N‐acetylcysteine prevents nitric oxide‐induced chondrocyte apoptosis and cartilage degeneration in an experimental model of osteoarthritis

We investigated whether N‐acetylcysteine (NAC), a precursor of glutathione, could protect rabbit articular chondrocytes against nitric oxide (NO)‐induced apoptosis and could prevent cartilage destruction in an experimental model of osteoarthritis (OA) in rats. Isolated chondrocytes were treated with various concentrations of NAC (0–2 mM). Apoptosis was induced by 0.75 mM sodium nitroprusside (SNP) dehydrate, which produces NO. Cell viability was assessed by MTT assay, while apoptosis was evaluated by Hoechst 33342 and TUNEL staining. Intracellular reactive oxygen species (ROS) and glutathione levels were measured, and expression of p53 and caspase‐3 were determined by Western blotting. To determine whether intraarticular injection of NAC prevents cartilage destruction in vivo, cartilage samples of an OA model were subjected to H&E, Safranin O, and TUNEL staining. NAC prevented NO‐induced apoptosis, ROS overproduction, p53 up‐regulation, and caspase‐3 activation. The protective effects of NAC were significantly blocked by buthionine sulfoximine, a glutathione synthetase inhibitor, indicating that the apoptosis‐preventing activity of NAC was mediated by glutathione. Using a rat model of experimentally induced OA, we found that NAC also significantly prevented cartilage destruction and chondrocyte apoptosis in vivo. These results indicate that NAC inhibits NO‐induced apoptosis of chondrocytes through glutathione in vitro, and inhibits chondrocyte apoptosis and articular cartilage degeneration in vivo. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:156–163, 2010     

骨关节炎软骨细胞凋亡调控基因的研究

目的　比较分析正常人及老年性骨关节炎患者软骨细胞bax和bcl 2的表达及细胞凋亡状况。　方法　取 9例骨关节炎患者的关节软骨做实验标本 ,以 6例无骨关节炎病史的意外死亡者关节软骨作为正常对照 ;采用逆转录 /聚合酶链反应 (RT PCR)方法检测bax和bcl 2mRNA表达 ,免疫组化检测bax和bcl 2蛋白 ;应用TUNEL方法进行凋亡细胞原位检测。　结果　骨关节炎患者和正常对照软骨细胞都能表达bax和bcl 2mRNA ;骨关节炎关节软骨细胞baxmRNA表达量较正常对照显著增高 (P <0 0 1) ,bcl 2mRNA表达量也高于正常对照组 (P <0 0 5 ) ,两组间bax/bcl 2表达量的比值差异无显著性意义 (P >0 0 5 ) ;免疫组化可检测到相应表达水平的蛋白 ;骨关节炎软骨细胞凋亡 (4%～ 14% )多于正常对照 (0～ 2 % )。　结论　软骨细胞凋亡受bax和bcl 2共同调节 ;bax和bcl 2的共同调节结果可能是OA患者软骨细胞凋亡增加 ,但凋亡率不高、病理过程进展缓慢的一个重要的原因     

Mechanisms of chondrocyte apoptosis.

This study addresses the occurrence and significance of chondrocyte apoptosis in the pathogenesis of cartilage destruction. Chondrocyte apoptosis can be induced in vitro by nitric oxide donors, but not by pro-inflammatory cytokines, such as IL-1 or TNF. A subset of chondrocytes, located in the superficial zone of cartilage, expresses the Fas antigen. Activation of the Fas receptor triggers apoptosis in these cells. In human and experimental osteoarthritis (OA) induced in rabbits by anterior cruciate ligament transection increased numbers of chondrocytes were undergoing apoptosis. Cartilage areas that contained apoptotic cells showed proteoglycan depletion and the number of apoptotic cells was significantly correlated with the levels of nitric oxide production and with the severity of OA. Articular cartilage is not vascularized and does not contain mononuclear phagocytes. There is, thus, no apparent mechanism for the clearance of apoptotic bodies. Chondrocyte-derived apoptotic bodies produced pyrophosphate and precipitated calcium. These results suggest that chondrocyte-derived apoptotic bodies express functional properties that may contribute to the pathologic cartilage degradation and calcification. Inhibition of chondrocyte apoptosis may be of therapeutic value after cartilage injury and in arthritis.     

The distribution of S-100 protein positive chondrocytes in the human articular cartilages under aging or diseased conditions]

There have been few reports on the localization of S-100 protein positive chondrocytes in the human articular cartilages. We studied 59 articular cartilages of the aged subjects, 65 osteoarthritic (OA) and 39 rheumatoid arthritic (RA) articular cartilages, to detect the histological localization of S-100 protein using immunoperoxidase method (ABC). The results obtained from normal cartilages demonstrated strongly positive cells representing hypertrophic chondrocytes in the perivascular areas of the neonatal articular cartilage and in the deep zone of the infant articular cartilage. The moderately positive cells were found in the intermediate zone of infant and adult articular cartilages. In mild OA, there were many positive chondrocytes in the intermediate zone with erosion of the surface layer, while in moderate or severe OA many strongly positive cells were found in clusters. The hypertrophic cells in the metaplastic cartilage arising from bone marrow in subjects with severe OA, or from pannus after RA were also positive. It is therefore, suggested that S-100 protein may be correlated with the metabolic activity of the cartilage matrix such as collagen and proteoglycan, as reported in the literature. S-100 protein further, appears to be useful for evaluating histologically the activity of cartilage repair in the pathologic human articular cartilages.     

DNA cytofluorometric analysis of chondrocytes in human articular cartilages under normal aging or arthritic conditions

OBJECTIVE: Since most chondrocytes in articular cartilage are in the resting phase (G0) of the cell cycle, it has been difficult to investigate their cell kinetics using 3H-thymidine autoradiography, or immunohistochemistry. In the present study, DNA cytofluorometry, which is useful to analyse the cell kinetics even for such inactive cell populations as in the G0 phase, was applied to human chondrocytes of the articular cartilages under normal aging and pathologic conditions such as osteoarthritis (OA), rheumatoid arthritis (RA), and aseptic necrosis (AN). DESIGN: The human articular cartilages for the study were obtained from autopsy and surgical materials. Fifty joints were used for the study of aging, 54 for the study of OA, 20 for studying RA, and 10 for AN study. The isolated chondrocytes were quickly prepared from fresh articular cartilages, using a combination method of enzymatic digestion with papain and collagenase, followed by mechanical cell separation by churning and homogenization. RESULTS: The DNA histograms obtained by cytofluorometry with propidium-iodide staining showed that most chondrocytes had diploid DNA content (2c) in all cartilages studied, suggesting that they were in the G0 phase. However, there were a few chondrocytes having tetraploid DNA content (4c) in the normally aged articular cartilages, and there were some cells having DNA content between 2c and 4c in the diseased cartilages. The former cells were considered to be G0-phase cells of the 4c chondrocytes, while the latter cells were considered to be in the DNA synthetic (S) phase or G2-phase of the 2c chondrocytes. The frequency of 4c chondrocytes in aged cartilage was significantly increased, compared to that in the young cartilage. In contrast to the normal cartilage, the frequency of S- and G2-phase cells, which was expressed as the S- G2 index, in diseased cartilages (OA, RA and AN) was significantly high (P< 0.0001). In OA cartilage, the S-G2 index was much higher in the severe or moderate stage than in the mild stage, suggesting that the chondrocytes in clusters may actively proliferate. CONCLUSION: These results showed that in normal articular cartilages most chondrocytes are in the G0 phase, while some became 4c polyploid cells, and that these G0-phase chondrocytes had a potential to proliferate under diseased conditions.     

Osteoarthritic cartilage fibrillation is associated with a decrease in chrondrocyte adhesion to fibronectin

Objective: Cartilage destruction in osteoarthritis (OA) is generally accepted as a failed repair process. Cell adhesion is implicated in tissue repair. Therefore, adhesion of OA chondrocytes to extracellular matrix proteins was investigated.Design: Using chondrocytes from human OA femoral head cartilage, adhesion to fibronectin and type II collagen of cells from distinct areas showing an intact cartilage surface or a fibrillated cartilage surface was studied. Modulation of chondrocyte adhesion by both protein kinase C (PKC) inhibitors and glucosamine sulfate (GS) was also investigated.Results: A significant (P<0.05) decrease in adhesion to fibronectin of chondrocytes from fibrillated cartilage, relative to those from grossly normal OA cartilage, was demonstrated. Adhesion to type II collagen was not modified by the chondrocyte origins (either from normal or fibrillated OA cartilage) Adhesion to fibronectin of cells from grossly intact cartilage was decreased by the addition of PKC and calmodulin-dependent kinase inhibitors, W7 and sphingosine, to the cell culture. Adhesion to fibronectin of chondrocytes from fibrillated cartilage was significantly (P<0.05) increased after glucosamine sulfate treatment.Conclusion: Fibrillation of cartilage from OA femoral head is associated with a defective adhesion of chondrocytes to fibronectin. The process is suggested to be dependent of PKC and/or calmodulin-dependent kinases and potentially reversible. Conceivably, it could play a role in OA cartilage destruction.     

治疗型关节炎护膝对实验性骨关节炎软骨细胞凋亡基因Bcl-2及p53的影响

目的：通过检测OA护膝对日本大耳白兔膝骨性关节炎软骨细胞凋亡基因Bcl-2、p53mRNA表达的影响,探讨OA护膝防治兔膝骨性关节炎软骨细胞凋亡的分子生物学机制。方法：健康6月龄日本大耳白兔54只,雌雄各半,空腹体重2～2．2kg,采用改良Huhh法复制膝骨性关节炎模型,随机分为6组,即正常组、模型组、对照组（微波组）、实验1组（电组）、实验2组（热组）、实验3组（护膝组）．正常组10只,常规饲养;模型组9只,造模后常规饲养;对照组9只,微波仪治疗30min,每日1次;实验1组9只,电（疏密波）治疗30min,每日1次;实验2组8只,热（热软膜）治疗30min,每日1次;实验3组9只,电热（OA护膝）治疗30min,每日1次,连续治疗16周时处死。采用荧光定量RT—PCR法检测各组膝关节软骨细胞Bcl-2、p53mRNA的表达水平。结果：16周时,各组所有抽提的兔关节软骨组织总RNA的OD260／OD280值均在1．80～2．00范围内,表明RNA纯度高：模型组、对照组、实验1组、实验2纽、实验3组关节软骨细胞p53的mRNA相对呈高表达,而关节软骨细胞Bcl-2的mRNA相对低表达,与正常组差异有统计学意义（P〈0．01）：关节软骨细胞Bcl-2、p53的mRNA相对表达水平,对照组、实验1组、实验2组、实验3组与模型组差异有统计学意义（P〈0．01）;对照组、实验1组、实验2组与实验3组差异有统计学意义（P〈0．01）。结论：OA护膝能提高关节软骨细胞Bcl-2 mRNA表达,减弱软骨细胞p53mRNA表达,从而抑制软骨细胞凋亡,延缓膝关节软骨的退变：     

Chondrocyte death by apoptosis is associated with cartilage matrix degradation

OBJECTIVES: To investigate the frequency of chondrocyte apoptosis in equine articular cartilage (AC) specimens and to examine the relationship between the process of cell death and the degree of cartilage degradation using a direct quantification of numbers of apoptotic cells and expression of active caspase-3. METHODS: AC from equine metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints was used and each joint was graded macroscopically for cartilage degradation (macroscopic osteoarthritis (OA) score). Cartilage sections were graded using a 'modified' Mankin scoring system. Apoptosis of chondrocytes in cartilage sections was assessed morphologically by appearance of apoptotic features (direct method) and by expression of active caspase-3 using indirect immunohistochemistry. RESULTS: The extent of apoptosis assessed by the direct method did not show any relationship with increasing severity of OA (P=0.72). However, there was a significant positive correlation between 'modified' Mankin score and apoptosis determined by caspase-3, with the extent of apoptosis found to increase linearly with increasing severity of OA (r=0.44, P=0.0043). Caspase-3 expression was found to be significantly higher in the superficial and middle zones than in the deep zone (P<0.001). In the superficial, middle and deep zones, expression of caspase-3 was significantly higher in the MCP joint than in the PIP joint (P=0.013, P=0.0018 and P=0.029, respectively). Within the MCP joints, apoptosis was higher in the lateral compartment compared to the medial (P=0.053). CONCLUSIONS: The data presented in this study demonstrate that chondrocyte apoptosis is positively associated with degree of cartilage matrix damage and that the extent of apoptosis varies with cartilage zones and mechanical loading environment of the joint.     

Localization of carboxy-terminal type II procollagen peptide (pCOL-II-C) in diseased cartilage

A well characterized rabbit polyclonal antibody against human carboxy-terminal type II procollagen peptide (pCOL-II-C) was used to study the immunolocalization of pCOL-II-C in articular cartilage obtained from patients with osteoarthritis (OA), rheumatoid arthritis (RA), and control non-diseased joints. In moderately degenerative OA cartilage, immunoreactive chondrocytes were observed in all layers, particularly along the margins of fibrillation and fissures, in chondrocyte clusters and in osteochondrophytes. The grade of immunostaining in OA correlated directly with Mankin's histological-histochemical scores of 0–7, but there was an inverse correlation between grade of immunostaining and Mankin's scores of 8–14. The grade of immunostaining was significantly higher in OA than in RA and normal control cartilage. Since type II collagen is a unique component of articular cartilage, localization of pCOL-II-C could reflect the increased synthesis of type II collagen by chondrocytes in diseased cartilage. This study was presented at the 8th Annual Meeting for Orthopaedic Research of the Japanese Orthopaedic Association in October 1993, at Matsumoto and at the 7th Annual Meeting of Japanese Society of Cartilage Metabolism in March 1994, at Hiroshima     

The role of mitochondria in osteoarthritis

Mitochondria are important regulators of cellular function and survival that may have a key role in aging-related diseases. Mitochondrial DNA (mtDNA) mutations and oxidative stresses are known to contribute to aging-related changes. Osteoarthritis (OA) is an aging-associated rheumatic disease characterized by articular cartilage degradation and elevated chondrocyte mortality. Articular cartilage chondrocytes survive and maintain tissue integrity in an avascular, low-oxygen environment. Recent ex vivo studies have reported mitochondrial dysfunction in human OA chondrocytes, and analyses of mitochondrial electron transport chain activity in these cells show decreased activity of Complexes I, II and III compared to normal chondrocytes. This mitochondrial dysfunction may affect several pathways that have been implicated in cartilage degradation, including oxidative stress, defective chondrocyte biosynthesis and growth responses, increased cytokine-induced chondrocyte inflammation and matrix catabolism, cartilage matrix calcification, and increased chondrocyte apoptosis. Mitochondrial dysfunction in OA chondrocytes may derive from somatic mutations in the mtDNA or from the direct effects of proinflammatory mediators such as cytokines, prostaglandins, reactive oxygen species and nitric oxide. Polymorphisms in mtDNA may become useful as biomarkers for the diagnosis and prognosis of OA, and modulation of serum biomarkers by mtDNA haplogroups supports the concept that mtDNA haplogroups may define specific OA phenotypes in the complex OA process.     

关节内骨折后受损软骨细胞凋亡的实验研究

目的：探讨关节内骨折后受损软骨的变化。方法：利用冲击在兔膝关节内造成骨折，利用TUNEL、流式细胞计数，透射电镜、DAPI染色等方法对受损软骨进行动态观察。结果：关节内骨折后2周，受损软骨细胞发生坏死及凋亡。1月后受损软骨细胞仍进行性凋亡。3－6月后，损伤软骨周围的细胞也发生凋亡，损伤部软骨细胞增生，其中部分增生细胞凋亡。结论：关节内骨折后软骨细胞进行性凋亡，可能是创伤后骨关节炎发生的原因之一。     

SOX2基因对骨关节炎软骨细胞凋亡的影响研究

目的 探讨SOX2基因对人骨关节炎（OA）软骨细胞凋亡的影响及机制。方法 以正常软骨组织作为对照,通过Western blotting检测OA软骨组织SOX2蛋白表达。从人OA中分离软骨细胞,参照Lipofectamine TM2000说明将重组体pcDNA3.1-SOX2及空载体pcDNA3.1转染软骨细胞,并设置空白对照组。AG490作为JAK2/STAT3信号通路抑制剂,各组细胞处理48 h,通过流式细胞术、ROS试剂盒分别检测各组细胞凋亡率及ROS水平。Western blotting检测JAK2、p-JAK2、STAT3和p-STAT3的蛋白相对表达量。结果 人OA软骨组织SOX2表达明显低于在正常软骨组织表达（0.065±0.009 vs 0.313±0.028, P<0.05）。转染pcDNA3.1-SOX2的OA软骨细胞SOX2表达明显高于空白组（0.556±0.048 vs 0.122±0.013, P<0.05）。pcDNA3.1-SOX2可明显降低OA软骨细胞凋亡率(3.11±0.42 vs 8.54±0.68)及ROS水平(23.46±2.15 vs 52.67±4.41),上调p-JAK2（0.142±0.013 vs 0.065±0.009）和p-STAT3表达（0.218±0.020 vs 0.126±0.015）（P<0.05）,AG490（15.23±1.13 vs 8.15±0.62）可诱导OA软骨细胞凋亡,而pcDNA3.1-SOX2可减弱AG490对OA软骨细胞凋亡促进作用（P<0.05）。结论 SOX2可抑制OA软骨细胞凋亡,其机制可能与激活JAK2/STAT3信号通路有关。     

Osteoarthritic cartilage fibrillation is associated with a decrease in chrondrocyte adhesion to fibronectin

OBJECTIVE: Cartilage destruction in osteoarthritis (OA) is generally accepted as a failed repair process. Cell adhesion is implicated in tissue repair. Therefore, adhesion of OA chondrocytes to extracellular matrix proteins was investigated. DESIGN: Using chondrocytes from human OA femoral head cartilage, adhesion to fibronectin and type II collagen of cells from distinct areas showing an intact cartilage surface or a fibrillated cartilage surface was studied. Modulation of chondrocyte adhesion by both protein kinase C (PKC) inhibitors and glucosamine sulfate (GS) was also investigated. RESULTS: A significant (P < 0.05) decrease in adhesion to fibronectin of chondrocytes from fibrillated cartilage, relative to those from grossly normal OA cartilage, was demonstrated. Adhesion to type II collagen was not modified by the chondrocyte origins (either from normal or fibrillated OA cartilage). Adhesion to fibronectin of cells from grossly intact cartilage was decreased by the addition of PKC and calmodulin-dependent kinase inhibitors, W7 and sphingosine, to the cell culture. Adhesion to fibronectin of chondrocytes from fibrillated cartilage was significantly (P < 0.05) increased after glucosamine sulfate treatment. CONCLUSION: Fibrillation of cartilage from OA femoral head is associated with a defective adhesion of chondrocytes to fibronectin. The process is suggested to be dependent of PKC and/or calmodulin-dependent kinases and potentially reversible. Conceivably, it could play a role in OA cartilage destruction.     

Apoptosis in osteoarthritis: morphology, mechanisms, and potential means for therapeutic intervention

Osteoarthritis (OA) is the most common chronic joint disorder found in the aging population and is primarily characterized by loss of articular cartilage. Because chondrocytes are essential for maintaining the normal matrix of cartilage, a central hypothesis in ongoing studies of cartilage degeneration in OA has focused on the potential role of cell death. Today, chondrocytic death in OA has been examined in various studies, most of which report an increased amount of apoptotic cell death in OA cartilage. Although the role of apoptosis in OA is not yet clear, the mechanism of apoptosis may offer potentially useful therapeutic targets for the management of this insidious disease.     

骨关节炎早中期关节软骨YKL-40表达与软骨细胞凋亡的关系

背景：软骨细胞凋亡是骨关节炎（osteoarthritis,OA）发病过程中重要的病理学特征。YKL-40是壳质酶蛋白家族的一种糖蛋白,但不具有壳质酶活性,在关节炎软骨、滑膜、巨噬细胞等均有表达,可能与炎症的状态、组织重塑等功能有关。YKL-40在OA早中期中的作用尚有待研究。目的：探讨OA早中期关节软骨YKL-40表达与软骨细胞凋亡率（apoptosisindex,AI）的关系。方法：通过前交叉韧带切断术（anterior cruciate ligament transaction,ACLT）建立SD大鼠膝关节OA模型,组织学评估软骨退变程度,采用改良Mankin评分系统进行评估,免疫组织化学法检测YKL-40的表达情况及软骨细胞AI,观察两个指标的表达特点,分析两者在此病变过程中的关系。结果：随造模时间的延长,软骨出现退变并逐渐加重,YKL-40的表达与软骨细胞AI呈正相关。结论：软骨细胞凋亡是OA早中期的重要事件,YKL-40可能为OA早中期病理过程中软骨细胞凋亡的重要影响因子。     

Hyaluronan inhibits matrix metalloproteinase‐13 in human arthritic chondrocytes via CD44 and P38

We investigated the effects of hyaluronan (HA) on interleukin‐1β (IL‐1β)‐stimulated matrix metalloproteinase (MMP)‐13 production in human chondrocytes from patients with osteoarthritis (OA) or rheumatoid arthritis (RA). Secreted levels of MMP‐13 in conditioned media were detected by immunoblotting, while intracellular MMP‐13 synthesis in articular cartilage was evaluated by immunofluorescence microscopic analysis. Mitogen‐activated protein kinases (MAPKs), p38, extracellular signal‐regulated kinases (ERK), and c‐jun NH2‐terminal kinase (JNK) were assessed by Western blotting. IL‐1β (2 ng/ml) stimulates the secretion of MMP‐13 in both OA and RA chondrocytes. Inhibition studies using specific MAPK inhibitors revealed that IL‐1β induced MMP‐13 via p38 in both OA and RA chondrocytes. HA down‐regulates IL‐1β‐stimulated MMP‐13 and phosphorylated p38 (p‐p38) in a dose‐dependent manner (0.1, 1, 2, and 4 mg/ml). When used at 4 mg/ml, HA inhibits p‐p38 phosphorylation by more than 60%. In response to IL‐1β, RA chondrocytes express a higher level of p‐p38 than that of OA chondrocytes. Inhibition of CD44, using a blocking antibody, significantly reversed the inhibitory effect of HA on both MMP‐13 and p‐p38. Our study clearly shows that HA inhibits IL‐1β‐induced MMP‐13 via its principal receptor, CD44, and subsequent intracellular p38 MAPK signaling in OA and RA chondrocytes. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:258–264, 2011     

关节游离体软骨细胞生物学特性的初步观察

目的观察关节游离体软骨细胞的生物学特性，寻找组织工程软骨种子细胞来源。方法分别取游离体软骨标本5例，正常软骨标本2例和骨关节炎软骨标本6例，组织学观察其细胞分布、尿嘧啶脱氧核苷末端转移酶介导的三磷酸脱氧核苷缺口标记法观察细胞凋亡，细胞分离培养观察原代软骨细胞形态，并进行比较。结果与正常软骨、骨关节炎软骨比较，游离体软骨细胞分布密度较高，细胞凋亡明显．体外培养时原代细胞已呈成纤维细胞样形态。结论游离体软骨细胞已具有成纤维细胞特性，不宜直接作为组织工程的种子细胞来源。     

Chondrocyte death during murine osteoarthritis

OBJECTIVE: To determine whether chondrocyte apoptosis occurs during the progression of osteoarthritis (OA) in the STR/ort mouse model of OA.METHODS: Serial cryostat sections were cut (10 microns) through the knee joint of young and old male STR/ort mice and graded for the severity of OA lesions. Age- and sex-matched CBA mice were used as controls. Apoptotic chondrocytes were detected using the TUNEL assay. Ultrastructural changes were examined using electron microscopy (EM). Expression of biochemical markers associated with apoptosis (bax, bcl-2 and caspases-3, -8 & -9) was investigated using immunohistochemistry.RESULTS: TUNEL assays on histological sections of STR/ort knee joints showed that the number of TUNEL-positive chondrocytes in the tibial medial articular cartilage correlated with the severity of the OA damage. These cells were located close to the lesional area. Only very occasional TUNEL positive chondrocytes were detected in either morphologically normal STR/ort cartilage or in control CBA cartilage. Ultrastructural analysis of chondrocytes neighboring focal osteoarthritic lesions in STR/ort tibial cartilage revealed an abundance of abnormal cells exhibiting numerous morphological changes. These resembled, but in some cases differed, from changes reported in classical apoptosis. The changes include abnormal distribution of chromatin, cell shrinkage, membrane blebbing and deposition of cell remnants (apoptotic bodies) in the lacuna space. Despite the TUNEL and EM changes, immunohistochemistry failed to detect any changes in the ratio of bax to bcl-2 in tibial chondrocytes of STR/ort mice. Both bcl-2 and bax levels decreased with age in morphologically normal STR/ort and control CBA cartilage. None of the caspases tested for was detected in tibial chondrocytes of either strain.CONCLUSION: Chondrocyte cell death is correlated with the progression of OA in STR/ort mice and has many of the morphological characteristics of classical apoptosis. Absence of changes in bax to bcl-2 ratio in STR/ort chondrocytes indicate that the mitochondrial pathway of apoptosis is unlikely to be involved. Failure to detect caspases could be due to low levels of enzyme expression, expression within a very brief time period, or to a caspase-independent mechanism of cell death.