Electron Microscopic Insights into Degenerating Lumbar Intervertebral Disc: A Prospective, Cohort Study

Abstract

Objective

Degenerative disc disease is a leading cause of chronic low back pain, yet radiological and histological assessments often fail to explain symptom severity. Existing studies rarely incorporate electron microscopy (EM), leaving a gap in knowledge regarding cellular and extracellular matrix (ECM) alterations at the ultrastructural level and their relationship with clinical outcomes. This study aimed to investigate ultrastructural changes in lumbar intervertebral disc tissue using EM and to correlate these findings with clinical symptoms and radiological features.

Materials and Methods

A prospective observational study was conducted in 50 patients undergoing lumbar discectomy. Disc tissue was analyzed using EM. Clinical parameters (visual analogue scale [VAS] score, symptom duration) and MRI features (Pfirrmann grading, Modic changes) were recorded and statistically correlated with EM findings.

Results

Chondrocyte clustering (88%) and intracellular inclusion bodies (56%) were the most common EM findings. Both were significantly associated with higher VAS scores (p = 0.02 and p = 0.009, respectively) and longer symptom duration. Chondrocyte clustering and inclusions increased with higher Pfirrmann grades (p = 0.015 and p = 0.022, respectively). No significant association was found between Modic changes and the study outcomes.

Conclusion

EM revealed critical ultrastructural changes in degenerating lumbar discs, with chondrocyte clustering and inclusions emerging as potential morphological markers of disease severity and chronicity.

Keywords

intervertebral disc degeneration - chondrocytes - discectomy - electron microscopy - low back pain

Introduction

Intervertebral disc degeneration is one of the most common causes of chronic low back pain and radiculopathy, affecting millions worldwide and posing a significant socioeconomic burden.[1] [2] The intervertebral disc, particularly the nucleus pulposus and annulus fibrosus, undergoes progressive biochemical and structural changes with age, mechanical stress, and injury.[3] While radiological imaging modalities, such as MRI and CT scans, have enabled noninvasive evaluation of disc degeneration, they provide only macroscopic information. Although essential for diagnosis and surgical planning, these imaging techniques often lack the resolution to assess ultrastructural changes occurring at the cellular and subcellular levels within the disc tissue.[4] [5]

Electron microscopy (EM) provides unparalleled resolution in analyzing biological tissues, allowing for the visualization of ultrastructural alterations that remain undetectable through conventional light microscopy or radiographic imaging.[6] The application of EM in evaluating intervertebral disc pathology remains relatively underexplored. Previous studies have primarily focused on histological grading and gross degenerative changes, with limited emphasis on the ultrastructural characteristics of disc material in correlation with clinical presentation.[7] Specifically, the association between electron microscopic changes in disc tissue and the duration and severity of patient symptoms has not been comprehensively investigated.

Degenerative disc disease (DDD) exhibits a spectrum of pathological alterations, ranging from early proteoglycan loss and disorganization of collagen fibers to advanced calcification, neovascularization, and cellular apoptosis.[8] [9] These microscopic changes likely precede and contribute to the macroscopic radiological findings observed in advanced disc degeneration. However, the progression and clinical significance of these changes remain poorly understood, particularly concerning symptomatology. Furthermore, variability in patient-reported symptoms, ranging from mild discomfort to debilitating pain, despite similar imaging findings, suggests that factors beyond radiological evidence may influence the clinical course of DDD.[10]

In an effort to bridge this gap in knowledge, the present study was designed to investigate the ultrastructural characteristics of intervertebral disc material obtained from patients undergoing discectomy. By employing EM, we aimed to evaluate the disc's microarchitectural features, including collagen fiber arrangement, cellular morphology, presence of degenerative debris, and other subcellular anomalies. We further sought to correlate these findings with the duration and severity of clinical symptoms and radiological grading of disc degeneration. This is one of the first studies attempting to establish a direct association between EM-based disc changes and clinical parameters in patients with lumbar disc herniation. Understanding these associations may enhance our comprehension of disc degeneration at a molecular level and offer potential insights into personalized treatment approaches based on the underlying pathology. Through this study, we hope to contribute to the growing body of knowledge in spinal pathology and advocate for integrating ultrastructural analysis into routine disc evaluation, where feasible, to improve diagnostic accuracy and therapeutic outcomes.

Materials and Methods

This prospective, observational study was conducted in the Department of Neurosurgery of a tertiary care center between January 2020 and December 2021. The primary aim was to analyze ultrastructural changes in excised lumbar intervertebral disc tissue using EM and correlate these findings with clinical symptoms and MRI-based radiological features in patients with lumbar disc disease.

Study design and ethical considerations: the study protocol was reviewed and approved by the Institutional Ethics Committee, and written informed consent was obtained from all participants. All procedures were conducted in accordance with the ethical standards of the Committee on Human Experimentation and conformed to the principles of the Declaration of Helsinki. No personal identifiers were used in the documentation or analysis of clinical data.

Subject selection: fifty consecutive patients presenting with classical signs and symptoms of lumbar disc herniation, confirmed on MRI, and scheduled for surgical intervention (discectomy) were included. These patients formed the study group. No control group was included due to ethical limitations associated with disc sampling from asymptomatic individuals.

Inclusion criteria for the study were: adult patients between 18 and 65 years of age with a clinical diagnosis of lumbar disc disease, presenting with low back pain with or without radiculopathy, who demonstrated MRI-confirmed lumbar disc herniation with degenerative changes, and were planned for surgical management via discoidectomy. Exclusion criteria consisted of patients with a history of previous spinal surgery at the affected disc level, those with infectious disc pathology such as tubercular discitis, known malignancy, or systemic diseases that could affect connective tissue structure, including rheumatoid arthritis and systemic lupus erythematosus. Patients with diabetes mellitus and those with a history of chronic smoking exceeding 10 pack-years were also excluded from the study.

Baseline demographic data, including age, sex, weight, and occupation, were recorded at the time of admission.

Clinical and radiological evaluation: a detailed clinical history was obtained, focusing on the duration and severity of symptoms. Pain severity was quantified using the visual analogue scale (VAS). All patients underwent lumbar spine MRI, and degeneration severity was assessed using the Pfirrmann grading system and Modic classification.[11] [12] ([Tables 1] and [2]). MRI evaluations were performed by two independent radiologists blinded to clinical and operative details to minimize assessment bias.

Table 1 Pfirrmann grading system for lumbar intervertebral disc degeneration based on MRI characteristics, including disc structure, nucleus-annulus distinction, signal intensity, and disc height

Grade	Structure	Distinction of nucleus and annulus	Signal Intensity	Height of disk
I	Homogeneous, bright white	Clear	Hyperintense, isointense to cerebrospinal fluid	Normal
II	Inhomogeneous, with or without horizontal bands	Clear	Hyperintense, isointense to cerebrospinal fluid	Normal
III	Inhomogeneous, gray	Unclear	Intermediate	Normal to slightly decreased
IV	Inhomogeneous, gray to black	Lost	Intermediate to hypointense	Normal to moderately decreased
V	Inhomogeneous, black	Lost	Hypointense	Collapsed disk space

Table 2 Modic classification of vertebral endplate changes on MRI, describing signal intensity patterns on T1- and T2-weighted images associated with different types of degenerative changes

Type I Modic change	Hypo-intense signal intensity of end-plates on T1-weighted image (T1WI) and hyper-intense signal on T2WI
Type II Modic change	Hyper-intense signal intensity on both T1WI and T2WI
Type III Modic change	Hypo-intense signal intensity on both T1WI and T2WI

Surgical technique and tissue handling: all patients underwent standard or minimally invasive lumbar discectomy under general anesthesia. The surgical approach was determined based on the location of the disc herniation and the surgeon's preference.

Immediately after excision, disc specimens were placed in Karnovsky's fixative, comprising 2.5% glutaraldehyde and 2% formaldehyde in 0.1 M sodium cacodylate buffer (pH 7.4). Specimens were stored at 4°C for up to 7 days due to logistical constraints before being processed. Secondary fixation was performed using 1% osmium tetroxide, followed by dehydration in graded ethanol and embedding in epoxy resin. Ultrathin sections were examined under a transmission electron microscope.

Data analysis and statistical methods: data were analyzed using SPSS version 25.0. Continuous variables (e.g., age, VAS scores) were expressed as mean ± standard deviation, while categorical variables were presented as frequencies and percentages. Depending on data normality, correlations between EM findings and clinical/radiological features were analyzed using Pearson or Spearman correlation coefficients. As appropriate, group comparisons were performed using student's t-test or the Mann–Whitney U test. Statistical significance was set at p < 0.05.

Results

This prospective study was conducted from January 2020 to December 2021 and included 50 patients (28 males, 22 females) presenting with lumbar disc disease. The majority of patients (31; 62%) were aged between 40 and 60 years, with 12 (24%) younger than 40 years and 7 (14%) older than 60 years. The most frequently affected disc level was L4–L5 in 26 patients (52%), followed by L5–S1 in 14 (28%) and L3–L4 in 10 (20%).

The median duration of symptoms was 12 months (range, 1–30 months). Pain severity, assessed by the VAS, revealed that 20 patients (40%) reported a score of 7 and another 20 (40%) reported a score of 8. Four patients (8%) each had scores of 6 and 9, while 2 patients 23 (4%) reported the maximum score of 10. Surgical intervention was performed exclusively in patients with VAS scores of 6 or higher.

MRI evaluation demonstrated that 31 patients (62%) had Pfirrmann grade 4 disc degeneration, 13 patients (26%) had grade 5, and 6 patients (12%) had grade 3. No patients exhibited 1 or 2 changes. Regarding Modic changes, 12 (24%) had type 1, 32 (64%) type 2, and 6 (12%) type 3 changes.

Electron microscopic findings: EM revealed consistent ultrastructural abnormalities in disc tissue:

• Chondrocyte clustering (defined as the grouping of two or more chondrocytes) was present in 44 patients (88%). ([Fig. 1])

• Intracellular inclusion bodies, indicating protein aggregates, were observed in 28 patients (56%). ([Fig. 2])

• Collagen abnormalities, characterized by irregular collagen bundles surrounding chondrocytes, were noted in 33 patients (66%).

• Chondron formation, characterized by clusters of two fused chondrocytes, was identified in 10 patients (20%). ([Fig. 3])

Fig. 1 Electron micrograph showing a cluster of three chondrocytes within the nucleus pulposus. An intracellular inclusion body is visible within one of the cells (indicated by the red arrow), suggesting progressive cellular degeneration. Surrounding the chondrocytes is an encircling extracellular matrix (blue arrow), indicative of ongoing matrix remodeling in response to degenerative changes.

Fig. 2 High-resolution electron micrograph of a degenerating chondrocyte containing prominent intracellular inclusion bodies (red arrow). The cell is enveloped by a dense and irregular pericellular matrix, reflecting cellular stress and altered matrix synthesis typical of advanced disc degeneration.

Fig. 3 Electron micrograph illustrating a chondron—two chondrocytes clustered within a shared pericellular capsule. This structural arrangement may represent a reparative adaptation or a degenerative phenotype, commonly observed in discs with osteophyte formation or chronic degenerative changes.

These EM features showed a positive correlation with pain severity and symptom duration ([Tables 3] and [4]).

Table 3 Correlation between electron microscopic findings in lumbar disc tissue and pain severity (VAS score)

Electron microscopic findings		Pain severity (VAS score)					p-Value	Test applied
		6	7	8	9	10
Chondrocyte clustering	Absent	2	3	1	0	0	0.02	Chi-square for trend
	Present	2	17	19	4	2
Inclusions	Absent	3	12	6	1	0	0.009
	Present	1	8	14	3	2
ECM and collagen abnormality	Absent	2	6	4	3	2	0.209
	Present	2	14	16	1	0

Abbreviations: ECM, extracellular matrix; VAS, visual analogue scale.

Note: The table shows the distribution of chondrocyte clustering, intracellular inclusions, and ECM/collagen abnormalities across VAS scores, along with corresponding p-values and statistical tests used.

Table 4 Comparison of symptom duration (in months) with the presence or absence of specific electron microscopic features in lumbar disc tissue

Duration (mo)	Electron microscopic feature		p-Value	Test performed
	Absent	Present
Chondrocyte clustering
Number	6	44	0.010	Mann–Whitney
Mean ± SD	6.3 ± 2.4	13.0 ± 5.9
Median	6	14
IQR	2.0	7.0
Range	3–10	5–28
Inclusions
Number	22	28	0.018	Mann–Whitney
Mean ± SD	8.5 ± 3.2	14.2 ± 6.3
Median	8	15
IQR	4.0	6.0
Range	4–16	6–30
ECM and collagen abnormalities
Number	17	33	0.075	Mann–Whitney
Mean ± SD	10.5 ± 5.8	13.0 ± 6.4
Median	10	12
IQR	4.0	5.5
Range	5–22	6–28

Abbreviations: ECM, extracellular matrix; IQR, interquartile range; SD, standard deviation.

Note: Data are presented as mean ± SD, median, interquartile range (IQR), and range. Statistical significance was assessed using the Mann–Whitney test.

Correlation with Pain Severity (VAS Score)

Chondrocyte clustering was significantly associated with higher VAS scores (p = 0.02, chi-square for trend). Inclusion bodies also correlated significantly with pain severity (p = 0.009). extracellular matrix (ECM) and collagen abnormalities showed no significant association with pain (p = 0.209).

Correlation with Symptom Duration

Patients with chondrocyte clustering had a median symptom duration of 14 months 29 (interquartile range [IQR]: 5–28), compared with 6 months (IQR: 3–10) in those without clustering (p = 0.010). Inclusion bodies appeared in patients with a median duration of 15 months (IQR: 6–30) versus 8 months (IQR: 4–16) when absent (p = 0.018). ECM abnormalities showed a trend toward longer duration but were not statistically significant (p = 0.075).

Correlation with MRI Findings

The prevalence of chondrocyte clustering increased with Pfirrmann grade: five out of six patients in grade 3, 28 out of 31 in grade 4, and 11 out of 13 in grade 5 (p = 0.015, chi-square for trend). Inclusion bodies were more common in higher grades (p = 0.022). ECM abnormalities were higher in severe degeneration but did not reach statistical significance (p = 0.09; [Table 5]). No statistically significant associations were found between EM findings and Modic change types (all p > 0.05; [Table 6]).

Table 5 Association between symptom duration and presence of electron microscopic features in lumbar disc specimens

Duration (mo)	Electron microscopic feature		p-Value	Test performed
	Absent	Present
Chondrocyte clustering
Number	6	44	0.010	Mann–Whitney
Mean ± SD	6.3 ± 2.4	13.0 ± 5.9
Median	6	14
IQR	2.0	7.0
Range	3–10	5–28
Inclusions
Number	22	28	0.018	Mann–Whitney
Mean ± SD	8.5 ± 3.2	14.2 ± 6.3
Median	8	15
IQR	4.0	6.0
Range	4–16	6–30
ECM and collagen abnormalities
Number	17	33	0.075	Mann–Whitney
Mean ± SD	10.5 ± 5.8	13.0 ± 6.4
Median	10	12
IQR	4.0	5.5
Range	5–22	6–28

Note: Duration is compared between patients with and without chondrocyte clustering, inclusion bodies, and ECM/collagen abnormalities. Results include mean ± SD, median, interquartile range (IQR), and range. Statistical analysis was performed using the Mann–Whitney U test.

Table 6 Correlation between electron microscopic features and Modic type changes on MRI

Electron microscopic feature		Modic type			p-Value	Test applied
		Type 1	Type 2	Type 3
Chondrocyte clustering	Absent	1	3	2	0.229	Chi-square for trend
	Present	11	29	4
	Odds ratio	1	0.88	0.18
Inclusions	Absent	3	10	9	0.19
	Present	6	15	7
	Odds ratio	1	0.75	0.26
ECM and collagen abnormality	Absent	4	7	6	0.12
	Present	5	18	10
	Odds ratio	1	0.31	0.33

Note: The table presents the distribution of chondrocyte clustering, inclusion bodies, and ECM/collagen abnormalities across Modic types 1, 2, and 3, along with corresponding p-values, odds ratios, and the statistical test applied (chi-square for trend).

Temporal Progression

Analysis suggests a temporal progression of degenerative ultrastructural changes detectable by EM: collagen abnormalities appear earliest (median duration: approximately 12 months), followed by chondrocyte clustering (approximately 14 months), and then appearance of inclusion bodies (approximately 15 months).

Osteophyte formation: osteophytes were identified in 10 patients (20%): six had osteophytes in both the anterior and posterior regions, and 4 had osteophytes only in the posterior region. All these patients exhibited chondrocyte clustering and chondron formation near osteophytes, indicating a possible role for chondrocytes in osteophyte development.

Discussion

This study offers ultrastructural insight into lumbar disc degeneration by correlating electron microscopic findings with clinical and imaging parameters. Notably, chondrocyte emerged as a predominant feature, observed in 88% of specimens, and was significantly associated with higher pain severity (VAS score) and longer symptom duration. This suggests that chondrocyte clustering may serve as a morphological marker of disease progression.

Chondrocyte clustering has been reported as a hallmark of intervertebral disc degeneration may indicate a proliferative response to mechanical stress or matrix breakdown products. Previous studies have described these clusters as a result of clonal expansion of chondrocytes in degenerated human discs, potentially triggered by biomechanical or inflammatory stimuli.[13] [14] The presence of clustering in a high percentage of our samples raises several considerations: (1) is clustering an early event in degeneration, (2) does it represent a failed repair mechanism, or (3) could clustered chondrocytes represent a progenitor pool lineage differentiation, such as osteogenic transformation near osteophytes?

Inclusion bodies, seen in 56% of cases, also correlated with pain and duration, often in specimens with more severe degeneration. These granular inclusions may consist of matrix degradation products and calcium phosphate deposits, which form due to impaired molecular diffusion in the dense avascular disc matrix.[5] Their presence might reflect chronicity or a failed attempt at cellular homeostasis.

ECM abnormalities, observed in 66% of patients, manifested as irregular collagen bundles surrounding chondrocytes. These were associated with symptom duration and trended with pain severity. The ECM is vital for maintaining the disc's biomechanical properties, and its disruption can lead to a reduction in tensile strength and elasticity. Changes in ECM composition during degeneration include increased collagen types I, III, VI, and X, as well as elastin deposition and proteoglycan fragmentation.[15] [16] These alterations may initiate or perpetuate inflammatory cascades, leading to progressive disc dysfunction. Prior EM studies have described similar changes in collagen organization, including encircling ECM layers and abnormal fibril formation.[17] [18] Such disorganization can disrupt the annulus fibrosus' lamellar continuity, impairing its mechanical integrity.

Osteophyte formation, observed in 20% of patients, was always accompanied by chondrocyte clustering and chondron formation at the osteophyte interface. This could imply a reparative or adaptive response, possibly mediated by chondrocyte transdifferentiation into osteoblast-like cells, a phenomenon reported in osteoarthritis and suggested in spinal degeneration.[19] [20] This observation, although preliminary, invites further molecular investigation into models. the signaling pathways governing such transitions.

Despite the novel insights, the study has limitations. The COVID-19 pandemic and logistical challenges limited the sample size to 50. Delays between tissue procurement and EM analysis may have impacted ultrastructural integrity. Moreover, the absence of a standardized grading system for EM findings in disc tissue limits broader comparisons. Lastly, the lack of a control group of healthy disc tissue prevents definitive temporal staging of the observed changes. Nevertheless, this study reinforces EM's utility in identifying ultrastructural changes in lumbar discs, surpassing the resolution of light microscopy in distinguishing viable versus necrotic chondrocytes.[14] Given the growing burden of lumbar degenerative disease, with global prevalence projected to rise in tandem with aging populations,[21] [22] the need for ultrastructural biomarkers becomes increasingly relevant. This study supports the hypothesis that degeneration is a progressive, multifactorial process with distinct morphological stages observable at the cellular level.

EM can be a powerful adjunct in understanding intervertebral disc degeneration. Chondrocyte clustering appears to be a pivotal early event, strongly associated with clinical severity and radiological grades. Longer symptom duration is linked with more pronounced ultrastructural changes, supporting a temporal evolution of degeneration. ECM abnormalities and granular inclusions further corroborate chronic matrix degradation and cellular stress. These findings prompt new hypotheses about chondrocyte behavior in degeneration, including potential roles in osteophyte formation and lineage plasticity. Future research should explore the molecular regulators of chondrocyte transdifferentiation and assess whether these pathways could be targeted therapeutically to halt or reverse disc degeneration. Larger, longitudinal studies with better-defined EM scoring systems are needed to validate these findings and enhance diagnostic precision in spinal disorders.

Conflict of Interest

None declared.

Authors' Contributions

P.L. contributed to conceptualization, methodology, formal analysis, and writing—original draft, review, and editing. C.K. contributed to formal analysis and writing—original draft, review, and editing. A.K.B.C., N.G., and H.S.: data curation and writing—review and editing. V.B. contributed to formal analysis, data curation, and writing—review and editing. D.S. contributed to conceptualization, supervision, and writing—review and editing.

Ethical Approval

The study has received approval from the Institutional Ethics Committee of Maulana Azad Medical College (MAMC), New Delhi, and adheres to the principles outlined in the Declaration of Helsinki.

• References

• 1 GBD 2015 Disease and Injury Incidence and Prevalence Collaborators. Global, regional, and national incidence, prevalence, and years lived with disability for 310 diseases and injuries, 1990–2015: a systematic analysis for the Global Burden of Disease Study 2015. Lancet 2017; 389 (10064): e1
• 2 Balagué F, Mannion AF, Pellisé F, Cedraschi C. Non-specific low back pain. Lancet 2012; 379 (9814) 482-491
• 3 Adams MA, Roughley PJ. What is intervertebral disc degeneration, and what causes it?. Spine 2006; 31 (18) 2151-2161
• 4 Modic MT, Ross JS. Lumbar degenerative disk disease. Radiology 2007; 245 (01) 43-61
• 5 Urban JP, Roberts S. Degeneration of the intervertebral disc. Arthritis Res Ther 2003; 5 (03) 120-130
• 6 Trout JJ, Buckwalter JA, Moore KC, Landas SK. Ultrastructure of the human intervertebral disc. I. Changes in notochordal cells with age. Tissue Cell 1982; 14 (02) 359-369
• 7 Boos N, Weissbach S, Rohrbach H, Weiler C, Spratt KF, Nerlich AG. Classification of age-related changes in lumbar intervertebral discs: 2002 Volvo Award in basic science. Spine 2002; 27 (23) 2631-2644
• 8 Buckwalter JA. Aging and degeneration of the human intervertebral disc. Spine 1995; 20 (11) 1307-1314
• 9 Boos N, Nerlich AG, Wiest I, von der Mark K, Aebi M. Immunolocalization of type X collagen in human lumbar intervertebral discs during ageing and degeneration. Histochem Cell Biol 1997; 108 (06) 471-480
• 10 Boden SD, Davis DO, Dina TS, Patronas NJ, Wiesel SW. Abnormal magnetic-resonance scans of the lumbar spine in asymptomatic subjects. A prospective investigation. J Bone Joint Surg Am 1990; 72 (03) 403-408
• 11 Pfirrmann CW, Metzdorf A, Zanetti M, Hodler J, Boos N. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine 2001; 26 (17) 1873-1878
• 12 Viswanathan VK, Shetty AP, Rajasekaran S. Modic changes - an evidence-based, narrative review on its patho-physiology, clinical significance and role in chronic back pain. J Clin Orthop Trauma 2020; 11 (06) 1169-1171
• 13 Pytel P, Wollmann RL, Fessler RG, Krausz TN, Montag AG. Degenerative spine disease: pathologic findings in 985 surgical specimens. Am J Clin Pathol 2006; 125 (02) 193-202
• 14 Johnson WE, Eisenstein SM, Roberts S. Cell cluster formation in degenerate lumbar intervertebral discs is associated with increased disc cell proliferation. Connect Tissue Res 2001; 42 (03) 197-207
• 15 Iatridis JC, Michalek AJ, Purmessur D, Korecki CL. Localized intervertebral disc injury leads to organ level changes in structure, cellularity, and biosynthesis. Cell Mol Bioeng 2009; 2 (03) 437-447
• 16 Brinjikji W, Diehn FE, Jarvik JG. et al. MRI findings of disc degeneration are more prevalent in adults with low back pain than in asymptomatic controls: a systematic review and meta-analysis. AJNR Am J Neuroradiol 2015; 36 (12) 2394-2399
• 17 Gruber HE, Hanley Jr EN. Analysis of aging and degeneration of the human intervertebral disc. Comparison of surgical specimens with normal controls. Spine 1998; 23 (07) 751-757
• 18 Antoniou J, Steffen T, Nelson F. et al. The human lumbar intervertebral disc: evidence for changes in the biosynthesis and denaturation of the extracellular matrix with growth, maturation, ageing, and degeneration. J Clin Invest 1996; 98 (04) 996-1003
• 19 Tang Y, Wu X, Lei W. et al. TGF-beta1-induced migration of bone mesenchymal stem cells couples bone resorption with formation. Nat Med 2009; 15 (07) 757-765
• 20 Zhou X, von der Mark K, Henry S, Norton W, Adams H, de Crombrugghe B. Chondrocytes transdifferentiate into osteoblasts in endochondral bone during development, postnatal growth and fracture healing in mice. PLoS Genet 2014; 10 (12) e1004820
• 21 United Nations. World Population Ageing 2019: Highlights. Department of Economic and Social Affairs, Population Division (2019).
• 22 Wu A, March L, Zheng X. et al. Global low back pain prevalence and years lived with disability from 1990 to 2017: estimates from the Global Burden of Disease Study 2017. Ann Transl Med 2020; 8 (06) 299

Publication History

Article published online:
27 February 2026