Cryopreservation of porcine articular cartilage: MRI and biochemical results after different freezing protocols

Chondrocyte Viability of Particulated Porcine Articular Cartilage Is Maintained in Tissue Storage After Cryoprotectant Exposure, Vitrification, and Tissue Warming

OBJECTIVE

Vitrification of articular cartilage (AC) is a promising technique which may enable long-term tissue banking of AC allografts. We previously developed a 2-step, dual-temperature, multi-cryoprotectant agent (CPA) loading protocol to cryopreserve particulated AC (1 mm3 cubes). Furthermore, we also determined that the inclusion of ascorbic acid (AA) effectively mitigates CPA toxicity in cryopreserved AC. Prior to clinical translation, chondrocytes must remain viable after tissue re-warming and before transplantation. However, the effects of short-term hypothermic storage of particulated AC after vitrification and re-warming are not documented. This study evaluated the chondrocyte viability of post-vitrified particulated AC during a 7-day tissue storage period at 4 °C. We hypothesized that porcine particulated AC could be stored for up to 7 days after successful vitrification without significant loss of cell viability, and these results would be enhanced when cartilage is incubated in storage medium supplemented with clinical grade AA.

DESIGN

Three experimental groups were examined at 5 time points: a fresh control (only incubated in medium), a vitrified - AA group, and a vitrified + AA group (N = 7).

RESULTS

There was a mild decline in cell viability but both treatment groups maintained a viability of greater than 80% viable cells which is acceptable for clinical translation.

CONCLUSION

We determined that particulated AC can be stored for up to 7 days after successful vitrification without a clinically significant decline in chondrocyte viability. This information can be used to guide tissue banks regarding the implementation of AC vitrification to increase cartilage allograft availability.

Subchondral bone plate thickness is associated with micromechanical and microstructural changes in the bovine patella osteochondral junction with different levels of cartilage degeneration

The influence of joint degeneration on the biomechanical properties of calcified cartilage and subchondral bone plate at the osteochondral junction is relatively unknown. Common experimental difficulties include accessibility to and visualization of the osteochondral junction, application of mechanical testing at the appropriate length scale, and availability of tissue that provides a consistent range of degenerative changes. This study addresses these challenges. A well-established bovine patella model of early joint degeneration was employed, in which micromechanical testing of fully hydrated osteochondral sections was carried out in conjunction with high-resolution imaging using differential interference contrast (DIC) optical light microscopy. A total of forty-two bovine patellae with different grades of tissue health ranging from healthy to mild, moderate, and severe cartilage degeneration, were selected. From the distal-lateral region of each patella, two adjacent osteochondral sections were obtained for the mechanical testing and the DIC imaging, respectively. Mechanical testing was carried out using a robotic micro-force acquisition system, applying compression tests over an array (area: 200 μm × 1000 μm, step size: 50 μm) across the osteochondral junction to obtain a stiffness map. Morphometric analysis was performed for the DIC images of fully hydrated cryo-sections. The levels of cartilage degeneration, DIC images, and the stiffness maps were used to associate the mechanical properties onto the specific tissue regions of cartilage, calcified cartilage, and subchondral bone plate. The results showed that there were up to 20% and 24% decreases (p < 0.05) in the stiffness of calcified cartilage and subchondral bone plate, respectively, in the severely degenerated group compared to the healthy group. Furthermore, there were increases (p < 0.05) in the number of tidemarks, bone spicules at the cement line, and the mean thickness of the subchondral bone plate with increasing levels of degeneration. The decreasing stiffness in the subchondral bone plate coupled with the presence of bone spicules may be indicative of a subchondral remodeling process involving new bone formation. Moreover, the mean thickness of the subchondral bone plate was found to be the strongest indicator of mechanical and associated structural changes in the osteochondral joint tissues.

Numerical Modeling of Heat and Mass Transfer during Cryopreservation Using Interval Analysis

In the paper, the numerical analysis of heat and mass transfer proceeding in an axially symmetrical articular cartilage sample subjected to the cryopreservation process is presented. In particular, a two-dimensional (axially symmetrical) model with imprecisely defined parameters is considered. The base of the heat transfer model is given by the interval Fourier equation and supplemented by initial boundary conditions. The phenomenon of cryoprotectant transport (Me2SO) through the extracellular matrix is described by the interval mass transfer equation. The liquidus-tracking (LT) method is used to control the temperature, which avoids the formation of ice regardless of the cooling and warming rates. In the LT process, the temperature decreases/increases gradually during addition/removal of the cryoprotectant, and the articular cartilage remains on or above the liquidus line so that no ice forms, independent of the cooling/warming rate. The discussed problem is solved using the interval finite difference method with the rules of directed interval arithmetic. Examples of numerical computations are presented in the final part of the paper. The obtained results of the numerical simulation are compared with the experimental results, realized for deterministically defined parameters.

Apparatus and Method for Rapid Detection of Acoustic Anisotropy in Cartilage

Purpose

Articular cartilage is known to be mechanically anisotropic. In this paper, the acoustic anisotropy of bovine articular cartilage and the effects of freeze-thaw cycling on acoustic anisotropy were investigated.

Methods

We developed apparatus and methods that use a magnetic L-shaped sample holder, which allowed minimal handling of a tissue, reduced the number of measurements compared to previous studies, and produced highly reproducible results.

Results

SOS was greater in the direction perpendicular to the articular surface compared to the direction parallel to the articular surface (N=17, P = 0.00001). Average SOS was 1,758 ± 107 m/s perpendicular to the surface, and 1,617 ± 55 m/s parallel to it. The average percentage difference in SOS between the perpendicular and parallel directions was 8.2% (95% CI: 5.4% to 11%). Freeze-thaw cycling did not have a significant effect on SOS (P>0.4).

Conclusion

Acoustic measurement of tissue properties is particularly attractive for work in our laboratory since it has the potential for nondestructive characterization of the properties of developing engineered cartilage. Our approach allowed us to observe acoustic anisotropy of articular cartilage rapidly and reproducibly. This property was not significantly affected by freeze-thawing of the tissue samples, making cryopreservation practical for these assays.

Composition, structure and tensile biomechanical properties of equine articular cartilage during growth and maturation

Articular cartilage undergoes structural and biochemical changes during maturation, but the knowledge on how these changes relate to articular cartilage function at different stages of maturation is lacking. Equine articular cartilage samples of four different maturation levels (newborn, 5-month-old, 11-month-old and adult) were collected (N = 25). Biomechanical tensile testing, Fourier transform infrared microspectroscopy (FTIR-MS) and polarized light microscopy were used to study the tensile, biochemical and structural properties of articular cartilage, respectively. The tensile modulus was highest and the breaking energy lowest in the newborn group. The collagen and the proteoglycan contents increased with age. The collagen orientation developed with age into an arcade-like orientation. The collagen content, proteoglycan content, and collagen orientation were important predictors of the tensile modulus (p < 0.05 in multivariable regression) and correlated significantly also with the breaking energy (p < 0.05 in multivariable regression). Partial least squares regression analysis of FTIR-MS data provided accurate predictions for the tensile modulus (r = 0.79) and the breaking energy (r = 0.65). To conclude, the composition and structure of equine articular cartilage undergoes changes with depth that alter functional properties during maturation, with the typical properties of mature tissue reached at the age of 5-11 months.

The Probable Explanation for the Low Friction of Natural Joints

The surface of an articular cartilage, coated with phospholipid (PL) bilayers, plays an important role in its lubrication and movement. Intact (normal) and depleted surfaces of the joint were modelled and the pH influence on the surface interfacial energy, wettability and friction were investigated. In the experiments, the deterioration of the PL bilayer was controlled by its wettability and the applied friction. The surrounding fluid of an undamaged articular cartilage, the synovial fluid, has a pH value of approximately 7.4. Buffer solutions were formulated to represent the synovial fluid with various pH values. It was found that the surface interfacial energy was stabilised at its lowest values when the pH varied between 6.5 and 9.5. These results suggested that as the PL bilayers deteriorated, the hydration repulsion mechanism became less effective as friction increased. The decreased number of bilayers changed the wettability and lowered PL lubricant properties.

Effects of cryopreservation on the depth-dependent elastic modulus in articular cartilage and implications for osteochondral grafting

Cryopreservation of articular cartilage is often used in storage of experimental samples and osteochondral grafts, but the depth-dependence and concentration of glycosaminoglycan (GAG) are significantly altered when cryogenically stored without a cryoprotectant, which will reduce cartilage stiffness and affect osteochondral graft function and long-term viability. This study investigates our ability to detect changes due to cryopreservation in the depth-dependent elastic modulus of osteochondral samples. Using a direct-visualization method requiring minimal histological alterations, unconfined stepwise stress relaxation tests were performed on four fresh (never frozen) and three cryopreserved (-20 °C) canine humeral head osteochondral slices 125 ± 5 μm thick. Applied force was measured and tissue images were taken at the end of each relaxation phase using a 4× objective. Intratissue displacements were calculated by tracking chondrocytes through consecutive images for various intratissue depths. The depth-dependent elastic modulus was compared between fresh and cryopreserved tissue for same-depth ranges using analysis of variance (ANOVA) with Tukey post-test with a 95% confidence interval. Cryopreservation was found to significantly alter the force-displacement profile and reduce the depth-dependent modulus of articular cartilage. Excessive collagen fiber folding occurred at 40-60% relative depth, producing a "black line" in cryopreserved tissue. Force-displacement curves exhibited elongated toe-region in cryopreserved tissue while fresh tissue had nonmeasurable toe-region. Statistical analysis showed significant reduction in the elastic modulus and GAG concentration throughout the tissue between same-depth ranges. This method of cryopreservation significantly reduces the depth-dependent modulus of canine humeral osteochondral samples.

Approaches to preserve human osteochondral allografts

Osteochondral defects may progress to osteoarthritis. Many attempts have been developed to overcome this issue, including osteochondral autografts and allografts. The goal of this study was to develop a new protocol for storage of human osteochondral allografts. Osteochondral plugs were randomly allocated in the following groups: control, immediate freezing up to -70 °C, cooling at 4 °C, and storage at 37 °C. Samples from the cooling at 4 °C and storage at 37 °C groups were stored in tubes containing medium plus human albumin and analyzed after 1, 3, and 14 days. The frozen groups' samples were cryopreserved for 1 year in cryotubes containing medium only (FM), medium plus human albumin (FA), and medium plus human albumin and glucose (FG) and were then analyzed. Analysis involved histological study with hematoxylin-eosin and Safranin O and a modified Live/Dead assay. In samples stored both at 37 and 4 °C, analysis showed statistically significant higher cellular mortality at 14 days compared to 1 and 3 days, but mortality in the 4 °C group was lower. In the freezing protocols, the FA group showed less cellular mortality than the FM and FG groups. Cooling at 4 °C offers better preservation capacity than storage at 37 °C, but both offer the capacity for preservation for 14 days. Adding human albumin to the storage medium is useful in reducing cellular mortality in samples frozen for 1 year.

Cell-tissue interactions in osteoarthritic human hip joint articular cartilage

Volume and morphology of chondrocytes in osteoarthritic human hip joint articular cartilage were characterized, and their relationship to tissue structure and function was determined. Human osteochondral articular cartilage samples (n=16) were obtained from the femoral heads of nine patients undergoing total hip arthroplasty due to osteoarthritis (OA). Superficial chondrocytes (N=65) were imaged in situ with a confocal laser scanning microscope at 37 °C. This was followed by the determination of the mechanical properties of the tissue samples, depth-wise characterization of cell morphology (height, width; N=385) as well as structure and composition of the tissues using light microscopy, digital densitometry, Fourier transform infrared microspectroscopy and polarized light microscopy. Significant correlations were found between the cell volume and the orientation angle associated with the collagen fibers (r=0.320, p=0.009) as well as between the cell volume and the initial dynamic modulus of the tissue (r=-0.305, p=0.013). Furthermore, the depth-dependent chondrocyte aspect ratio (height/width) correlated significantly with the orientation angle of the collagen fibers and with the tissue's proteoglycan content (r=0.261 and r=0.228, respectively, p<0.001). Our findings suggest that the orientation angle of the collagen fibers primarily controls chondrocyte volume and shape in osteoarthritic human hip joint articular cartilage.

Site-dependent changes in structure and function of lapine articular cartilage 4 weeks after anterior cruciate ligament transection

OBJECTIVE

The aim of this study was to investigate the site-dependent changes in the structure and function of articular cartilage in the lapine knee joint at a very early stage of osteoarthritis (OA), created experimentally by anterior cruciate ligament transection (ACLT).

METHODS

Unilateral ACLT was performed in eight mature New Zealand white rabbits. ACL transected and contralateral (C-L) joints were prepared for analysis at 4 weeks after ACLT. Three rabbits with intact joints were used as a control group (CNTRL). Femoral groove, medial and lateral femoral condyles, and tibial plateaus were harvested and used in the analysis. Biomechanical tests, microscopy and spectroscopy were used to determine the biomechanical properties, composition and structure of the samples. A linear mixed model was chosen for statistical comparisons between the groups.

RESULTS

As a result of ACLT, the equilibrium and dynamic moduli were decreased primarily in the femoral condyle cartilage. Up to three times lower moduli (P < 0.05) were observed in the ACLT group compared to the control group. Significant (P < 0.05) proteoglycan (PG) loss in the ACLT joint cartilage was observed up to a depth of 20-30% from the cartilage surface in femoral condyles, while significant PG loss was confined to more superficial regions in tibial plateaus and femoral groove. The collagen orientation angle was increased (P < 0.05) up to a cartilage depth of 60% by ACLT in the lateral femoral condyle, while smaller effects, but still significant, were observed at other locations. The collagen content was increased (P < 0.05) in the middle and deep zones of the ACLT group compared to the control group samples, especially in the lateral femoral condyle.

CONCLUSION

Femoral condyle cartilage experienced the greatest structural and mechanical alterations in very early OA, as produced by ACLT. Degenerative alterations were observed especially in the superficial collagen fiber organization and PG content, while the collagen content was increased in the deep tissue of femoral condyle cartilage. The current findings provide novel information of the early stages of OA in different locations of the knee joint.

Role of cells in freezing-induced cell-fluid-matrix interactions within engineered tissues

During cryopreservation, ice forms in the extracellular space resulting in freezing-induced deformation of the tissue, which can be detrimental to the extracellular matrix (ECM) microstructure. Meanwhile, cells dehydrate through an osmotically driven process as the intracellular water is transported to the extracellular space, increasing the volume of fluid for freezing. Therefore, this study examines the effects of cellular presence on tissue deformation and investigates the significance of intracellular water transport and cell-ECM interactions in freezing-induced cell-fluid-matrix interactions. Freezing-induced deformation characteristics were examined through cell image deformetry (CID) measurements of collagenous engineered tissues embedded with different concentrations of MCF7 breast cancer cells versus microspheres as their osmotically inactive counterparts. Additionally, the development of a biophysical model relates the freezing-induced expansion of the tissue due to the cellular water transport and the extracellular freezing thermodynamics for further verification. The magnitude of the freezing-induced dilatation was found to be not affected by the cellular water transport for the cell concentrations considered; however, the deformation patterns for different cell concentrations were different suggesting that cell-matrix interactions may have an effect. It was, therefore, determined that intracellular water transport during freezing was insignificant at the current experimental cell concentrations; however, it may be significant at concentrations similar to native tissue. Finally, the cell-matrix interactions provided mechanical support on the ECM to minimize the expansion regions in the tissues during freezing.

Effects of Freeze-Thaw Cycle with and without Proteolysis Inhibitors and Cryopreservant on the Biochemical and Biomechanical Properties of Articular Cartilage

OBJECTIVE

We investigated the effects of freeze-thawing on the properties of articular cartilage.

DESIGN

The reproducibility of repeated biomechanical assay of the same osteochondral sample was first verified with 11 patellar plugs from 3 animals. Then, 4 osteochondral samples from 15 bovine patellae were divided into 4 groups. The reference samples were immersed in phosphate-buffered saline (PBS) containing proteolysis inhibitors and biomechanically tested before storage for further analyses. Samples of group 1 were biomechanically tested before and after freeze-thawing in PBS in the absence and those of group 2 in the presence of inhibitors. Samples of the group 3 were biomechanically tested in PBS-containing inhibitors, but frozen in 30% dimethyl sulfoxide/PBS and subsequently tested in PBS supplemented with the inhibitors. Glycosaminoglycan contents of the samples and immersion solutions were analyzed, and proteoglycan structures examined with SDS-agarose gel electrophoresis.

RESULTS

Freeze-thawing decreased slightly dynamic moduli in all 3 groups. The glycosaminoglycan contents and proteoglycan structures of the cartilage were similar in all experimental groups. Occasionally, the diffused proteoglycans were partly degraded in group 1. Digital densitometry revealed similar staining intensities for the glycosaminoglycans in all groups. Use of cryopreservant had no marked effect on the glycosaminoglycan loss during freeze-thawing.

CONCLUSION

The freeze-thawed cartilage samples appear suitable for the biochemical and biomechanical studies.

Measurement of spatiotemporal intracellular deformation of cells adhered to collagen matrix during freezing of biomaterials

Preservation of structural integrity inside cells and at cell-extracellular matrix (ECM) interfaces is a key challenge during freezing of biomaterials. Since the post-thaw functionality of cells depends on the extent of change in the cytoskeletal structure caused by complex cell-ECM adhesion, spatiotemporal deformation inside the cell was measured using a newly developed microbead-mediated particle tracking deformetry (PTD) technique using fibroblast-seeded dermal equivalents as a model tissue. Fibronectin-coated 500 nm diameter microbeads were internalized in cells, and the microbead-labeled cells were used to prepare engineered tissue with type I collagen matrices. After a 24 h incubation the engineered tissues were directionally frozen, and the cells were imaged during the process. The microbeads were tracked, and spatiotemporal deformation inside the cells was computed from the tracking data using the PTD method. Effects of particle size on the deformation measurement method were tested, and it was found that microbeads represent cell deformation to acceptable accuracy. The results showed complex spatiotemporal deformation patterns in the cells. Large deformation in the cells and detachments of cells from the ECM were observed. At the cellular scale, variable directionality of the deformation was found in contrast to the one-dimensional deformation pattern observed at the tissue scale, as found from earlier studies. In summary, this method can quantify the spatiotemporal deformation in cells and can be correlated to the freezing-induced change in the structure of cytosplasm and of the cell-ECM interface. As a broader application, this method may be used to compute deformation of cells in the ECM environment for physiological processes, namely cell migration, stem cell differentiation, vasculogenesis, and cancer metastasis, which have relevance to quantify mechanotransduction.

Cryopreservation of articular cartilage

Cryopreservation has numerous practical applications in medicine, biotechnology, agriculture, forestry, aquaculture and biodiversity conservation, with huge potentials for biological cell and tissue banking. A specific tissue of interest for cryopreservation is the articular cartilage of the human knee joint for two major reasons: (1) clinically, there exists an untapped potential for cryopreserved cartilage to be used in surgical repair/reconstruction/replacement of injured joints because of the limited availability of fresh donor tissue and, (2) scientifically, successful cryopreservation of cartilage, an avascular tissue with only one cell type, is considered a stepping stone for transition from biobanking cell suspensions and small tissue slices to larger and more complicated tissues. For more than 50years, a great deal of effort has been directed toward understanding and overcoming the challenges of cartilage preservation. In this article, we focus mainly on studies that led to the finding that vitrification is an appropriate approach toward successful preservation of cartilage. This is followed by a review of the studies on the main challenges of vitrification, i.e. toxicity and diffusion, and the novel approaches to overcome these challenges such as liquidus tracking, diffusion modeling, and cryoprotective agent cocktails, which have resulted in the recent advancements in the field.

Initial application of EPIC-μCT to assess mouse articular cartilage morphology and composition: effects of aging and treadmill running

OBJECTIVE

The current study was undertaken to adapt Equilibrium Partitioning of an Ionic Contrast agent via microcomputed tomography (EPIC-μCT) to mouse articular cartilage (AC), which presents a particular challenge because it is thin (30 μm) and has a small volume (0.2-0.4 mm(3)), meaning there is only approximately 2-4 μg of chondroitin sulfate (CS) glycosaminoglycan per joint surface cartilage.

DESIGN

Using 6 μm isotropic voxels and the negatively charged contrast agent ioxaglate (Hexabrix), we optimized contrast agent concentration and incubation time, assessed two methods of tissue preservation (formalin fixation and freezing), examined the effect of ex vivo chondroitinase ABC digestion on X-ray attenuation, assessed accuracy and precision, compared young and skeletally mature cartilage, and determined patterns of degradation in a murine cartilage damage model induced by treadmill running.

RESULTS

The optimal concentration of the contrast agent was 15%, formalin fixation was preferred to freezing, and 2 h of incubation was needed to reach contrast agent equilibrium with formalin-fixed specimens. There was good agreement with histologic measurements of cartilage thickness, although μCT over-estimated thickness by 13% (5 μm) in 6-week-old mice. Enzymatic release of 0.8 μg of chondrotin sulfate (about 40% of the total) increased X-ray attenuation by 17%. There was a 15% increase in X-ray attenuation in 14-week-old mice compared to 6-week-old mice (P < 0.001) and this corresponded to 65% decrease in CS content at 14 weeks. The older mice also had reductions of 33% in cartilage thickness and 44% in cartilage volume (P < 0.001). Treadmill running induced a 16% decrease in cartilage thickness (P = 0.012) and a 12% increase in X-ray attenuation (P = 0.006) in 14-week-old mice.

CONCLUSION

This technique enables non-destructive visualization and quantification of murine femoral AC in three dimensions with anatomic specificity and should prove to be a useful new tool in studying degeneration of cartilage in mouse models.

Effects of freezing-induced cell-fluid-matrix interactions on the cells and extracellular matrix of engineered tissues

The two most significant challenges for successful cryopreservation of engineered tissues (ETs) are preserving tissue functionality and controlling highly tissue-type dependent preservation outcomes. In order to address these challenges, freezing-induced cell-fluid-matrix interactions should be understood, which determine the post-thaw cell viability and extracellular matrix (ECM) microstructure. However, the current understanding of this tissue-level biophysical interaction is still limited. In this study, freezing-induced cell-fluid-matrix interactions and their impact on the cells and ECM microstructure of ETs were investigated using dermal equivalents as a model ET. The dermal equivalents were constructed by seeding human dermal fibroblasts in type I collagen matrices with varying cell seeding density and collagen concentration. While these dermal equivalents underwent an identical freeze/thaw condition, their spatiotemporal deformation during freezing, post-thaw ECM microstructure, and cellular level cryoresponse were characterized. The results showed that the extent and characteristics of freezing-induced deformation were significantly different among the experimental groups, and the ETs with denser ECM microstructure experienced a larger deformation. The magnitude of the deformation was well correlated to the post-thaw ECM structure, suggesting that the freezing-induced deformation is a good indicator of post-thaw ECM structure. A significant difference in the extent of cellular injury was also noted among the experimental groups, and it depended on the extent of freezing-induced deformation of the ETs and the initial cytoskeleton organization. These results suggest that the cells have been subjected to mechanical insult due to the freezing-induced deformation as well as thermal insult. These findings provide insight on tissue-type dependent cryopreservation outcomes, and can help to design and modify cryopreservation protocols for new types of tissues from a pre-developed cryopreservation protocol.

Influence of knee positions on T2, T*2, and dGEMRIC mapping in porcine knee cartilage

We examined the influence of flexed knee positions on cartilage MR assessments. Sagittal T(2), T*(2), and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) maps of the femoral cartilage were obtained in eight 6-month-old porcine femorotibial joints in the extended knee position (position A: flexion 0 degrees and femoral shaft in parallel with the amplitude of static field), flexed knee position (position B: flexion 40 degrees and femoral shaft oriented at 40 degrees to the amplitude of static field), and oblique-placed knee-extended position (position C: flexion 0 degrees and femoral shaft oriented at 40 degrees to the amplitude of static field). Comparison of the MR parameters between positions A and C showed isolated influence of the magic-angle effect, and comparison between positions A and B showed effects of knee flexion. Proteoglycan and hydroxyproline content in cartilage specimen at each region of interest had no significant correlation with T(2), T*(2), and dGEMRIC values. At the central zone, located on a weight-bearing area and parallel to the amplitude of static field, T(2)/T*(2)/dGEMRIC values increased by 6.8/11/0.8% at position C and by 24/44/31% at position B compared with position A. There was a significant increase in T(2) and T*(2) values at position B compared with those at position A. The substantial changes in T(2), T*(2), and dGEMRIC were shown in response to knee flexion, presumably due to the compounding influence of the magic-angle effect and change in the intra-articular biomechanical condition.

Damages to the extracellular matrix in articular cartilage due to cryopreservation by microscopic magnetic resonance imaging and biochemistry

To investigate the damages to the extracellular matrix in articular cartilage due to cryopreservation, the depth-dependent concentration profiles of glycosaminoglycans (GAGs) in 34 cartilage specimens from canine humeral heads were imaged at 13-mum pixel resolution using the in vitro version of the dGEMRIC protocol in microscopic MRI (microMRI). In addition, a biochemical assay was used to determine the GAG loss from the tissue to the solution where the tissue was immersed. For specimens that had been frozen at -20 degrees C or -80 degrees C without any cryoprotectant, a significant loss of GAG (as high as 56.5%) was found in cartilage, dependent upon the structural zones of the tissue and the conditions of cryopreservation. The cryoprotective abilities of dimethyl sulfoxide (DMSO) as a function of its concentration in saline and storage temperature were also investigated. A 30% DMSO concentration was sufficient in preventing the reduction of GAG in the tissue at the -20 degrees C storage temperature, but a 50% concentration of DMSO was necessary for the -80 degrees C cryopreservation. These imaging results were verified by the biochemical analysis.

Quantitative second harmonic generation imaging of cartilage damage

Cartilage damage was studied using non-invasive multiphoton-excited autofluorescence and quantitative second harmonic generation (SHG) microscopy. Two cryopreservation techniques based upon freezing and vitrification methods, respectively, were employed to determine whether or not the collagen fiber structure of full thickness porcine articular cartilage was affected by cryopreservation and whether the level of collagen damage could be determined quantitatively in non-processed (non-fixed, non-sliced, non-stained) tissues. Multiphoton-induced autofluorescence imaging revealed the presence of chondrocytes, as well as collagenous structures in all fresh, vitrified and frozen cryopreserved cartilage samples. SHG imaging of the frozen cryopreserved specimens showed a dramatic loss of mean gray value intensities when compared to both fresh and vitrified tissues (P<0.05), indicating structural changes of the extracellular matrix, in particular the deformation and destruction of the collagen fibers in the analyzed articular cartilage. A 0.9974 correlation coefficient was observed between the metabolic cell activity assessed by the alamarBlue technique, and retention of collagen structure between the three experimental groups. These studies suggest that multiphoton-induced autofluorescence imaging combined with quantitative SHG signal profiling may prove to be useful tools for the investigation of extracellular matrix changes in preserved cartilage, giving insights on the structural quality prior to implantation.