Dependence of crack shape in loaded articular cartilage on the collagenous structure

Collagen integrity of the annulus fibrosus in degenerative disc disease individuals quantified with collagen hybridizing peptide

Introduction

Degenerative disc disease (DDD) is accompanied by structural changes in the intervertebral discs (IVD). Extra-cellular matrix degradation of the annulus fibrosus (AF) has been linked with degeneration of the IVD. Collagen is a vital component of the IVD. Collagen hybridizing peptide (CHP) is an engineered protein that binds to degraded collagen, which we used to quantify collagen damage in AF. This method was used to compare AF samples obtained from donors with no DDD to AF samples from patients undergoing surgery for symptomatic DDD.

Methods

Fresh AF tissue was embedded in an optimal cutting temperature compound and cryosectioned at a thickness of 8 μm. Hematoxylin and Eosin staining was performed on sections for general histomorphological assessment. Serial sections were stained with Cy3-conjugated CHP and the mean fluorescence intensity and areal fraction of Cy3-positive staining were averaged for three regions of interest (ROI) on each CHP-stained section.

Results

Increases in mean fluorescence intensity (p = 0.0004) and percentage of positively stained area (p = 0.00008) with CHP were detected in DDD samples compared to the non-DDD samples. Significant correlations were observed between mean fluorescence intensity and percentage of positively stained area for both non-DDD (R = 0.98, p = 5E-8) and DDD (R = 0.79, p = 0.0012) samples. No significant differences were detected between sex and the lumbar disc level subgroups of the non-DDD and DDD groups. Only tissue pathology (non-DDD versus DDD) influenced the measured parameters. No three-way interactions between tissue pathology, sex, and lumbar disc level were observed.

Discussion and Conclusions

These findings suggest that AF collagen degradation is greater in DDD samples compared to non-DDD samples, as evidenced by the increased CHP staining. Strong positive correlations between the two measured parameters suggest that when collagen degradation occurs, it is detected by this technique and is widespread throughout the tissue. This study provides new insights into the structural alterations associated with collagen degradation in the AF that occur during DDD.

Microscale investigation of the anisotropic swelling of cartilage tissue and cells in response to hypo-osmotic challenges

Tissue swelling represents an early sign of osteoarthritis, reflecting osmolarity changes from iso- to hypo-osmotic in the diseased joints. Increased tissue hydration may drive cell swelling. The opposing cartilages in a joint may swell differently, thereby predisposing the more swollen cartilage and cells to mechanical injuries. However, our understanding of the tissue-cell interdependence in osmotically loaded joints is limited as tissue and cell swellings have been studied separately. Here, we measured tissue and cell responses of opposing patellar (PAT) and femoral groove (FG) cartilages in lapine knees exposed to an extreme hypo-osmotic challenge. We found that the tissue matrix and most cells swelled during the hypo-osmotic challenge, but to a different extent (tissue: <3%, cells: 11%-15%). Swelling-induced tissue strains were anisotropic, showing 2%-4% stretch and 1%-2% compression along the first and third principal directions, respectively. These strains were amplified by 5-8 times in the cells. Interestingly, the first principal strains of tissue and cells occurred in different directions (60-61° for tissue vs. 8-13° for cells), suggesting different mechanisms causing volume expansion in the tissue and the cells. Instead of the continuous swelling observed in the tissue matrix, >88% of cells underwent regulatory volume decrease to return to their pre-osmotic challenge volumes. Cell shapes changed in the early phase of swelling but stayed constant thereafter. Kinematic changes to tissue and cells were larger for PAT cartilage than for FG cartilage. We conclude that the swelling-induced deformation of tissue and cells is anisotropic. Cells actively restored volume independent of the surrounding tissues and seemed to prioritize volume restoration over shape restoration. Our findings shed light on tissue-cell interdependence in changing osmotic environments that is crucial for cell mechano-transduction in swollen/diseased tissues.