生长因子在关节软骨损伤修复中的应用进展

关节是人体重要的运动器官,其组织结构特殊,成熟关节软骨组织没有血供,一旦损伤难以自愈.临床上常用的关节软骨损伤修复手段有微骨裂术、关节软骨移植、关节置换术、软骨组织工程等,但这些方法的修复效果都不理想.生长因子是体内组织分泌的一种具有生物活性的物质,可促进细胞生长、增殖、迁移和分化.软骨发育过程中有许多生长因子参与,如成纤维细胞生长因子(FGF)、骨形态发生蛋白(BMP)、胰岛素样生长因子(IGF)等.研究显示,在关节软骨修复过程中加入外源性生长因子可有效促进关节软骨损伤的修复.对目前应用于关节软骨损伤修复研究中的生长因子进行综述,讨论这些生长因子在关节软骨发育及其在关节软骨修复中的作用,分析生长因子在关节软骨修复应用中存在的问题.     

Distribution of chondrocytes containing alpha-smooth muscle actin in human normal, osteoarthrotic, and transplanted articular cartilage

The aim of our study was to evaluate the occurrence of chondrocytes containing alpha-smooth muscle actin in human normal and diseased cartilage. Immunohistochemistry using monoclonal antibodies for alpha-smooth actin, muscle-specific actin, S-100 protein, CD 34, and desmin was performed on samples of human articular cartilage obtained at autopsy following sudden death, during total hip and knee replacement for osteoarthritis, or after femoral neck fracture in patients without symptoms of osteoarthritis. Moreover, the layers of residual cartilage from chondral posttraumatic defects obtained during preoperative arthroscopy and of newly formed cartilage after autologous-chondrocyte transplantation (Hyalograft C) obtained during second-look arthroscopy were also examined by immunohistochemistry and RT PCR. Our study showed that a significant percentage of articular chondrocytes express alpha-smooth muscle actin in healthy, diseased, and regenerated articular cartilage. Alpha-actin positive chondrocytes (18%) were observed predominantly in the upper zone of normal articular cartilage. By contrast, only approximately 10% of cartilage cells in the deep region stained for this contractile actin isoform. Actin-positive chondrocytes (myochondrocytes) are formed predominantly in response to injury to the osteoarthrotic cartilage, at sites of defective healing, and in newly formed cartilage after autologous chondrocyte transplantation. Fibrocartilage is present in some of these conditions, and it is known that this tissue contains chondrocytes with actin. The presence of myochondrocytes in the surface layer of normal articular cartilage indicates that this region probably plays an important role in maintaining cartilage integrity. Myochondrocytes may utilize the contractile actin isoform in manipulating the extracellular matrix of articular cartilage. It is also possible that actin-containing chondrocytes have a higher potential for regeneration in contrast to chondrocytes that do not contain this contractile material in their cytoplasm.     

Lectin-binding in normal and fibrillated articular cartilage of human patellae

Summary Fluorescein-isothiocyanate (FITC) labeled lectins were used to study the distribution of specific binding-sites in histological sections of normal and fibrillated articular cartilage of human patellae.It has been shown that normal articular cartilage reveals lectin binding-sites for Concanavalin A (Con A) and wheat germ agglutinin (WGA), but not for soybean agglutinin (SBA), peanut agglutinin (PNA) and Ulex europaeus agglutinin (UEA).In fibrillated cartilage the distribution pattern of Con A and WGA is completely changed. SBA, PNA and UEA show a distinct staining pattern in particular in the fibrillated areas of degenerated cartilage. Lectin-staining of the extracellular matrix and the chondrocytes in both normal and fibrillated cartilage did not show any correlation with material that was either PAS- or Alcian blue-positive. In comparison with the conventional PAS- and Alcian blue reaction lectin-staining proved to be superior.Visualization of intra- and extracellular glycoconjugate-changes in normal and fibrillated cartilage in areas with no PAS and/or Alcian blue staining indicates that all layers of the cartilage are involved in the pathological process.It is evident that lectins can demonstrate minute differences between normal and arthrotic cartilage and we therefore conclude that lectins are sensitive and specific tools for the study of degenerative joint diseases.     

Development of a biocomposite to fill out articular cartilage lesions. Light, scanning and transmission electron microscopy of sheep chondrocytes cultured on a collagen I/III sponge

The regenerative capacity of hyaline articular cartilage is limited. Thus, lesions of this tissue are a proarthrotic factor, and up to now the conservative treatment of cartilage lesions and arthrosis does not yield satisfying results. Therefore, autologous transplantation of articular chondrocytes is being investigated in a variety of different assays. The aim of our study was to create a mechanically stable cell-matrix implant with viable and active chondrocytes which could serve to fill out articular lesions created in the knees of sheep. For this purpose, articular cartilage was collected from knee lesions, chondrocytes were liberated enzymatically and seeded in culture flasks and cultured till confluency. Cells were then trypsinized and grown on a type I/III collagen matrix (Chondro-Gide™, Geistlich Biomaterials, Wolhusen, Switzerland) for 3, 6 and 10 days before being fixed and embedded for electron microscopy by routine methods. Scanning electron microscopy was performed after dehydration in acetone, critical point drying and sputter-coating with gold-paladium.

Light microscopically, clusters of chondrocytes can be seen on the surface of the matrix with a few cells growing into the matrix. Transmission electron microscopic photographs yield a rather differentiated chondrocyte-like appearance, which is evidence of a matrix-induced redifferentiation after dedifferentiation during the growth period in the culture flasks. Scanning electron microscopic results show large, flattened chondrocytes without signs of differentiation on plastic, whereas chondrocytes grown on the Chondro-Gide™ sponge show a more roundish aspect wrapping firmly around the collagen fibrils, exhibiting numerous contacts with the matrix. This cell-matrix biocomposite can now serve to fill out articular cartilage lesions created in the knees of sheep.     

Origin-associated features of chondrocytes in mouse Meckel''s cartilage and costal cartilage: an in vitro study

Using a cell culture method, we histochemically and immunohistochemically investigated whether chondrocytes deriving from different origins, such as Meckel's or costal cartilages, express similar phenotypic characteristics. Chondrocytes isolated enzymatically from Meckel's and costal cartilages of 17-day embryonic mice both actively proliferated and formed cartilage nodules consisting of toluidine blue-positive proteoglycans and type II collagen. Both deposited calcified cartilaginous matrix as revealed by alkaline phosphatase (ALPase) activity and alizarin red staining throughout 3 weeks in culture. Immunostaining for osteopontin (OP), osteocalcin (OC), and osteonectin (ON) revealed that chondrocytes from both cartilages were positive for their proteins, but type I collagen was detected only in cells transforming from Meckel's chondrocytes late in the culture. Electron microscopy demonstrated that although costal and Meckel's chondrocytes had typical chondrocytic features during 2 weeks in culture, Meckel's chondrocytes transformed into osteocytic cells that produced thick, banded type I collagen fibrils. In contrast, costal chondrocytes maintained typical hypertrophic morphology throughout the final stage of culture. The present study suggests that Meckel's chondrocytes derived from neural crest-ectomesenchyme retain osteogenic potential, and differ from costal chondrocytes originating from mesoderm.     

The mechanical and material properties of elderly human articular cartilage subject to impact and slow loading

The mechanical properties of articular cartilage vary enormously with loading rate, and how these properties derive from the composition and structure of the tissue is still unclear. This study investigates the mechanical properties of human articular cartilage at rapid rates of loading, compares these with measurements at slow rates of loading and explores how they relate to the gross composition of the tissue. Full-depth femoral head cartilage biopsies were subjected to a slow, unconfined compression test followed by an impact at an energy of 78.5 mJ and velocity 1.25 m s⁻¹. The modulus was calculated from the slope of the loading curve and the coefficient of restitution from the areas under the loading and unloading curves.Tissue composition was measured as water, collagen and glycosaminoglycan contents. The maximum dynamic modulus ranged from 25 to 150 MPa. These values compared with 1–3 MPa measured during quasi-static loading. The coefficient of restitution was 0.502 (0.066) (mean (standard deviation)) and showed no site variation. Water loss was not detectable. Composition was not strongly associated with modulus; water and collagen contents together predicted about 25% of the variance in modulus.     

Senescent chondrogenic progenitor cells derived from articular cartilage of knee osteoarthritis patients contributes to senescence-associated secretory phenotype via release of IL-6 and IL-8

ObjectivesDespite the presence of chondrogenic progenitor cells (CPCs) in knee osteoarthritis patients they are unable to repair the damaged cartilage. This study aimed to evaluate the oxidative stress, cellular senescence, and senescence-associated secretory phenotype (SASP) in the CPCs derived from osteoarthritic cartilage and compare with the CPCs of healthy articular cartilage.MethodsIsolated CPCs were characterized based on phenotypic expression of stem cell markers, clonogenicity, and tri-lineage differentiation assay. Production of ROS was measured using DCFDA assay. Cellular senescence in CPCs was assessed by senescence-associated beta-galactosidase assay and expression of senescence markers at the gene level using real-time PCR. Morphological features associated with senescent OA-CPCs were studied using scanning electron microscopy. To study SASP, the production of inflammatory cytokines was assessed in the culture supernatant using a flow-cytometer based cytometric bead array.ResultsOA-CPCs exhibited elevated ROS levels along with a relatively high percentage of senescent cells compared to non-OA CPCs, and a positive correlation exists between ROS production and senescence. The morphological assessment of senescent CPCs revealed increased cell size and multiple nuclei in senescent OA-CPCs. These results were further validated by elevated expression of senescence genes p16, p21, and p53. Additionally, culture supernatant of senescent OA-CPCs expressed IL-6 and IL-8 cytokines indicative of SASP.ConclusionsDespite exhibiting similar expression of stem cell markers and clonogenicity, CPCs undergo oxidative stress in diseased knee joint leading to increased production of intracellular ROS in chondrogenic progenitor cells that support cellular senescence. Further, senescence in OA-CPCs is mediated via the release of pro-inflammatory cytokines, IL-6 and IL-8.     

Some biochemical characteristics and water exchange in human articular cartilage in osteoarthrosis

Rearrangement of intra- and intermolecular bonds in collagen molecule, disaggregation of proteoglycans and their elimination from cartilage involved in osteoarthrosis are responsible for water accumulation and its increased mobility in cartilage.     

Morphological characteristics of the life cycle of resting cartilage cells in mouse rib investigated in intrasplenic isografts

Summary Resting cartilages taken from 2-day-old mouse ribs were transplanted into spleens in order to carry out morphological investigations of the life cycles of their chondrocytes. The explants were isografted for periods of up to 60 days and examined at light and electron microscopic levels, using von Kossa's reaction or osmium-potassium ferrocyanide (OPF) fixation. By day 3 after transplantation, resting cartilage containing immature chondrocytes was well adapted to splenic tissue and by 7 days after transplantation these chondrocytes had changed into early hypertrophic chondrocytes containing large vacuoles, glycogen aggregates and abundant secretory organelles. It was also demonstrated by von Kossa's reaction that the initial calcification occurred in the territorial matrix during this period. In spite of the hypertrophic chondrocytes in the central zone being surrounded by an extensively calcified matrix during days 14–21 after transplantation, these cells had well-preserved organized organelles, except that Golgi-associated elements and endoplasmic reticulum revealed a tendency toward degenerative changes. With increased duration of the grafting period, from 30–60 days, the calcification zone progressed gradually, and the number of hypertrophic chondrocytes embedded in the calcified matrix decreased considerably. By day 60, degenerating hypertrophic chondrocytes of two types were distinguished: flattened cells containing large vacuoles, poorly developed Golgi apparatuses, and rough endoplasmic reticulum; and shrunken dark cells displaying terminal hypertrophy. During the present study, we observed no vascular invasion into the calcified matrix, or appearance of bone-related cells, and the morphological changes from the resting chondrocytes to cellular hypertrophy accompanied by the formation of a calcified matrix were observed at day 60. These findings indicate that resting cartilage cells of the mouse have the capacity for terminal differentiation when transplanted into the spleen.     

N,N-dicarboxymethyl chitosan as delivery agent for bone morphogenetic protein in the repair of articular cartilage

Bone morphogenetic protein (BMP), associated with N,N-dicarboxymethyl chitosan, is used to induce or facilitate the repair of articular cartilage lesions. This association is intended for the synergistic potentiation of the respective biological effects. Data show that BMP-7 enhances the in vivo proliferation of cells with chondrocytes phenotype in the articular environment, leading to partial healing of the articular surface of the lesions. N,N-dicarboxymethyl chitosan is found to be useful as a molecular carrier or drug delivery agent.     

Correlation between the surface area of synovial membrane and the surface area of articular cartilage in synovial joints of the mouse and human

Background  Articular cartilage is avascular. Evidence suggests that in mature animals synovial fluid is the dominant source of nutrition for articular cartilage. If so, there might be a direct correlation between the surface area of synovial membrane and the surface area of articular cartilage in synovial joints. Methods  The hindlimb joints of young adult Swiss Webster mice were prepared for histology and sagitally sectioned at regular intervals. The surface area of synovial membrane and the surface area and volume of articular cartilage were calculated from serial photomicrographs of knee, ankle, and first metatarsophalangeal (MTP) joints using ImageJ software and Cavalieri’s method. Sagittal E12 slices from numerous synovial joints in a single human cadaver were similarly analysed. Results  There was a strong statistically significant positive linear correlation between the surface area of synovial membrane and articular cartilage in the hindlimb joints of the mouse (r = 0.96, 95% CI 0.90–0.98, P < 0.0001). A similarly strong highly statistically significant corelation was observed between the surface area of synovial membrane and volume of articular cartilage. This relationship was also observed across a wider range of synovial joints in the human (r = 0.83, 95% CI 0.48–0.95, P = 0.0009). All analyses remained highly statistically significant after adjusting the standard errors and consequent P values for the linear models based on multiple observations in the same subject. Conclusions  This study demonstrates for the first time that there is a direct positive linear correlation between the surface area of synovial membrane and the surface area of articular cartilage in synovial joints in the mouse and human. These novel findings support the concept that the nutrition of mature articular cartilage is dependent on synovial fluid and may also explain why some joints communicate with surrounding bursae. Perhaps more consideration should be given to synovial membrane when studying the pathology of articular cartilage.     

The distribution of Notch receptors and their ligands during articular cartilage development

We examined the distribution of Notch family members and their ligands during the development of articular cartilage and the growth plate. Notch 1 was expressed by the chondrocytes of the developing articular surface but became increasingly restricted to the deeper layers after birth whilst expression of this family member was restricted to hypertrophic chondrocytes in the growth plate. Notch 2 and 4, Delta and Jagged 2 showed a broadly similar distribution, being present throughout the articular cartilage during development and becoming increasingly restricted to deeper layers with age. Hypertrophic chondrocytes within the growth plate also expressed Notch 2 and 4, Delta and Jagged 2 (which was also expressed in prehypertrophs). Notch 3 and Jagged 1 were absent from developing articular cartilage but were present in deeper layers at later time points (> 1 month) and both receptor and ligand were expressed in hypertrophic chondrocytes at all ages examined. These results highlight the complex Notch signalling interactions that result in the formation of the heterogeneous articular cartilage and allow for the co-ordinated ossification and elongation of the growth plate. Mechanisms by which these processes are controlled are discussed in light of recent advances in the understanding of Notch signalling pathways.     

The surface contour of articular cartilage in an intact, loaded joint

The friction coefficients measured in diarthrodial joints are small. Theories of joint lubrication attribute this efficiency to entrapment or movement of synovial fluid, yet anatomical models of the surface are based on studies of isolated fragments of cartilage, not functional joints. To investigate the functional interrelationship of joint surfaces and synovial fluid, the ultrastructure of loaded joints was examined. Twenty-four New Zealand white rabbit knee joints were loaded either statically or moved ex vivo using simulated muscle forces and then plunge-frozen under load. After fixation in the frozen/loaded state by freeze-substitution fixation, the medial joint compartments were embedded in epoxy resin while still articulated. Bone was trimmed away from the articular surfaces, permitting the cartilage to be sectioned for light and electron microscopy. These joint surfaces were then compared with controls which were not loaded, not moved or had been disarticulated prior to embedding. Articular surfaces of loaded joints were smooth at magnifications from ×35 to ×7500, whereas the tibial surfaces of nonloaded joints were irregular. Small pools of joint fluid were observed at the meniscal edge and beneath the anterior horn of the meniscus. At magnifications of ×40000, the joint surfaces were separated by a uniform 100 nm space containing fluid. An amorphous, electron dense articular surface lamina was present but, when loaded, was thicker and flatter than previously reported. No surface pits or bumps were visible in embedded, loaded joints. This is the first ultrastructural study of intact loaded joints. The findings suggest that fluid film lubrication is present in diarthrodial joints, but the fluid sequestration postulated in several models is not apparent.     

人外周血内皮祖细胞的分离、培养及鉴定

背景：内皮祖细胞是成熟内皮细胞的前体细胞,具有新生血管和新生内皮化作用,在许多方面均有广泛应用前景,但其生物学特征及鉴定方法仍存争议。 目的：探索从人外周血分离培养内皮祖细胞的方法并鉴定其生物学特征。 方法：外周采血后应用密度梯度离心法分离成人外周血单核细胞,在内皮细胞全培养基重悬后接种于纤维连接蛋白包被的培养瓶中,体外培养扩增获取人内皮祖细胞并观察其形态变化、生长增殖潜能及细胞表面抗原表达情况,并通过细胞一氧化氮分泌功能测定及体外血管形成实验检测其功能学特征。 结果与结论：体外诱导培养后,六七天形成纺锤样细胞簇,两三周黏附细胞发育形成鹅卵石样外观细胞,逐渐融合呈外生性生长。在相同培养条件下,与人主动脉内皮细胞相比人内皮祖细胞具有高的增殖潜能。人内皮祖细胞表达CD31、CD34、CD144、KDR,表现为典型内皮细胞系表型,此外细胞可摄取ac-LDL并结合UEA-Ⅰ。在功能上内皮祖细胞可分泌一氧化氮并可在Matrigel中形成管腔样结构。提示通过密度梯度离心法分离人外周血单核细胞体外黏附诱导培养可获取人外周血内皮祖细胞;细胞形态、增殖能力、生物表型特征结合细胞功能学的综合性鉴定方法用于内皮祖细胞的鉴定具有一定意义。     

Influences of the depth-dependent material inhomogeneity of articular cartilage on the fluid pressurization in the human knee

The material properties of articular cartilage are depth-dependent, i.e. they differ in the superficial, middle and deep zones. The role of this depth-dependent material inhomogeneity in the poromechanical response of the knee joint has not been investigated with patient-specific joint modeling. In the present study, the depth-dependent and site-specific material properties were incorporated in an anatomically accurate knee model that consisted of the distal femur, femoral cartilage, menisci, tibial cartilage and proximal tibia. The collagen fibers, proteoglycan matrix and fluid in articular cartilage and menisci were considered as distinct constituents. The fluid pressurization in the knee was determined with finite element analysis. The results demonstrated the influences of the depth-dependent inhomogeneity on the fluid pressurization, compressive stress, first principal stress and strain along the tissue depth. The depth-dependent inhomogeneity enhanced the fluid support to loading in the superficial zone by raising the fluid pressure and lowering the compressive effective stress at the same time. The depth-dependence also reduced the tensile stress and strain at the cartilage–bone interface. The present 3D modeling revealed a complex fluid pressurization and 3D stresses that depended on the mechanical contact and relaxation time, which could not be predicted by existing 2D models from the literature. The greatest fluid pressure was observed in the medial condyle, regardless of the depth-dependent inhomogeneity. The results indicated the roles of the tissue inhomogeneity in reducing deep tissue fractures, protecting the superficial tissue from excessive compressive stress and improving the lubrication in the joint.     

Development of articular cartilage and the metaphyseal growth plate: the localization of TRAP cells, VEGF, and endostatin

During long bone development the original cartilaginous model in mammals is replaced by bone, but at the long bone endings the avascular articular cartilage remains. Before the articular cartilage attains structural maturity it undergoes reorganization, and molecules such as vascular endothelial growth factor (VEGF) and endostatin could be involved in this process. VEGF attracts blood vessels, whereas endostatin blocks their formation. The present study therefore focused on the spatio-temporal localization of these two molecules during the development of the articular cartilage. Furthermore, we investigated the distribution of the chondro/osteoclasts at the chondro-osseous junction of the articular cartilage with the subchondral bone. Mice served as our animal model, and we examined several postnatal stages of the femur starting with week (W) 4. Our results indicated that during the formation of the articular cartilage, VEGF and endostatin had an overlapping localization. The former molecule was, however, down-regulated, whereas the latter was uniformly intensely localized until W12. At the chondro-osseous junction, the number of tartrate-resistant acid phosphatase (TRAP)-positive chondro/osteoclasts declined with increasing age. Until W3 the articular cartilage was not well organized but at W8 it appeared structurally mature. At that time only a few TRAP cells were present, indicative of a low resorptive activity at the chondro-osseous junction. Subsequently, we examined the metaphyseal growth plate that is closed when skeletal maturity is attained. Within its hypertrophic zone, localization of endostatin and VEGF was observed until W6 and W8, respectively. At the chondro-osseous junction of the growth plate, chondro/osteoclasts remained numerous until W12 to allow for its complete resorption. According to former findings, VEGF is critical for a normal skeleton development, whereas endostatin has almost no effect on this process. In conclusion, our findings suggest that both VEGF and endostatin play a role in the structural reorganization of the articular cartilage and endostatin may be involved in the maintenance of its avascularity. In the growth plate, however, endostatin does not appear to counteract VEGF, allowing vascular invasion of hypertrophic cartilage and bone growth.     

The elastic network of articular cartilage: an immunohistochemical study of elastin fibres and microfibrils

The elastic network of articular cartilage was investigated by immunohistochemistry using specific antibodies to elastin and fibrillin‐1. Articular cartilage was dissected from defined regions of bovine metacarpophalangeal joints. Elastin fibres and microfibrils were dual‐immunostained by labelling with distinct fluorescent dyes. A conventional fluorescence microscope combined with a polarized light filter was used to study the organization and degree of colocalization of elastin fibres, microfibrils and of the collagen network. We observed an elaborately organized elastic network. In the uppermost superficial zone, where few cells were present, elastin fibres and microfibrils formed a dense three dimensional network showing some degree of colocalization. The thickness and organization of this elastic network varied dramatically from region to region and was most extensive in the metacarpal palmar region. In the middle and deep zones, very few elastin fibres were observed but microfibrils formed a network in the inter‐territorial matrix and dense network around the cells. Our finding of a three dimensional network of dense, well organized elastin fibres and microfibrils in the surface zone of the articular cartilage matrix, and a dense network of microfibrils around the cells deeper into the tissue suggests the elastic network could play both a mechanical and a biological role in articular cartilage.     

Adaptive filtering, modelling and classification of knee joint vibroarthrographic signals for non-invasive diagnosis of articular cartilage pathology

Interpretation of vibrations or sound signals emitted from the patellofemoral joint during movement of the knee, also known as vibroarthrography (VAG), could lead to a safe, objective, and non-invasive clinical tool for early detection, localisation, and quantification of articular cartilage disorders. In this study with a reasonably large database of VAG signals of 90 human knee joints (51 normal and 39 abnormal), a new technique for adaptive segmentation based on the recursive least squares lattice (RLSL) algorithm was developed to segment the non-stationary VAG signals into locally-stationary components; the stationary components were then modelled autoregressively, using the Burg-Lattice method. Logistic classification of the primary VAG signals into normal and abnormal signals (with no restriction on the type of cartilage pathology) using only the AR coefficients as discriminant features provided an accuracy of 68.9% with the leave-one-out method. When the abnormal signals were restricted to chondromalacia patella only, the classification accuracy rate increased to 84.5%. The effects of muscle contraction interference (MCI) on VAG signals were analysed using signals from 53 subjects (32 normal and 21 abnormal), and it was found that adaptive filtering of the MCI from the primary VAG signals did not improve the classification accuracy rate. The results indicate that VAG is a potential diagnostic tool for screening for chondromalacia patella.     

Chondrocytes synthesize type I collagen and accumulate the protein in the matrix during development of rat tibial articular cartilage

The present study was designed to investigate whether or not chondrocytes in articular cartilage express type I collagen in vivo under physiological conditions. Expressions of the gene and the phenotype of type I collagen were examined in rat tibial articular cartilage in the knee joint during development. Knee joints of Wistar rats at 1, 5, and 11 weeks postnatal were fixed in 4% paraformaldehyde with or without 0.5% glutaraldehyde and decalcified in 10% EDTA. After the specimens were embedded in paraffin and serial sections made, adjacent sections were processed for immunohistochemistry and in situ hybridization for type I collagen. The epiphysis of the tibia was composed of cartilage in week-1 rats. Formation of articular cartilage was in progress in week 5 as endochondral ossification proceeded and was completed in week 11. Anti-type I collagen antibody stained only the superficial area of the epiphysis in week 1, but the immunoreactivity was expanded into the deeper region of the articular cartilage with development in weeks 5 and 11. Hybridization signals for pro-alpha 1 (I) collagen were seen in some of chondrocytes in the epiphysis of the week-1 tibia. The most intense signals were identified in chondrocytes in week 5 and the signals appeared weaker in week 11. The present study demonstrated that chondrocytes synthesize type I collagen and accumulate the protein in the matrix during development of the articular cartilage.