained Joint Loading on TMJ Disc Nutrient Environment

The temporomandibular joint (TMJ) disc nutrient environment profoundly affects cell energy metabolism, proliferation, and biosynthesis. Due to technical challenges of in vivo measurements, the human TMJ disc extracellular nutrient environment under load, which depends on metabolic rates, solute diffusion, and disc morphometry, remains unknown. Therefore, the study objective was to predict the TMJ disc nutrient environment under loading conditions using combined experimental and computational modeling approaches. Specifically, glucose consumption and lactate production rates of porcine TMJ discs were measured under varying tissue culture conditions (n = 40 discs), and mechanical strain-dependent glucose and lactate diffusivities were measured using a custom diffusion chamber (n = 6 discs). TMJ anatomy and loading area were obtained from magnetic resonance imaging of healthy human volunteers (n = 11, male, 30 ± 9 y). Using experimentally determined nutrient metabolic rates, solute diffusivities, TMJ anatomy, and loading areas, subject-specific finite element (FE) models were developed to predict the 3-dimensional nutrient profiles in unloaded and loaded TMJ discs (unloaded, 0% strain, 20% strain). From the FE models, glucose, lactate, and oxygen concentration ranges for unloaded healthy human TMJ discs were 0.6 to 4.0 mM, 0.9 to 5.0 mM, and 0% to 6%, respectively, with steep gradients in the anterior and posterior bands. Sustained mechanical loading significantly reduced nutrient levels (P < 0.001), with a critical zone in which cells may die representing approximately 13.5% of the total disc volume. In conclusion, this study experimentally determined TMJ disc metabolic rates, solute diffusivities, and disc morphometry, and through subject-specific FE modeling, revealed critical interactions between mechanical loading and nutrient supply and metabolism for the in vivo human TMJ disc. The results suggest that TMJ disc homeostasis may be vulnerable to pathological loading (e.g., clenching, bruxism), which impedes nutrient supply. Given difficulties associated with direct in vivo measurements, this study provides a new approach to systematically investigate homeostatic and degenerative mechanisms associated with the TMJ disc.

Inhibition of both endplate nutritional pathways results in intervertebral disc degeneration in a goat model

Background

The vertebral endplate route was demonstrated to be the main pathway for nutrition to the intervertebral disc. However, it is still a controversial issue on whether the blocking of the endplate nutritional pathway could result in intervertebral disc degeneration (IDD) in animal models. The aim was therefore to investigate the effect of the inhibition of both endplate nutritional pathways by bone cement injection on the IDD in a goat model.

Methods

Two lumbar intervertebral discs (L2–3 and L3–4) in eight 24-month-old goats were blocked in both endplate nutritional pathways by cement injection, and the other two lumbar intervertebral discs (L1–2 and L4–5) remained intact as normal controls. Effective blocking area percentage in nucleus pulposus (NP) was calculated, and X-rays, magnetic resonance imaging (MRI), and histology studies were performed at 4, 12, 24, and 48 weeks after operation.

Results

The mean effective blocking area percentage was 60.7 ± 5.3%. Imaging examinations at the time of 48 weeks after blocking the endplate nutritional pathways showed obvious IDD, with larger disc height reduction and higher degrees of disc degeneration grading compared with the normal controls. Histological examinations including HE, Masson’s trichrome, Sirius Red, and proteoglycan stainings also confirmed the degenerative changes of the blocked discs.

Conclusions

The endplate nutritional route could be inhibited by blocking both endplate pathways with cement injection in a goat model. The severe inhibition in the endplate nutritional pathways may result in IDD.

Matrix modification for enhancing the transport properties of the human cartilage endplate to improve disc nutrition

Poor solute transport through the cartilage endplate (CEP) impairs disc nutrition and could be a key factor that limits the success of intradiscal biologic therapies. Here we demonstrate that treating the CEP with matrix metalloproteinase-8 (MMP-8) reduces the matrix constituents that impede solute uptake and thereby improves nutrient diffusion. Human CEP tissues harvested from four fresh cadaveric lumbar spines (age range: 38-66 years old) were treated with MMP-8. Treatment caused a dose-dependent reduction in sGAG, localized reductions to the amount of collagen, and alterations to collagen structure. These matrix modifications corresponded with 16-24% increases in the uptake of a small solute (376 Da). Interestingly, the effects of MMP-8 treatment depended on the extent of non-enzymatic glycation: treated CEPs with high concentrations of advanced glycation end products (AGEs) exhibited the lowest uptake compared to treated CEPs with low concentrations of AGEs. Moreover, AGE concentrations were donor-specific, and the donor tissues with the highest AGE concentrations appeared to have lower uptake than would be expected based on the initial amounts of collagen and sGAG. Finally, increasing solute uptake in the CEP improved cell viability inside diffusion chambers, which supports the nutritional relevance of enhancing the transport properties of the CEP. Taken together, our results provide new insights and in vitro proof-of-concept for a treatment approach that could improve disc nutrition for biologic therapy: specifically, matrix reduction by MMP-8 can enhance solute uptake and nutrient diffusion through the CEP, and AGE concentration appears to be an important, patient-specific factor that influences the efficacy of this approach.

Effects of dynamic loading on solute transport through the human cartilage endplate

Nutrient and metabolite transport through the cartilage endplate (CEP) is important for maintaining proper disc nutrition, but the mechanisms of solute transport remain unclear. One unresolved issue is the role of dynamic loading. In comparison to static loading, dynamic loading is thought to enhance transport by increasing convection. However, the CEP has a high resistance to fluid flow, which could limit solute convection. Here we measure solute transport through site-matched cadaveric human lumbar CEP tissues under static vs. dynamic loading, and we determine how the degree of transport enhancement from dynamic loading depends on CEP porosity and solute size. We found that dynamic loading significantly increased small and large solute transport through the CEP: on average, dynamic loading increased the transport of sodium fluorescein (376 Da) by a factor of 1.85 ± 0.64 and the transport of a large dextran (4000 Da) by a factor of 4.97 ± 3.05. Importantly, CEP porosity (0.65 ± 0.07; range: 0.47-0.76) strongly influenced the degree of transport enhancement. Specifically, for both solutes, transport enhancement was greater for CEPs with low porosity than for CEPs with high porosity. This is because the CEPs with low porosity were susceptible to larger improvements in fluid flow under dynamic loading. The CEP becomes less porous and less hydrated with aging and as disc degeneration progresses. Together, these findings suggest that as those changes occur, dynamic loading has a greater effect on solute transport through the CEP compared to static loading, and thus may play a larger role in disc nutrition.

Contribution of the endplates to disc degeneration

Purpose of review

The endplates form the interface between the rigid vertebral bodies and compliant intervertebral discs. Proper endplate function involves a balance between conflicting biomechanical and nutritional demands. This review summarizes recent data that highlight the importance of proper endplate function and the relationships between endplate dysfunction, adjacent disc degeneration, and axial low back pain.

Recent findings

Changes to endplate morphology and composition that impair its permeability associate with disc degeneration. Endplate damage also associates with disc degeneration, and the progression of degeneration may be accelerated and the chronicity of symptoms heightened when damage coincides with evidence of adjacent bone marrow lesions.

Summary

The endplate plays a key role in the development of disc degeneration and low back pain. Clarification of the mechanisms governing endplate degeneration and developments in clinical imaging that enable precise evaluation of endplate function and dysfunction will distinguish the correlative vs. causative nature of endplate damage and motivate new treatments that target pathologic endplate function.

COMPUTATION OF THE DYNAMIC COMPRESSION EFFECTS IN SPINE DISCS USING INTEGRAL METHODS

The computational modeling using integral methods of dynamic loading and its effects on the nutrients transport in spine discs is addressed in this work. The numerical simulation and analysis was carried out using the Boundary Element Method (BEM) and a 3D model (axisymmetric) of the disc. The boundary model was discretized using linear interpolated elements and a multi-region approach. Concentration and production of three nutrients as lactate, oxygen and glucose were obtained. The maximum lactate concentration was observed very close to the interface between the nucleus and the inner annulus. A relatively simple model discretized with 130 boundary elements yielded very similar results to these coming from more complex FEM-based models. The numerical efforts in the domain and boundary discretizations were optimized using the BEM. Our results are in good agreement with those obtained using with finite element-based models. As expected, the dynamic loading increased the oxygen–glucose consumption and the lactate production, thus leading to a poor oxygen–glucose concentration at the nucleus of the disc. All of that is a favorable environment for a disc degeneration mechanism to be developed.

ENERGY SUPPLY AND DEMAND IN THE INTERVERTEBRAL DISC

ABSTRACT The intervertebral disc (IVD) is one of the parts of the body most commonly affected by disease, and it is only recently that we have come closer to understanding the reasons for its degeneration, in which nutrient supply plays a crucial role. In this literature review, we discuss the basic principles and characteristics of energy supply and demand to the IVD. Specifically, we review how different metabolites influence IVD cell activity, the effects of mechanical loading on IVD cell metabolism, and differences in energy metabolism of the annulus fibrous and nucleus pulposus cell phenotypes. Determining the factors that influence nutrient supply and demand in the IVD will enhance our understanding of the IVD pathology, and help to elucidate new therapeutic targets for IVD degeneration treatment.

Quantifying Baseline Fixed Charge Density in Healthy Human Cartilage Endplate: A Two-point Electrical Conductivity Method

STUDY DESIGN

Regional measurements of fixed charge densities (FCDs) of healthy human cartilage endplate (CEP) using a two-point electrical conductivity approach.

OBJECTIVE

The aim of this study was to determine the FCDs at four different regions (central, lateral, anterior, and posterior) of human CEP, and correlate the FCDs with tissue biochemical composition.

SUMMARY OF BACKGROUND DATA

The CEP, a thin layer of hyaline cartilage on the cranial and caudal surfaces of the intervertebral disc, plays an irreplaceable role in maintaining the unique physiological mechano-electrochemical environment inside the disc. FCD, arising from the carboxyl and sulfate groups of the glycosaminoglycans (GAG) in the extracellular matrix of the disc, is a key regulator of the disc ionic and osmotic environment through physicochemical and electrokinetic effects. Although FCDs in the annulus fibrosus (AF) and nucleus pulposus (NP) have been reported, quantitative baseline FCD in healthy human CEP has not been reported.

METHODS

CEP specimens were regionally isolated from human lumbar spines. FCD and ion diffusivity were concurrently investigated using a two-point electrical conductivity method. Biochemical assays were used to quantify regional GAG and water content.

RESULTS

FCD in healthy human CEP was region-dependent, with FCD lowest in the lateral region (P = 0.044). Cross-region FCD was 30% to 60% smaller than FCD in NP, but similar to the AF and articular cartilage (AC). CEP FCD (average: 0.12 ± 0.03 mEq/g wet tissue) was correlated with GAG content (average: 31.24 ± 5.06 μg/mg wet tissue) (P = 0.005). In addition, the cross-region ion diffusivity in healthy CEP (2.97 ± 1.00 × 10 cm/s) was much smaller than the AF and NP.

CONCLUSION

Healthy human CEP acts as a biomechanical interface, distributing loads between the bony vertebral body and soft disc tissues and as a gateway impeding rapid solute diffusion through the disc.

LEVEL OF EVIDENCE

N/A.

A computational measurement of cartilaginous endplate structure using ultrashort time-to-echo MRI scanning

BACKGROUND AND OBJECTIVE

Ultrashort time-to-echo (UTE) MRI scanning has been applied to observe the cartilaginous endplate (CEP) in intervertebral disc. CEP plays a critical role in IVD health and disease. Nevertheless, current measurements of CEP based on UTE MRI technique are still by manual segmentation, and observation of signal abnormality was usually time-consuming and often disturbed by subjective bias. This study hence proposed an efficient way to harvest the global parameters of CEP after UTE MRI scanning.

METHODS

Ex-vivo UTE-MRI scanning was performed using 12 goat lumbar spine specimens. After the UTE-MRI data were collected, the computational method for CEP segmentation and assessment was developed. Global view of CEP, e.g., surface morphology as well as distributions of thickness and signal intensity, were measured. Histological staining of the CEP as well as manual CEP segmentation was then conducted to validate the accuracy.

RESULTS

Segmentation of CEP by the proposed method presented a good agreement with manual measurement, with mean Jaccard index of 0.7296 and mean Cohen's Kappa coefficient of 0.8286. The processing time for CEP segmentation and property measurements was 59.2s which was much shorter than the manual measurement.

CONCLUSIONS

This newly-developed technique is able to qualitatively and quantitatively assess the CEP structure, which is very valuable for the clinicians and researchers to accurately evaluate the endplate health after UTE MRI scanning.

[Expression and significance of hypoxia-inducible factor 1α in endplate chondrocytes of rats]

Objective

To explore the expression and significance of hypoxia-inducible factor 1α (HIF-1α) in endplate chondrocytes, and to study the relations between HIF-1α expression and endplate chondrocytes apoptosis.

Methods

Eight Sprague Dawley rats were selected to obtain the L _1-5 intervertebral disc endplate; the endplate chondrocytes were isolated by enzyme digestion method, and the endplate chondrocytes at passage 3 were cultured under 20% O ₂ condition (group A), and under 0.5% O ₂ condition (group B). Cell morphology was observed by inverted phase contrast microscope and cell apoptosis was detected using flow cytometry after cultured for 24 hours; the mRNA expression of HIF-1α was detected by real-time fluorescent quantitative PCR, the protein expressions of HIF-1α, Bax, and Bcl-2 by Western blot. Gene clone technology to design and synthesize two siRNAs based on the sequence of HIF-1α mRNA. HIF-1α specific RNAi sequence compound was constructed and transfected into cells. The transfected endplate chondrocytes at passage 3 were cultured under 0.5% O ₂ condition in group C and group D (HIF-1α gene was silenced). After cultured for 24 hours, cells were observed via immunofluorescence staining of HIF-1α, and cell apoptosis was detected using flow cytometry. Meanwhile, the mRNA expressions of HIF-1α, collagen type II (COL II), Aggrecan, and SOX9 were detected by real-time fluorescent quantitative PCR, and the protein expressions of HIF-1α, Bax, and Bcl-2 by Western blot.

Results

At 24 hours after culture, small amount of vacuoles necrotic cells could be observed in group A and group B; there was no significant difference in apoptosis rate between groups A and B ( t=1.026, P=0.471), and HIF-1α mRNA and protein expressions in group B were significantly higher than those in group A ( t=22.672, P=0.015; t=18.396, P=0.013), but, there was no significant difference in protein expressions of Bax and Bcl-2 between groups A and B ( t=0.594, P=0.781; t=1.251, P=0.342). The number of vacuolar necrosis cells in group D was significantly higher than that in group C, and HIF-1α positive cells were observed in group D. The apoptosis rate of group D was significantly higher than that of group C ( t=27.143, P=0.002). The mRNA expressions of HIF-1α, COL II, Aggrecan, and SOX9 in group D were significantly lower than those in group C ( t=21.097, P=0.015; t=34.829, P=0.002; t=18.673, P=0.022; t=31.949, P=0.007). The protein expressions of HIF-1α and Bcl-2 in group D were significantly lower than those in group C ( t=37.648, P=0.006; t=16.729, P=0.036), but the protein expression of Bax in group D was significantly higher than that in group C ( t=25.583, P=0.011).

Conclusion

HIF-1α mRNA expression is up-regulated under hypoxia condition, which will increase the hypoxia tolerance of endplate chondrocytes. Cell apoptosis is suppressed by the activation of HIF-1α in endplate chondrocytes under hypoxia condition.