Vascularization of the human intervertebral disc: A scoping review

Intervertebral discs (IVDs) are often referred to as the largest avascular structures of the human body, yet a collective resource characterizing the vascularization of the IVD does not exist. To address this gap, the objective of this study was to conduct a comprehensive search of the literature to review and summarize current knowledge of the prevalence and localization of blood supply in human IVDs, with a scoping review. A comprehensive search of peer-reviewed publications on the topic of IVD vascularization in humans was conducted across six electronic databases: PubMed, EMBASE, MEDLINE, Scopus, Web of Science, and BIOSIS Previews. Studies of humans were included regardless of age, sex, ethnicity, and health status, with the exception of IVD herniation. Two independent reviewers screened titles and abstracts and full-texts according to eligibility criteria. The review was conducted and reported according to Preferred Reporting Items for Systematic Reviews Extension for Scoping Reviews guidelines. Our search yielded 3122 articles, with 22 articles meeting the inclusion criteria. The study samples ranged in age from fetal to >90 years and included both sexes, various health statuses, and used different methodologies (eg, histology, medical imaging, and gross dissection) to assess vasculature. Overall, consistent observations were that (a) the nucleus pulposus of the IVD is avascular throughout life, (b) both the cartilage endplates and annulus fibrosus receive considerable blood supply early in life that diminishes over the lifespan, and (c) vascular ingrowth into the cartilage endplates and inner layers of the annulus fibrosus is commonly associated with damaged or disrupted tissue, irrespective of age. Histology and immunohistochemistry are often used to report vascularization of the IVD. The body of the current literature suggests that the IVD should not be generalized as an avascular tissue. Instead, vascularization of the IVD differs based on the constituent tissues, their age, and state of degeneration or damage.

High glucose-induced excessive reactive oxygen species promote apoptosis through mitochondrial damage in rat cartilage endplate cells

Diabetes mellitus (DM) is an important factor in intervertebral disc degeneration (IDD). Apoptosis of cartilage endplate (CEP) cells is one of the initiators of IDD. However, the effects of high glucose on CEP cells are still unknown. Therefore, we conducted the present study to evaluate the effects of high glucose on CEP cells and to identify the mechanisms of those effects. Rat CEP cells were isolated and cultured in 10% foetal bovine serum (FBS, normal control) or high-glucose medium (10% FBS + 0.1 M glucose or 10% FBS + 0.2 M glucose, experimental conditions) for 1 or 3 days. In addition, CEP cells were treated with 0.2 M glucose for 3 days in the presence or absence of alpha-lipoic acid (ALA, 0.15 M). Flow cytometry was performed to identify and quantify the degree of apoptosis. The expression of reactive oxygen species (ROS) was assessed by flow cytometry, and mitochondrial damage (mitochondrial membrane potential) was assessed by fluorescence microscopy. Furthermore, the expression levels of cleaved caspase-3, cleaved caspase-9, Bcl-2, Bax, and cytochrome c were evaluated by Western blotting. High glucose significantly increased apoptosis and ROS accumulation in CEP cells in a dose- and time-dependent manner. Meanwhile, a disrupted mitochondrial membrane potential was detected in rat CEP cells cultured in the two high glucose concentrations. Incubating in high glucose enhanced the expression levels of cleaved caspase-3, cleaved caspase-9, Bax, and cytochrome c but decreased the level of the anti-apoptotic protein Bcl-2. ALA inhibited the expression of cleaved caspase-3, cleaved caspase-9, Bax, and cytochrome c but enhanced the expression of Bcl-2. ALA also prevented disruption of the mitochondrial membrane potential in CEP cells. This study demonstrates that high glucose-induced excessive reactive oxygen species promote mitochondrial damage, thus causing apoptosis in rat CEP cells in a dose- and time-dependent manner. ALA could prevent mitochondrial damage and apoptosis caused by high glucose in CEP cells. The results suggest that appropriate blood glucose control may be the key to preventing IDD in diabetic patients. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2476-2483, 2018.

Cartilage Endplate Thickness Variation Measured by Ultrashort Echo-Time MRI Is Associated With Adjacent Disc Degeneration

STUDY DESIGN

A magnetic resonance imaging study of human cadaver spines.

OBJECTIVE

To investigate associations between cartilage endplate (CEP) thickness and disc degeneration.

SUMMARY OF BACKGROUND DATA

Damage to the CEP is associated with spinal injury and back pain. However, CEP morphology and its association with disc degeneration have not been well characterized.

METHODS

Ten lumbar motion segments with varying degrees of disc degeneration were harvested from six cadaveric spines and scanned with magnetic resonance imaging in the sagittal plane using a T2-weighted two-dimensional (2D) sequence, a three-dimensional (3D) ultrashort echo-time (UTE) imaging sequence, and a 3D T1ρ mapping sequence. CEP thicknesses were calculated from 3D UTE image data using a custom, automated algorithm, and these values were validated against histology measurements. Pfirrmann grades and T1ρ values in the disc were assessed and correlated with CEP thickness.

RESULTS

The mean CEP thickness calculated from UTE images was 0.74 ± 0.04 mm. Statistical comparisons between histology and UTE-derived measurements of CEP thickness showed significant agreement, with the mean difference not significantly different from zero (P = 0.32). Within-disc variation of T1ρ (standard deviation) was significantly lower for Pfirrmann grade 4 than Pfirrmann grade 3 (P < 0.05). Within-disc variation of T1ρ and adjacent CEP thickness heterogeneity (coefficient of variation) had a significant negative correlation (r = -0.65, P = 0.04). The standard deviation of T1ρand the mean CEP thickness showed a moderate positive correlation (r = 0.40, P = 0.26).

CONCLUSION

This study demonstrates that quantitative measurements of CEP thickness measured from UTE magnetic resonance imaging are associated with disc degeneration. Our results suggest that variability in CEP thickness and T1ρ, rather than their mean values, may serve as valuable diagnostic markers for disc degeneration.

LEVEL OF EVIDENCE

N/A.

Coming together is a beginning: the making of an intervertebral disc

The intervertebral disc (IVD) is a complex fibrocartilaginous structure located between the vertebral bodies that allows for movement and acts as a shock absorber in our spine for daily activities. It is composed of three components: the nucleus pulposus (NP), annulus fibrosus, and cartilaginous endplate. The characteristics of these cells are different, as they produce specific extracellular matrix (ECM) for tissue function and the niche in supporting the differentiation status of the cells in the IVD. Furthermore, cell heterogeneities exist in each compartment. The cells and the supporting ECM change as we age, leading to degenerative outcomes that often lead to pathological symptoms such as back pain and sciatica. There are speculations as to the potential of cell therapy or the use of tissue engineering as treatments. However, the nature of the cells present in the IVD that support tissue function is not clear. This review looks at the origin of cells in the making of an IVD, from the earliest stages of embryogenesis in the formation of the notochord, and its role as a signaling center, guiding the formation of spine, and in its journey to become the NP at the center of the IVD. While our current understanding of the molecular signatures of IVD cells is still limited, the field is moving fast and the potential is enormous as we begin to understand the progenitor and differentiated cells present, their molecular signatures, and signals that we could harness in directing the appropriate in vitro and in vivo cellular responses in our quest to regain or maintain a healthy IVD as we age.

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.

Influence of biochemical composition on endplate cartilage tensile properties in the human lumbar spine

Endplate cartilage integrity is critical to spine health and is presumably impaired by deterioration in biochemical composition. Yet, quantitative relationships between endplate biochemical composition and biomechanical properties are unavailable. Using endplate cartilage harvested from human lumbar spines (six donors, ages 51-67 years) we showed that endplate biochemical composition has a significant influence on its equilibrium tensile properties and that the presence of endplate damage associates with a diminished composition-function relationship. We found that the equilibrium tensile modulus (5.9 ± 5.7 MPa) correlated significantly with collagen content (559 ± 147 µg/mg dry weight, r(2)  = 0.35) and with the collagen/GAG ratio (6.0 ± 2.1, r(2)  = 0.58). Accounting for the damage status of the adjacent cartilage improved the latter correlation (r(2)  = 0.77) and indicated that samples with adjacent damage such as fissures and avulsions had a diminished modulus-collagen/GAG relationship (p = 0.02). Quasi-linear viscoelastic relaxation properties (C, t1 , and t2 ) did not correlate with biochemical composition. We conclude that reduced matrix quantity decreases the equilibrium tensile modulus of human endplate cartilage and that characteristics of biochemical composition that are independent of matrix quantity, that is, characteristics related to matrix quality, may also be important.

Characterization of the human intervertebral disc cartilage endplate at the molecular, cell, and tissue levels

Given the importance of the cartilage endplate (CEP) in low back pain (LBP), there is a need to characterize the human CEP at the molecular, cell, and tissue levels to inform treatment strategies that target it. The goal of this study was to characterize the structure, matrix composition, and cell phenotype of the human CEP compared with adjacent tissues within the intervertebral joint: the nucleus pulposus (NP), annulus fibrosus (AF), and articular cartilage (AC). Isolated CEP, NP, AF, and AC tissues and cells were evaluated for cell morphology, matrix composition, collagen structure, glycosaminoglycan content, and gene and protein expression. The CEP contained elongated cells that mainly produce a collagen-rich interterritorial matrix and a proteoglycan-rich territorial matrix. The CEP contained significantly fewer glycosaminoglycans than the NP tissue. Significant differences in matrix and cell marker gene expression were observed between CEP and NP or AF, with the greatest differences between CEP and AC. We were able to distinguish NP from CEP cells using collagen-10 (COLX), highlighting COLX as a potential CEP marker. Our findings suggest that at the cell and tissue levels, the CEP demonstrates both similarities and differences when compared with NP, AF, and hyaline AC. This study highlights a unique structure, matrix composition, and cell phenotype for the human CEP and can help to inform regenerative strategies that target the intervertebral disc joint in chronic LBP.

The expression of osteoprotegerin is required for maintaining the intervertebral disc endplate of aged mice

OBJECTIVE

Human chondrocytes and annulus fibrosus cells of intervertebral disc (IVD) express osteoprotegerin (OPG), but the effect of OPG on the pathogenesis of IVD degeneration remains unknown. Here we assessed the phenotype change of IVD in OPG(-/-) mice.

METHODS

The IVDs from 12-, 20-, and 28-week-old OPG(-/-) mice and WT controls were subjected to histologic analyses including TRAP staining for osteoclasts, immunostaining for OPG and type I collagen protein expression, and TUNEL staining for apoptosis. The IVD tissues were also subjected to real time RT-PCR for mRNA expression of genes for osteoblast-osterix, ALP, and osteocalcin; for osteoclasts-trap, rank, mmp9 and cathepsin K, and for chondrocytes-aggrecan, mmp13 and Col10.

RESULTS

OPG protein expresses at the cells of endplate cartilage and annulus fibrosis in IVDs of WT mice. Compared to WT mice, OPG(-/-) mice developed aging related cartilage loss and bony tissue appearance at the endplate. Stating from 20 weeks of age, IVDs from OPG(-/-) mice expressed significantly increased mmp13 and Col10 levels, which is associated with increased osteoblast number and elevated expression of osteoblast marker genes. Furthermore, TRAP+ osteoclasts were presented in the endplate cartilage of OPG(-/-) mice. These osteoclasts localized adjacently to and erosion into the cartilage. Increased expression of RANK, mmp9 and cathepsin k was detected in OPG(-/-) IVDs.

CONCLUSIONS

OPG at IVD plays an important role for maintaining the integrity of endplate cartilage during aging by preventing endplate cartilage from osteoclast-mediated resorption.

CircSNHG5 Sponges Mir-495-3p and Modulates CITED2 to Protect Cartilage Endplate From Degradation

Background

Intervertebral disc degeneration (IDD) is a highly prevalent degenerating disease that produces tremendous amount of low back and neck pain. The cartilage endplate (CEP) is vitally important to intervertebral discs in both physiological and pathological conditions. In addition, circular RNAs (circRNAs) have been shown to be involved in the regulation of various diseases, including IDD. However, the particular role of circRNAs in cervical vertebral CEP degeneration remains unclear. Here, we examined the unique role of circRNAs in CEP of patients with cervical fracture and degenerative cervical myelopathy (DCM).

Methods

Human competitive endogenous RNA (ceRNA) microarray was performed by previous research. Western blot (WB), immunofluorescence (IF), quantitative RT-PCR (qRT-PCR), luciferase assay, and fluorescence in situ hybridization (FISH) were employed to analyze the function of circSNHG5 and its downstream effectors, miR-495-3p, and CITED2.

Results

We demonstrated that circSNHG5 expression was substantially low in degenerative CEP tissues. Knockdown of circSNHG5 in chondrocytes resulted in a loss of cell proliferation and followed by degradation of extracellular matrix (ECM). In addition, circSNHG5 was shown to sponge miR-495-3p and modulate the expression of the downstream gene CITED2. This mechanism of action was further validated via overexpression and knockdown of CITED2.

Conclusion

Our findings identified a novel circSNHG5-miR-495-3p axis responsible for IDD progression. Future investigations into IDD therapy may benefit from targeting this axis.

Comparison of Oxygen Consumption Rates of Nondegenerate and Degenerate Human Intervertebral Disc Cells

STUDY DESIGN

In vitro measurements of the oxygen consumption rates (OCR) of human intervertebral disc (IVD) cells.

OBJECTIVE

The aim of this study was to determine the differences in the OCR of nondegenerate and degenerate human annulus fibrosus (AF), nucleus pulposus (NP), and cartilage endplate (CEP) cells at different glucose concentrations.

SUMMARY OF BACKGROUND DATA

The avascular nature of the IVD creates a delicate balance between rate of nutrient transport through the matrix and rate of disc cell consumption necessary to maintain tissue health. Previous studies have shown a dependence of OCR for animal (e.g., bovine and porcine) IVD cells on oxygen level and glucose concentration. However, the OCR of nondegenerate human IVD cells compared to degenerate human IVD cells at different glucose concentrations has not been investigated.

METHODS

IVD cells were isolated from the AF, NP, and CEP regions of human cadaver spines and surgical samples. The changes in oxygen concentration were recorded when cells were sealed in a metabolic chamber. The OCR of cells was determined by curve fitting using the Michaelis-Menton equation.

RESULTS

Under identical cell culture conditions, the OCR of degenerate human IVD cells was three to five times greater than that of nondegenerate human IVD cells. The degenerate IVD cells cultured in low-glucose medium (1 mmol/L) exhibited the highest OCR compared to degenerate cells cultured at higher glucose levels (i.e., 5 mmol/L, 25 mmol/L), whereas no significant differences in OCR were found among the nondegenerate IVD cells for all glucose levels.

CONCLUSION

Considering the significantly higher OCR and unique response to glucose of degenerate human IVD cells, the degeneration of the IVD is associated with a cell phenotypic change related to OCR. The OCR of IVD cells reported in this study will be valuable for understanding human IVD cellular behavior and tissue nutrition in response to disc degeneration.

LEVEL OF EVIDENCE

N/A.

MRI quantification of human spine cartilage endplate geometry: Comparison with age, degeneration, level, and disc geometry

Geometry is an important indicator of disc mechanical function and degeneration. While the geometry and associated degenerative changes in the nucleus pulposus and the annulus fibrosus are well-defined, the geometry of the cartilage endplate (CEP) and its relationship to disc degeneration are unknown. The objectives of this study were to quantify CEP geometry in three dimensions using an MRI FLASH imaging sequence and evaluate relationships between CEP geometry and age, degeneration, spinal level, and overall disc geometry. To do so, we assessed the MRI-based measurements for accuracy and repeatability. Next, we measured CEP geometry across a larger sample set and correlated CEP geometric parameters to age, disc degeneration, level, and disc geometry. The MRI-based measures resulted in thicknesses (0.3-1 mm) that are comparable to prior measurements of CEP thickness. CEP thickness was greatest at the anterior/posterior (A/P) margins and smallest in the center. The CEP A/P thickness, axial area, and lateral width decreased with age but were not related to disc degeneration. Age-related, but not degeneration-related, changes in geometry suggest that the CEP may not follow the progression of disc degeneration. Ultimately, if the CEP undergoes significant geometric changes with aging and if these can be related to low back pain, a clinically feasible translation of the FLASH MRI-based measurement of CEP geometry presented in this study may prove a useful diagnostic tool. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1410-1417, 2016.

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.

Tidemark Avulsions are a Predominant Form of Endplate Irregularity

STUDY DESIGN

Descriptive histologic and magnetic resonance imaging study of human cadaveric spines.

OBJECTIVE

To identify and characterize common endplate pathologies to form a histologic foundation for an etiology-based classification system.

SUMMARY OF BACKGROUND DATA

Irregularities at the spinal disc-vertebra interface are associated with back pain and intervertebral disc herniation injuries. However, there is currently a lack of consensus regarding terminology for classification. This limits the potential for advancing understanding of back pain mechanisms, and prohibits meaningful comparisons for identifying priorities for prevention and treatment. Prior classification systems largely rely on observations from clinical imaging, which may miss subtle pathologic features.

METHODS

Fifteen cadaveric spines with moderate to severe disc degeneration were obtained and scanned with MRI in the sagittal plane using two-dimensional T1-weighted and T2-weighted fast spin-echo sequences. Eighty-nine lumbar and lower thoracic bone-disc-bone motion segments were extracted, fixed, sectioned, and stained for histologic evaluation. Focal endplate irregularities were identified and categorized based on features that inferred causation. The presence, type, and anatomic location were recorded. A classification system with three major categories of focal endplate irregularities was created.

RESULTS

Disc-vertebra avulsion and vertebral rim degeneration were more common than subchondral nodes: 50% of irregularities were classified as rim degeneration (75/150), 35% were classified as avulsions (52/150), and 15% were classified as nodes (23/150). Ninety percent of avulsions were subclassified as "tidemark avulsions," a highly prevalent form of endplate irregularity in which the outer annulus separates from the vertebra at the tidemark. These tidemark avulsions have not been previously described, yet are visible on T2-weighted MRI as high-intensity regions.

CONCLUSION

This study provides histologic basis for a system to classify focal endplate irregularities. Included is a previously unidentified but prevalent finding of tidemark avulsions, which are visible with both histology and magnetic resonance imaging. These observations will help clinicians better organize patients into meaningful groups to facilitate diagnosis, treatment, and clinical research.

LEVEL OF EVIDENCE

3.

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.

Intradiscal treatment of the cartilage endplate for improving solute transport and disc nutrition

Poor nutrient transport through the cartilage endplate (CEP) is a key factor in the etiology of intervertebral disc degeneration and may hinder the efficacy of biologic strategies for disc regeneration. Yet, there are currently no treatments for improving nutrient transport through the CEP. In this study we tested whether intradiscal delivery of a matrix-modifying enzyme to the CEP improves solute transport into whole human and bovine discs. Ten human lumbar motion segments harvested from five fresh cadaveric spines (38-66 years old) and nine bovine coccygeal motion segments harvested from three adult steers were treated intradiscally either with collagenase enzyme or control buffer that was loaded in alginate carrier. Motion segments were then incubated for 18 h at 37 °C, the bony endplates removed, and the isolated discs were compressed under static (0.2 MPa) and cyclic (0.4-0.8 MPa, 0.2 Hz) loads while submerged in fluorescein tracer solution (376 Da; 0.1 mg/ml). Fluorescein concentrations from site-matched nucleus pulposus (NP) samples were compared between discs. CEP samples from each disc were digested and assayed for sulfated glycosaminoglycan (sGAG) and collagen contents. Results showed that enzymatic treatment of the CEP dramatically enhanced small solute transport into the disc. Discs with enzyme-treated CEPs had up to 10.8-fold (human) and 14.0-fold (bovine) higher fluorescein concentration in the NP compared to site-matched locations in discs with buffer-treated CEPs (p < 0.0001). Increases in solute transport were consistent with the effects of enzymatic treatment on CEP composition, which included reductions in sGAG content of 33.5% (human) and 40% (bovine). Whole disc biomechanical behavior-namely, creep strain and disc modulus-was similar between discs with enzyme- and buffer-treated CEPs. Taken together, these findings demonstrate the potential for matrix modification of the CEP to improve the transport of small solutes into whole intact discs.

Advances and Prospects in Biomaterials for Intervertebral Disk Regeneration

Low-back and neck-shoulder pains caused by intervertebral disk degeneration are highly prevalent among middle-aged and elderly people globally. The main therapy method for intervertebral disk degeneration is surgical intervention, including interbody fusion, disk replacement, and diskectomy. However, the stress changes caused by traditional fusion surgery are prone to degeneration of adjacent segments, while non-fusion surgery has problems, such as ossification of artificial intervertebral disks. To overcome these drawbacks, biomaterials that could endogenously regenerate the intervertebral disk and restore the biomechanical function of the intervertebral disk is imperative. Intervertebral disk is a fibrocartilaginous tissue, primarily comprising nucleus pulposus and annulus fibrosus. Nucleus pulposus (NP) contains high water and proteoglycan, and its main function is absorbing compressive forces and dispersing loads from physical activities to other body parts. Annulus fibrosus (AF) is a multilamellar structure that encloses the NP, comprises water and collagen, and supports compressive and shear stress during complex motion. Therefore, different biomaterials and tissue engineering strategies are required for the functional recovery of NP and AF based on their structures and function. Recently, great progress has been achieved on biomaterials for NP and AF made of functional polymers, such as chitosan, collagen, polylactic acid, and polycaprolactone. However, scaffolds regenerating intervertebral disk remain unexplored. Hence, several tissue engineering strategies based on cell transplantation and growth factors have been extensively researched. In this review, we summarized the functional polymers and tissue engineering strategies of NP and AF to endogenously regenerate degenerative intervertebral disk. The perspective and challenges of tissue engineering strategies using functional polymers, cell transplantation, and growth factor for generating degenerative intervertebral disks were also discussed.

Impact of sufficient contact between the autograft and endplate soon after surgery to prevent nonunion at 12 months following posterior lumbar interbody fusion

OBJECTIVE

Nonunion after posterior lumbar interbody fusion (PLIF) is associated with poor long-term outcomes in terms of health-related quality of life. Biomechanical factors in the fusion segment may influence spinal fusion rates. There are no reports on the relationship between intervertebral union and the absorption of autografts or vertebral endplates. Therefore, the purpose of this retrospective study was to evaluate the risk factors of nonunion after PLIF and identify preventive measures.

METHODS

The authors analyzed 138 patients who underwent 1-level PLIF between 2016 and 2018 (75 males, 63 females; mean age 67 years; minimum follow-up period 12 months). Lumbar CT images obtained soon after the surgery and at 6 and 12 months of follow-up were examined for the mean total occupancy rate of the autograft, presence of a translucent zone between the autograft and endplate (more than 50% of vertebral diameter), cage subsidence, and screw loosening. Complete intervertebral union was defined as the presence of both upper and lower complete fusion in the center cage regions on coronal and sagittal CT slices at 12 months postoperatively. Patients were classified into either union or nonunion groups.

RESULTS

Complete union after PLIF was observed in 62 patients (45%), while nonunion was observed in 76 patients (55%). The mean total occupancy rate of the autograft immediately after the surgery was higher in the union group than in the nonunion group (59% vs 53%; p = 0.046). At 12 months postoperatively, the total occupancy rate of the autograft had decreased by 5.4% in the union group and by 11.9% in the nonunion group (p = 0.020). A translucent zone between the autograft and endplate immediately after the surgery was observed in 14 and 38 patients (23% and 50%) in the union and nonunion groups, respectively (p = 0.001). The nonunion group had a significantly higher proportion of cases with cage subsidence and screw loosening at 12 months postoperatively in comparison to the union group (p = 0.010 and p = 0.009, respectively).

CONCLUSIONS

A lower occupancy rate of the autograft and the presence of a translucent zone between the autograft and endplate immediately after the surgery were associated with nonunion at 12 months after PLIF. It may be important to achieve sufficient contact between the autograft and endplate intraoperatively for osseous union enhancement and to avoid excessive absorption of the autograft. The achievement of complete intervertebral union may decrease the incidence of cage subsidence or screw loosening.

Inhibition of Rac1 activity by NSC23766 prevents cartilage endplate degeneration via Wnt/β-catenin pathway

Cartilage endplate (CEP) degeneration has been considered as one of important factors related to intervertebral disc degeneration (IVDD). Previous researches have showed that Rac1 played a pivotal role in chondrocyte differentiation. However, the effect of Rac1 during the process of CEP degeneration remains unclear. Herein, we explored the effect of Rac1 on CEP degeneration and elucidated the underlying molecular mechanism. We found expression of Rac1‐GTP increased in human‐degenerated CEP tissue and IL‐1β‐stimulated rat endplate chondrocytes (EPCs). Our study revealed that Rac1 inhibitor NSC23766 treatment promoted the expression of collagen II, aggrecan and Sox‐9, and decreased the expression of ADTAMTS5 and MMP13 in IL‐1β‐stimulated rat EPCs. Moreover, we also found that NSC23766 could suppress the activation of Wnt/β‐catenin pathway, suggesting that the beneficial effects of Rac1 inhibition in EPCs are mediated through the Wnt/β‐catenin signalling. Besides, puncture‐induced rats models showed that NSC23766 played a protective role on CEP and disc degeneration. Collectively, these findings demonstrated that Rac1 inhibition delayed the EPCs degeneration and its potential mechanism may be associated with Wnt/β‐catenin pathway regulation, which may help us better understand the association between Rac1 and CEP degeneration and provide a promising strategy for delaying the progression of IVDD.

Degenerative Nucleus Pulposus Cells Derived Exosomes Promoted Cartilage Endplate Cells Apoptosis and Aggravated Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration is a complex multifactorial disease model, which pathogenesis has not been fully defined. There are few studies on the information interaction between nucleus pulposus (NP) cells and cartilage endplate (CEP) cells. Exosomes, as a carrier of information communication between cells, have become a research hotspot recently. The purpose of this study was to explore whether degenerative NP cells-derived exosomes promoted CEP cells apoptosis and aggravated IVD degeneration. The degenerative NP cells model was induced by TNFα. NPC exosomes were isolated from the supernatant of the NP cell culture medium. The viability of NP cells and CEP cells was examined by CCK-8 assays. The exosomes were identified by TEM, NTA, and western blot. Extracellular matrix (ECM) metabolism was measured by cellular immunofluorescence and qRT-PCR. Apoptosis was detected by flow cytometry and TUNEL. X-ray and magnetic resonance imaging (MRI), as well as hematoxylin-eosin (H&E), Safranine O-Green staining was adopted to evaluate IVD degeneration grades. TNFα had a minor impact on NPC viability but inhibited ECM synthesis and promoted ECM degradation. TNFα-NPC-Exo had less effect on CEPC proliferation but promoted CEPC apoptosis and affect ECM metabolism, inhibiting aggrecan and collagen II expression and enhancing MMP-3 expression. TNFα-NPC-Exo aggravates IVD degeneration in a rat model and promoted CEPC apoptosis. In conclusion, this study demonstrated that degenerated NPC-exosome could induce apoptosis of CEPCs, inhibit ECM synthesis, and promote ECM degradation. In addition, it was proved that degenerated NPC-exosome aggravates IVD degeneration.

PTEN inhibitor VO-OHpic protects endplate chondrocytes against apoptosis and calcification via activating Nrf-2 signaling pathway

Cartilage endplate (CEP) degeneration and calcification is an important contributor to the onset and pathogenesis of intervertebral disc degeneration (IDD). However, the underlying mechanisms of CEP degeneration remain elusive, let alone according treatment strategies to prevent CEP degeneration. Phosphatase and tensin homolog (PTEN) is a tumor suppressor gene that promotes cell apoptosis, and recent studies indicated that PTEN is overexpressed in degenerated intervertebral disc. However, whether direct inhibition of PTEN attenuates CEP degeneration and IDD development remains largely unknown. In the present study, our in vivo experiments demonstrated that VO-OHpic could attenuate IDD progression and CEP calcification. We also found that VO-OHpic inhibited oxidative stress induced chondrocytes apoptosis and degeneration by activating Nrf-2/HO-1 pathway, thus promoted parkin mediated mitophagy process and inhibited chondrocytes ferroptosis, alleviated redox imbalance and eventually improved cell survival. Nrf-2 siRNA transfection significantly reversed the protective effect of VO-OHpic on endplate chondrocytes. In conclusion, our study demonstrated that inhibition of PTEN with VO-OHpic attenuates CEP calcification and IDD progression. Moreover, VO-OHpic protects endplate chondrocytes against apoptosis and degeneration via activating Nrf-2/HO-1 mediated mitophagy process and ferroptosis inhibition. Our results suggest that VO-OHpic may be a potential effective medicine for IDD prevention and 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.

Investigating the role of culture conditions on hypertrophic differentiation in human cartilage endplate cells

Cartilage endplate degeneration/calcification has been linked to the onset and progression of intervertebral disc degeneration and there is a critical need to understand mechanisms, such as hypertrophic differentiation, of cartilage endplate degeneration/calcification to inform treatment strategies for discogenic back pain. In vitro cell culture conditions capable of inducing hypertrophic differentiation are used to study pathophysiological mechanisms in articular chondrocytes, but culture conditions capable of inducing a hypertrophic cartilage endplate cell phenotype have yet to be explored. The goal of this study was to investigate the role of culture conditions capable of inducing hypertrophic differentiation in articular chondrocytes on hypertrophic differentiation in human cartilage endplate cells. Isolated human cartilage endplate cells were cultured as pellets for 21 days at either 5% O₂ (physiologic for cartilage) or 20.7% O₂ (hyperoxic) and treated with 10% fetal bovine serum or Wnt agonist, two stimuli used to induce hypertrophic differentiation in articular chondrocytes. Cartilage endplate cells did not exhibit a hypertrophic cell morphology in response to fetal bovine serum or Wnt agonist but did display other hallmarks of chondrocyte hypertrophy and degeneration such as hypertrophic gene and protein expression, and a decrease in healthy proteoglycans and an increase in fibrous collagen accumulation. These findings demonstrate that cartilage endplate cells take on a degenerative phenotype in response to hypertrophic stimuli in vitro, but do not undergo classical changes in morphology associated with hypertrophic differentiation regardless of oxygen levels, highlighting potential differences in the response of cartilage endplate cells versus articular chondrocytes to the same stimuli.

Extracellular Vesicles Derived From Stem Cells in Intervertebral Disc Degeneration

Intervertebral disc degeneration (IVDD) is the leading cause of low back pain related to degradation of cartilaginous tissues, mainly resulting from oxidative stress, cell apoptosis, and extracellular matrix degradation. Extracellular vesicles (EVs) exist in all bodily fluids and can be produced by all types of cells. Stem cell-derived EVs (SC-EVs), which are the main paracrine components of stem cells, have gained significant attention in the field of regenerative medicine. Over the past years, accumulating evidence indicates the therapeutic and diagnostic potentials of EVs in IVDD. The main mechanisms involve the induction of regenerative phenotypes, apoptosis alleviation, and immune modulation. In addition, the efficiency of SC-EVs can be enhanced by choosing appropriate donor cells and cell phenotypes, optimizing cell culture conditions, or engineering EVs to deliver drugs and targeting molecules. Given the importance and novelty of SC-EVs, we give an overview of SC-EVs and discuss the roles of SC-EVs in IVDD.

Role of non‑coding RNAs in cartilage endplate (Review)

Cartilage endplate (CEP) degeneration is considered one of the major causes of intervertebral disc degeneration (IDD), which causes non-specific neck and lower back pain. In addition, several non-coding RNAs (ncRNAs), including long ncRNAs, microRNAs and circular RNAs have been shown to be involved in the regulation of various diseases. However, the particular role of ncRNAs in CEP remains unclear. Identifying these ncRNAs and their interactions may prove to be is useful for the understanding of CEP health and disease. These RNA molecules regulate signaling pathways and biological processes that are critical for a healthy CEP. When dysregulated, they can contribute to the development disease. Herein, studies related to ncRNAs interactions and regulatory functions in CEP are reviewed. In addition, a summary of the current knowledge regarding the deregulation of ncRNAs in IDD in relation to their actions on CEP cell functions, including cell proliferation, apoptosis and extracellular matrix synthesis/degradation is presented. The present review provides novel insight into the pathogenesis of IDD and may shed light on future therapeutic approaches.

[Effect of resveratrol on high mobility group box-1 protein signaling pathway in cartilage endplate degeneration caused by inflammation]

Objective

To investigate the effect of resveratrol (RES) on inflammation-induced cartilage endplate (CEP) degeneration, and its regulatory mechanism on high mobility group box-1 protein (HMGB1) signaling pathway.

Methods

The intervertebral CEP cells of Sprague Dawley (SD) rats aged 3 weeks were extracted and identified by toluidine blue staining and immunofluorescence staining of rabbit anti-rat collagen type Ⅱ. The cell counting kit 8 (CCK-8) method was used to screen the optimal concentration of RES on intervertebral CEP cells. Gene chip analysis was used to determine the target of RES on intervertebral CEP cells. Interleukin 1β (IL-1β) was used to construct the intervertebral CEP cell degeneration model caused by inflammation and the 7-8-week-old SD rat intervertebral disc degeneration model, and pcDNA3.1-HMGB1 (pcDNA3.1) was used as the control of RES effect. Flow cytometry and TUNEL staining were used to detect the apoptotic rate of intervertebral CEP cells and rat intervertebral disc tissue cells, respectively. ELISA kit was used to detect the content of interleukin 10 (IL-10) and tumor necrosis factor α (TNF-α) in the cell supernatant and rat serum. Western blot was used to detect the expressions of HMGB1, extracellular signal-regulated protein kinase (ERK), phosphorylated ERK (p-ERK), B cell lymphoma/leukemia 2 gene (Bcl-2), and Bcl-2-associated X protein (Bax).

Results

The extracted cells were identified as rat intervertebral CEP cells. CCK-8 method screened out the highest activity of intervertebral CEP cells treated with 30 μmol/L RES. The gene chip analysis confirmed that the HMGB1-ERK signal was the target of RES. Both cell experiments and animal experiments showed that RES treatment can significantly down-regulate the apoptosis rate of intervertebral CEP cells, inhibit the release of TNF-α, and increase the content of IL-10; and down-regulate the expressions of HMGB1, p-ERK, and Bax, and increase Bcl-2; and pcDNA3.1 could partially reverse these effects of RES, and the differences were all significant (P<0.05).

Conclusion

RES can significantly inhibit the apoptosis of intervertebral CEP cells induced by inflammation, which is related to inhibiting the expression of HMGB1.