Biochemical characteristics and biological significance of the genetically-distinct collagens

Decellularization compromises mechanical and structural properties of the native trachea

Tracheal replacement using tissue engineering technologies offers great potential to improve previously intractable clinical interventions, and interest in this area has increased in recent years. Many engineered airway constructs currently rely on decellularized native tracheas to serve as the scaffold for tissue repair. Yet, mechanical failure leading to airway narrowing and collapse remains a major cause of morbidity and mortality following clinical implantation of decellularized tracheal grafts. To understand better the factors contributing to mechanical failure in vivo, we characterized the histo-mechanical properties of tracheas following two different decellularization protocols, including one that has been used clinically. All decellularized tracheas deviated from native mechanical behavior, which may provide insights into observed in vivo graft failures. We further analyzed protein content by western blot and analyzed microstructure by histological staining and found that the specific method of decellularization resulted in significant differences in the depletion of proteoglycans and degradation of collagens I, II, III, and elastin. Taken together, this work demonstrates that the heterogeneous architecture and mechanical behavior of the trachea is severely compromised by decellularization. Such structural deterioration may contribute to graft failure clinically and limit the potential of decellularized native tracheas as viable long-term orthotopic airway replacements.

Gelatin as It Is: History and Modernity

The data concerning the synthesis and physicochemical characteristics of one of the practically important proteins-gelatin, as well as the possibilities of its practical application, are systematized and discussed. When considering the latter, emphasis is placed on the use of gelatin in those areas of science and technology that are associated with the specifics of the spatial/molecular structure of this high-molecular compound, namely, as a binder for the silver halide photographic process, immobilized matrix systems with a nano-level organization of an immobilized substance, matrices for creating pharmaceutical/dosage forms and protein-based nanosystems. It was concluded that the use of this protein is promising in the future.

Gelatin Matrix as Functional Biomaterial for Immobilization of Nanoparticles of Metal-Containing Compounds

The data concerning the synthesis and physicochemical characteristics of specific functional biomaterials-biopolymer-immobilized matrix systems based on gelatin as an array and chemical compounds, which include atoms of various metal elements-are systematized and discussed. The features of this biopolymer which determine the specific properties of the immobilized matrix systems formed by it and their reactivity, are noted. Data on gelatin-immobilized systems in which immobilized substances are elemental metals and coordination compounds formed as a result of redox processes, nucleophilic/electrophilic substitution reactions, and self-assembly (template synthesis), are presented. The possibilities of the practical use of metal-containing gelatin-immobilized systems are promising for the future; in particular, their potential in medicine and pharmacology as a vehicle for "targeted" drug delivery to various internal organs/tissues of the body, and, also, as potential biosensors is noted.

Collagen fibril formation in vitro: From origin to opportunities

Sometimes, to move forward, it is necessary to look back. Collagen type I is one of the most commonly used biomaterials in tissue engineering and regenerative medicine. There are a variety of collagen scaffolds and biomedical products based on collagen have been made, and the development of new ones is still ongoing. Materials, where collagen is in the fibrillar form, have some advantages: they have superior mechanical properties, higher degradation time and, what is most important, mimic the structure of the native extracellular matrix. There are some standard protocols for the formation of collagen fibrils in vitro, but if we look more carefully at those methods, we can see some controversies. For example, why is the formation of collagen gel commonly carried out at 37 ​°C, when it was well investigated that the temperature higher than 35 ​°C results in a formation of not well-ordered fibrils? Biomimetic collagen materials can be obtained both using culture medium or neutralizing solution, but it requires a deep understanding of all of the crucial points. One of this point is collagen extraction method, since not every method retains the ability of collagen to reconstitute native banded fibrils. Collagen polymorphism is also often overlooked in spite of the appearance of different polymorphic forms during fibril formation is possible, especially when collagen blends are utilized. In this review, we will not only pay attention to these issues, but we will overview the most prominent works related to the formation of collagen fibrils in vitro starting from the first approaches and moving to the up-to-date recipes.

Picrosirius Red Staining: Revisiting Its Application to the Qualitative and Quantitative Assessment of Collagen Type I and Type III in Tendon

Collagen has a major role in the structural organization of tendons. Picrosirius red (PSR) staining viewed under polarized light microscopy is the standard method to evaluate the organization of collagen fibers in tissues. It is also used to distinguish between type I and type III collagen in tissue sections. However, accurate analysis and interpretation of PSR images are challenging because of technical factors and historical misconceptions. The aim of this study was to clarify whether collagen types I and III can be distinguished by PSR staining in rat Achilles tendons, using double immunohistochemistry as the positive control. Our findings showed that PSR staining viewed with polarized light microscopy was suitable for qualitative and quantitative assessment of total collagen but was not able to distinguish collagen types. We found it critical to use a polarizing microscope equipped with a rotating stage; tendon section orientation at 45° with respect to crossed polarizers was optimal for the qualitative and quantitative assessment of collagen organization. Immunohistochemistry was superior to PSR staining for detection of collagen type III. We also compared formalin and Bouin solution as fixatives. Both produced similar birefringence, but formalin-fixed tendons provided higher quality histological detail with both hematoxylin-eosin and immunostaining.

PICROSIRIUS RED AND MASSON’S TRICHROME STAINING TECHNIQUES AS TOOLS FOR DETECTION OF COLLAGEN FIBERS IN THE SKIN OF DOGS WITH ENDOCRINE DERMATOPATHOLOGIES

Abstract Canine endocrinopathies, such as hypothyroidism and hyperadrenocortism,induce typical dermatological alterations. Collagen fibers are significant for the maintenance of structural integrity,as well as in the determination of tissue function. This study aimed at assessing the coloration caused by Picrosirius Red staining under circular polarization and Masson Trichrome staining, as tools to quantify the total collagen in the skin of dogs exhibiting endocrine dermatopathies. Skin samples taken from dogs with hypothyroidism and hyperadrenocorticism were stained using Hematoxylin and Eosin (HE), Masson’s Trichrome (MT) and Picrosirius Red (PSR). The animals with hyperadrenocorticism revealed a higher percentage of collagen area than did the animals with hypothyroidism PSRp (hypothyroidism = 43.76 ± 0.8255 and hyperadrenocorticism = 47.08 ± 0.8584). The percentage of the collagen area using MT between the groups is given in parenthesis (hypothyroidism = 52.06 ± 0.9371, hyperadrenocorticism = 61 ± 0.7529 and control = 56.88 ± 0.64) (p <0.05). It is therefore, evident that the special stains employed are useful in estimating the percentage of collagen area in the skin, as they revealed that dogs with hypothyroidism had lower collagen deposition whereas those animals with hyperadrenocorticism showed higher quantity of collagen in the dermis.

Raman Spectroscopy as a Tool for Quality and Sterility Analysis for Tissue Engineering Applications like Cartilage Transplants

At present, the production of tissue engineered cartilage requires the concurrent production of two identical transplants. One transplant is used for destructive quality control and the second one is implanted into the patient. A non-invasive characterization of such tissue engineering samples would be a promising tool to achieve a production process of just one transplant that is both implanted and tested. Raman spectroscopy is a method that satisfies this requirement by analyzing cells without lysis, fixation or the use of any chemicals. This pure optical technique is based on inelastic scattering of laser photons by molecular vibrations of biopolymers. Characteristic peaks in Raman spectra of cells could be assigned to typical biochemical molecules present in biological samples. For the analysis of chondrocytes present in cartilage transplants, the determination of the cell vitality as well as the discrimination of another cell type have been studied by Raman spectroscopy. Another bottleneck in such biological processes under GMP conditions is sterility control, as most of the commonly used methods require long cultivation times. Raman spectroscopy provides a good alternative to conventional methods in terms of time saving. In this study, the potential of Raman spectroscopy as a quality and sterility control tool for tissue engineering applications was studied by analyzing and comparing the spectra of cell and bacteria cultures.

Modeling IL-1 induced degradation of articular cartilage

In this study, we develop a computational model to simulate the in vitro biochemical degradation of articular cartilage explants sourced from the femoropatellar grooves of bovine calves. Cartilage explants were incubated in culture medium with and without the inflammatory cytokine IL-1α. The spatio-temporal evolution of the cartilage explant's extracellular matrix components is modelled. Key variables in the model include chondrocytes, aggrecan, collagen, aggrecanase, collagenase and IL-1α. The model is first calibrated for aggrecan homeostasis of cartilage in vivo, then for data on (explant) controls, and finally for data on the IL-1α driven proteolysis of aggrecan and collagen over a 4-week period. The model was found to fit the experimental data best when: (i) chondrocytes continue to synthesize aggrecan during the cytokine challenge, (ii) a one to two day delay is introduced between the addition of IL-1α to the culture medium and subsequent aggrecanolysis, (iii) collagen degradation does not commence until the total concentration of aggrecan (i.e. both intact and degraded aggrecan) at any specific location within the explant becomes ≤ 1.5 mg/ml and (iv) degraded aggrecan formed due to the IL-1α induced proteolysis of intact aggrecan protects the collagen network while collagen degrades in a two-step process which, together, significantly modulate the collagen network degradation. Under simulated in vivo conditions, the model predicts increased aggrecan turnover rates in the presence of synovial IL-1α, consistent with experimental observations. Such models may help to infer the course of events in vivo following traumatic joint injury, and may also prove useful in quantitatively evaluating the efficiency of various therapeutic molecules that could be employed to avoid or modify the course of cartilage disease states.

Biophysical cues and cell behavior: the big impact of little things

The extracellular matrix is composed of a variety of proteins, polysaccharides, and glycosaminoglycans that self-assemble into a hierarchical order of nanometer- to micrometer-scale fibrils and fibers. The shapes, sizes, and elasticity present within this highly ordered meshwork regulate behaviors in most cell types. It has been well documented that cellular migration, proliferation, differentiation, and tissue development are all influenced by matrix geometries and compliance, but how these external biophysical cues are translated into activated intracellular signaling cascades remains poorly understood. Fortunately, technological improvements in artificial substrate fabrication have provided biologists with tools to test cellular interactions within controlled three-dimensional environments. Here, we review cellular responses to biophysical cues and discuss their clinical relevancy and application. We focus especially on integrative approaches that aim to first characterize the properties of specific extracellular matrices and then precisely fabricate biomimetic materials to elucidate how relevant cells respond to the individual biophysical cues present in their native tissues. Through these types of comprehensive studies, biologists have begun to understand and appreciate how exceedingly small features can have a significant impact on the regulation, development, and homeostasis of cells and tissues.

Thiol-ene mediated neoglycosylation of collagen patches: a preliminary study

Despite the relevance of carbohydrates as cues in eliciting specific biological responses, the covalent surface modification of collagen-based matrices with small carbohydrate epitopes has been scarcely investigated. We report thereby the development of an efficient procedure for the chemoselective neoglycosylation of collagen matrices (patches) via a thiol-ene approach, between alkene-derived monosaccharides and the thiol-functionalized material surface. Synchrotron radiation-induced X-ray photoelectron spectroscopy (SR-XPS), Fourier transform-infrared (FT-IR), and enzyme-linked lectin assay (ELLA) confirmed the effectiveness of the collagen neoglycosylation. Preliminary biological evaluation in osteoarthritic models is reported. The proposed methodology can be extended to any thiolated surface for the development of smart biomaterials for innovative approaches in regenerative medicine.

Development of a novel immunoassay for the measurement of type II collagen neoepitope generated by collagenase cleavage

BACKGROUND

To evaluate cartilage degeneration in arthritis, we developed a novel enzyme-linked immunosorbent assay (ELISA) with the capacity to determine urinary concentrations of type II collagen neoepitope (CIINE) generated by collagenase cleavage.

METHODS

Two monoclonal antibodies, 20A10 and 6G4, were generated. Of these antibodies, 20A10 recognized CIINE regardless of hydroxylation of Pro⁷⁷¹, and 6G4 recognized the type II collagen-specific region adjacent to the neoepitope. A sandwich ELISA was constructed using these antibodies.

RESULTS

The ELISA positively determined CIINE concentrations from human and dog urine samples, and from tissue culture supernatant of rat and bovine cartilage. Validation with human urine samples revealed that the ELISA had a detection limit of 100 pmol/l, with intra- and inter-assay coefficients of less than 15%. Recovery of extraneously added CIINE peptide to human urine samples was 83.1-104%. Measurement with the ELISA demonstrated that urine samples from OA patients contained CIINE at significantly higher concentrations compared with those from healthy controls (P<0.01).

CONCLUSIONS

The ELISA can determine the CIINE concentration in human urine sensitively and accurately. This assay may also be useful to determine the concentration of CIINE of various animal samples.

Celebration of the 50th anniversary of the foundation of the French society for connective tissue research. Its short history in the frame of the origin and development of this discipline

The science of connective tissues has (at least) a double origin. Collagen, their major constituent was first studied in conjunction with the leather industry. Acid mucopolysaccharides (now glycosaminoglycans) were characterised by (bio)-chemists interested in glycoconjugates. They joined mainly hospital-based rheumatology departments. Later started the study of elastin with the discovery of elastases and of connective tissue-born (structural) glycoproteins. Besides rhumatologists and leather-chemists mainly pathologists became involved in this type of research, followed closely by ophthalmology research. The first important meetings of these diverse specialists were organised under the auspices of NATO, first in Saint-Andrew's in GB in 1964 and a few years later (1969) in Santa Margareta, Italy. With the discovery of fibronectin, a "structural glycoprotein", started the study of cell-matrix interactions, reinforced by the identification of cell-receptors mediating them and the "cross-talk" between cells and matrix constituents. The first initiative to organise societies for this rapidly growing discipline was that of Ward Pigman in New York in 1961, restricted however to glycol-conjugates. Next year, in 1962 was founded the first European Connective Tissue Society in Paris: the "Club français du tissu conjonctif", which played a crucial role in the establishment of schools, laboratories, national and international meetings in the major cities of France: Paris, Lyon, Reims, Caen,Toulouse. A second European society was born in Great Britain, and at a joint meeting with the French society at the Paris Pasteur Institute, was founded in 1967 by these societies the Federation of European Connective Tissue Societies (FECTS). Their meetings, organised every second year, drained a wide attendance from all over the world. An increasing number of young scientists joined since then this branch of biomedical discipline with several international journals devoted to connective tissue research, to matrix biology. The increasing number and quality of the young generation of scientists engaged in research related to the extracellular matrix or better Biomatrix and cell-matrix interactions is a further guarantee for the continued interest in this crucial field of science at the interface of basic and medically oriented research.

The tissue-engineered auricle: past, present, and future

The reconstruction, repair, and regeneration of the external auricular framework continue to be one of the greatest challenges in the field of tissue engineering. To replace like with like, we should emulate the native structure and composition of auricular cartilage by combining a suitable chondrogenic cell source with an appropriate scaffold under optimal in vitro and in vivo conditions. Due to the fact that a suitable and reliable substitute for auricular cartilage has yet to be engineered, hand-carved autologous costal cartilage grafts and ear-shaped porous polyethylene implants are the current treatment modalities for auricular reconstruction. However, over the last decade, significant advances have been made in the field of regenerative medicine and tissue engineering. A variety of scaffolds and innovative approaches have been investigated as alternatives to using autologous carved costal cartilage or porous polyethylene implants. A review of recent developments and the current state of the art and science is presented, focusing on scaffolds, cell sources, seeding densities, and mechanical characteristics of tissue-engineered auricular cartilage.

Modulation of collagen II fiber formation in 3-D porous scaffold environments

Collagen II, a major extracellular matrix component in cartilaginous tissues, undergoes fibrillogenesis under physiological conditions. The present study explored collagen II fiber formation in solution and in two- (coverslip) and three-dimensional (scaffold) environments under different incubation conditions. These conditions include variations in adsorption buffers, the presence of 1-ethyl-3-(3-dimenthylaminopropyl) carbodiimide/N-hydroxysuccinimide crosslinker and the nature of the material surfaces. We extend our observations of collagen II fiber formation in two dimensions to develop an approach for the formation of a fibrillar collagen II network throughout surface-modified polylactide-co-glycolide porous scaffolds. Morphologically, the collagen II network is similar to that present in native articular cartilage. Biological validation of the resultant optimized functional scaffold, using rat bone marrow-derived mesenchymal stem cells, shows appreciable cell infiltration throughout the scaffold with enhanced cell spreading at 24h post-seeding. This economic and versatile approach is thus believed to have significant potential in cartilage tissue engineering applications.

Cross-linking of type I collagen with microbial transglutaminase: identification of cross-linking sites

Collagen is a popular biomaterial. To deal with its lack of thermal stability and its weak resistance to proteolytic degradation, collagen-based materials are stabilized via different cross-linking procedures. Regarding the potential toxicity of residual cross-linking agents, enzyme-mediated cross-linking would provide an alternative and nontoxic method for collagen stabilization. The results of this study show that type I collagen is a substrate for mTG. However, epsilon-(gamma-glutamyl)lysine cross-links are only incorporated at elevated temperatures when the protein is partially or completely denatured. A maximum number of 5.4 cross-links per collagen monomer were found for heat-denatured collagen. Labeling with the primary amine monodansylcadaverine revealed that at least half of the cross-links are located within the triple helical region of the collagen molecule. Because the triple helix is highly ordered in its native state, this finding might explain why the glutamine residues are inaccessible for mTG under nondenaturing conditions.

Effect of polarized light emitting diode irradiation on wound healing

BACKGROUND

We propose a new phototherapy using polarized light from light emitting diode (LED). The purpose of this study is to clarify the effect of polarized LED irradiation on wound healing.

METHODS

Five groups were classified: control (C), unpolarized (U), linearly polarized (L), right circularly polarized (RC), and left circularly polarized (LC) LED irradiation. In vitro study, fibroblast cell cultures were irradiated, and cellular proliferation was evaluated with a WST-8 assay. In vivo study, full-thickness skin defect of 20 mm diameter was created on the dorsal side of rats. The ratio of the residual wound area was measured, and expression of type 1 and type 3 procollagen mRNA in granulation tissue was determined by real-time reverse transcription polymerase chain reaction method.

RESULTS

The cellular proliferation rates of group RC and L were significantly higher than other groups. The ratio of the residual wound area of group RC and L was significantly reduced than group C and U. Expression of type 1 procollagen mRNA in group RC was found to be significantly increased about 1.5-fold in comparison with the group C. There were no significant differences for type 3 procollagen.

CONCLUSIONS

The right circularly polarized light and linearly polarized light promoted the process of wound healing by increasing the proliferation of fibroblasts, and the right circularly polarized light increased the expression of type 1 procollagen mRNA. The effectiveness of right circularly polarized light suggests that some optical active material, which has a circular dichroic spectrum, takes part in a biochemical reaction.

Characteristics of collagen fibrils in the entire equine superficial digital flexor tendon

The superficial digital flexor tendon (SDFT) is one of the longest tendons in the horse. In racehorses, disturbance of the locomotor functions of the SDFT occurs most frequently in the central area of the mid-metacarpal region. While many studies have investigated the equine SDFT, there are no reports to date of the morphological characteristics of collagen fibrils in the central and peripheral areas of each of the three regions that comprise the entire tendon: the myotendinous junction (MTJ), the mid-metacarpal region (mM) and the osteotendinous junction (OTJ). Mass average diameter (MAD), which provides functional information on the mean collagen fibril diameter and tensile strength of the tendon, was found to be smaller in the central area than in the peripheral area of all three regions. The MAD value was lowest in both the central and peripheral areas in the MTJ region, and tended to increase generally in a distal direction in the OTJ. The OTJ is important parts that unite with the bone. We conclude that morphological structure suggested that it corresponds to biomechanical function in some region of the equine SDFT.

Review of Injectable Cartilage Engineering Using Fibrin Gel in Mice and Swine Models

More than a decade of work has been devoted to engineering cartilage for articular surface repair. This review covers the use of fibrin gel polymer as an injectable scaffold for generating new cartilage matrix from isolated articular chondrocytes beginning with studies in mice and culminating in an applied study in swine joints. These studies began with developing a formulation of fibrin that was injectable and promoted cartilage matrix formation. Subsequent studies addressed the problems of volume loss after the scaffolds were placed in vivo by adding lyophilized cartilage matrix. Additional studies focused on the ability of isolated chondrocytes to heal and repair cartilage in a model that could be biomechanically tested. In conclusion, this series of studies demonstrated that fibrin gel is a suitable polymer gel for generating new cartilage matrix from articular chondrocytes. The new matrix is capable of forming mechanical bonds between cartilage disks and can lead to healing and integration. Armed with these results, implantation of fibrin-cell constructs into defects in swine knees showed new cartilage formation and filling of the defects. Continuing work in these models with fibrin and other polymerizable hydrogels could result in a suitable cell-based therapy for articular cartilage lesions.

Effects of Ingestion of Collagen Peptide on Collagen Fibrils and Glycosaminoglycans in the Dermis

In order to investigate the effects of collagen peptide ingestion on fibroblasts and the extracellular matrix in the dermis, collagen peptide was administered orally to pigs at 0.2 g/kg body weight/d for 62 d, and its effects were compared with those of lactalbumin and water controls. Fibroblast density, and diameter and density of collagen fibrils were significantly larger in the collagen peptide group than in the lactalbumin and water control groups. The two major components of dermal glycosaminoglycans, hyaluronic acid and dermatan sulfate, which are present in the inter-fibrillar space, did not differ significantly among the three groups. However, the ratio of dermatan sulfate, which is derived from fibril-bound decorin, was largest in the collagen peptide group. These results suggest that ingestion of collagen peptide induces increased fibroblast density and enhances formation of collagen fibrils in the dermis in a protein-specific manner.

Producing a Flexible Tissue-Engineered Cartilage Framework Using Expanded Polytetrafluoroethylene Membrane as a Pseudoperichondrium

BACKGROUND

Both native and engineered cartilage is brittle and fractures easily without perichondrium. The aim of this study was to understand the role of the perichondrium and try to enhance the flexible properties of tissue-engineered cartilage using expanded polytetrafluoroethylene (ePTFE) membrane as a pseudoperichondrium.

METHODS

The study was conducted in two phases. In phase I, native swine auricular cartilage of different thicknesses was studied by histologic evaluation and failure testing. Next, isolated perichondrium was bonded to native cartilage slices using fibrin glue or Dermabond and tested to failure. In phase II, swine auricular chondrocytes were suspended in fibrin glue. The chondrocyte-fibrin glue composites were then bound to expanded polytetrafluoroethylene membrane in two trilaminar configurations: In group EC-1, the membrane was in the center, whereas it was on the surfaces in group EC-2. Specimens were implanted into nude mice for 4 weeks, 8 weeks, 12 weeks, and 8 months and subjected to histologic evaluation and failure testing.

RESULTS

In phase I, the results demonstrated that perichondrium securely bonded to the cartilage plays an important role in maintaining the flexible nature of elastic cartilage. In phase II, failure testing revealed that specimens in group EC-1 (expanded polytetrafluoroethylene core) were fractured during bending and destroyed after torsion, whereas those in group EC-2 (cartilage core) returned to their original shape without fracturing even after rigorous torsion. Histologic analysis demonstrated that transplanted chondrocytes penetrated into the microporous structure of expanded polytetrafluoroethylene and created a bond to it.

CONCLUSION

It is possible to engineer flexible cartilage using expanded polytetrafluoroethylene as a pseudoperichondrium.

Formation of Cartilage in Congenital Bicuspid Aortic Valves of Syrian Hamsters (Mesocricetus auratus)

The formation, topographical location and calcification of cartilage in congenital bicuspid aortic valves of 235 Syrian hamsters aged 0--173 days were studied by histological, histochemical and immunohistochemical techniques. In all animals the aortic valve was bicuspid; it had two leaflets, ventral and dorsal, each supported by its own aortic sinus. In 141 valves, a more or less developed raphe was located in the ventral sinus. The remaining 94 valves were devoid of any raphe. The first sign of valvular chondrogenesis was the presence of small groups of cells embedded in a type II collagen-positive extracellular matrix. These cellular groups, which appeared as early as 2 days after birth, became converted into hyaline cartilage or fibrocartilage. A considerable proportion (67%) of the valvular cartilages developed within the first 6 weeks of life. The cartilaginous tissue was capable of forming anywhere along the attachments of the valve leaflets to their supporting sinuses. However, statistical analyses substantiated the observation that the bases of the sinuses and raphes were the valvular regions particularly prone to the development of cartilage. At these sites, the cartilage was usually hyaline and often became calcified. The findings were consistent with the assumption that intense mechanical stimulation plays an important role in the formation of the valvular cartilage. Moreover, these findings supplied new evidence that in the cardiac semilunar valves of Syrian hamsters, cartilage formation does not involve the aggregation of large numbers of cells before their differentiation into chondrocytes. The valvular hyaline cartilages appear to act as competent pivots, resisting mechanical tensions generated during the cardiac cycle. Deposition of calcium in the matrix can be regarded as a reinforcement process of the cartilaginous tissue. Finally, it is hypothesized that the formation of cartilage in the aortic valves of hamsters prevents dystrophic calcification of the valve, a pathological change that causes aortic stenosis in man, especially in patients with a bicuspid aortic valve.

Formation of cartilaginous foci in the central fibrous body of the heart in Syrian hamsters (Mesocricetus auratus)

The formation of cartilage in the mammalian heart has been studied in the aortic and pulmonary valves. The chondrogenetic process that takes place in the cardiac skeleton is still unknown. The present study was designed to illustrate the ontogeny of cartilaginous foci occurring in the central fibrous body of the Syrian hamster (Mesocricetus auratus) heart. Hearts from 472 animals aged 0-708 days were examined using histological, histochemical and immunohistochemical techniques. Cartilage was present in the central fibrous body of 118 (25%) specimens. A further 104 hamsters were used for the detection of calcific deposits in the central fibrous body. Six (5.8%) showed calcified cartilage. The first sign related to chondrogenesis was the presence of small groups of cells embedded in a type II collagen-positive extracellular matrix. These cellular groups, which can appear as early as 2 days after birth, differentiate into hyaline cartilage or, less frequently, into fibrocartilage. The highest production of cartilaginous foci takes place between days 40 and 80. Thereafter, formation of new foci is uncommon. This indicates that appearance of cartilage in the central fibrous body of the heart is not a consequence of cardiac aging. The cartilaginous foci seem to act as pivots resisting mechanical tensions generated during the cardiac cycle. Deposition of calcium in the extracellular matrix of the foci can be regarded as a reinforcement of the cartilaginous tissue.

Injectable tissue-engineered cartilage with different chondrocyte sources

Injectable engineered cartilage that maintains a predictable shape and volume would allow recontouring of craniomaxillofacial irregularities with minimally invasive techniques. This study investigated how chondrocytes from different cartilage sources, encapsulated in fibrin polymer, affected construct mass and volume with time. Swine auricular, costal, and articular chondrocytes were isolated and mixed with fibrin polymer (cell concentration of 40 x 10 cells/ml for all groups). Eight samples (1 cm x 1 cm x 0.3 cm) per group were implanted into nude mice for each time period (4, 8, and 12 weeks). The dimensions and mass of each specimen were recorded before implantation and after explantation. Ratios comparing final measurements and original measurements were calculated. Histological, biochemical, and biomechanical analyses were performed. Histological evaluations (n = 3) indicated that new cartilaginous matrix was synthesized by the transplanted chondrocytes in all experimental groups. At 12 weeks, the ratios of dimension and mass (n = 8) for auricular chondrocyte constructs increased by 20 to 30 percent, the ratios for costal chondrocyte constructs were equal to the initial values, and the ratios for articular chondrocyte constructs decreased by 40 to 50 percent. Constructs made with auricular chondrocytes had the highest modulus (n = 3 to 5) and glycosaminoglycan content (n = 4 or 5) and the lowest permeability value (n = 3 to 5) and water content (n = 4 or 5). Constructs made with articular chondrocytes had the lowest modulus and glycosaminoglycan content and the highest permeability value and water content (p < 0.05). The amounts of hydroxyproline (n = 5) and DNA (n = 5) were not significantly different among the experimental groups (p > 0.05). It was possible to engineer injectable cartilage with chondrocytes from different sources, resulting in neocartilage with different properties. Although cartilage made with articular chondrocytes shrank and cartilage made with auricular chondrocytes overgrew, the injectable tissue-engineered cartilage made with costal chondrocytes was stable during the time periods studied. Furthermore, the biomechanical properties of the engineered cartilage made with auricular or costal chondrocytes were superior to those of cartilage made with articular chondrocytes, in this model.

Formation of cartilage in aortic valves of Syrian hamsters

The formation of cartilage in aortic valves of Syrian hamsters was studied using histological, histochemical and immunohistochemical techniques. The sample consisted of 281 specimens aged 0-363 days, all of which had a normal (tricuspid) aortic valve. The first sign of valvular chondrogenesis is the presence of small groups of cells embedded in a type II collagen-positive matrix. These groups of cells, which can appear as early as one day after birth, increase in size and differentiate into hyaline cartilage or fibrocartilage. From the fourth day of life, all hamsters examined displayed cartilaginous foci in the aortic valve. They were located along the fibrous attachments of the valve leaflets to their respective sinuses, including the valve commissures. A considerable proportion (76%) of cartilages formed within the first 40 days of life, that is during the period of time in which the histogenesis of the valve takes place. The present observations are consistent with the assumption that in mammals, the precursors of the aortic valve chondrocytes are neural crest-derived cells. Results of a statistical analysis substantiate that the incidence is significantly higher in (1) the territory that comprises the collagenous condensation of the ventral commissure and the ventro-lateral and proximal fibrous attachments of the right leaflet to its sinus, and (2) the proximal fibrous attachment of dorsal leaflet to its sinus. These findings together with data in the literature concerning the distribution of stress in each leaflet-sinus assembly of the valve during the cardiac cycle, suggest that mechanical action might play an inductive role in the formation of the cartilaginous tissue in the aortic valve of mammals. In addition, they point to the possibility that locally intense mechanical stimulation is responsible for the differentiation of the anticipated cartilaginous tissue into hyaline cartilage.

Tissue engineering of cartilage

The primary goal of engineering cartilage as a therapeutic approach is to restore the physiological conditions of an affected or defective tissue in the body. Cartilage tissue is distributed widely in the human body and possesses an organization related to the specific demand of a particular anatomical region. In selecting the proper material for engineering cartilage, the functional demands of the replacement tissue must be considered. In summary, there is a multitude of scaffolds, naturally occurring and synthetic, that are suitable for engineering cartilage. Investigators have shown that the characteristics of the neocartilage differ significantly depending upon which scaffold is used. There are also large differences when a single scaffold is tested in vitro as opposed to in vivo. Moreover, the addition of other materials internally or externally to the cartilage composite influences the physical and biomechanical properties of the newly formed tissue. The results achieved so far are extremely encouraging and motivate further investigative efforts in the field. The biochemical composition and, more importantly, the biomechanical properties of the native tissue still represent the ideal replacement tissue.

Formation of cartilage in the heart of the Spanish terrapin, Mauremys leprosa (Reptilia, Chelonia)

Cartilaginous deposits are regularly present in the heart of several reptilian, avian, and mammalian species. The formation of these extraskeletal cartilages has been studied in birds and mammals, but not in reptiles. The aim here was to elucidate this question in the Spanish terrapin. Hearts from 23 embryos belonging to Yntema (1968) developmental stages 17 to 26 and eight terrapins age 3 months to 10 years were examined using histological, histochemical, and immunohistochemical techniques. In the heart of the Spanish terrapin (Mauremys leprosa), chondrogenesis can start during embryonic life. Cartilaginous tissue develops from a mesenchymal cellular condensation that extends along the aorticopulmonary septum and the incipient pars fibrosa of the ventricular horizontal septum. This cellular condensation, which is smooth muscle alpha-actin (SMalpha-actin)-negative and type II collagen-negative during stages 17 to 22, acts as a prechondrogenic condensation. In stage 23, production of type II collagen begins in the central core of the condensation and gradually spreads toward its periphery. The type II collagen-positive (chondrogenic) cellular condensation remains devoid of perichondrium prior to birth. Thereafter, it converts into hyaline cartilage that extends along the proximal part of the aorticopulmonary septum and the pars fibrosa of the horizontal septum. Our findings are consistent with the assumption that, as in birds and mammals, the precursors of the cardiac chondrocytes in chelonians are neural crest-derived cells of nonmuscular nature. In addition, they point to the possibility that cells from the neural crest populate the embryonic pars fibrosa of the horizontal septum, thereby contributing to its alignment with the aorticopulmonary septum. In the present species, a second cartilaginous deposit of a hyaline nature extends along the sinus wall of the right semilunar valve of the right aorta, penetrating the fibrous cushion that constitutes the proximal support of the corresponding valve leaflet. This cartilage develops after birth, between the third and eighteenth month of life; its morphogenetic origin is unclear. The cartilaginous foci occurring in hearts of Spanish terrapin appear to act as pivots resisting mechanical tensions generated during the cardiac cycle. In the specimens examined there was no sign of replacement of the cardiac cartilages by bone tissue.

Similarity between the major collagens of cuttlefish cranial cartilage and cornea

Invertebrates possess unique collagen-containing connective tissue elements, the biochemistry of which is not clearly understood. We previously reported the occurrence of a novel heterotrimeric type V/XI like collagen in the cranial cartilage of the cuttlefish Sepia officinalis. We report here the purification of the three chains by ion exchange chromatography and the physicochemical characteristics of this collagen. This collagen shared substantial similarity to the collagen purified from the cornea of S. officinalis, with respect to chain composition, cyanogen bromide peptide profile and amino acid composition. The mobility of the C3 chain was retarded in the corneal collagen, which also had an increased glycine content and a smaller ratio of hydroxylysine to lysine, together with a reduction in bound carbohydrates. The cartilage collagen had a higher denaturation temperature than corneal collagen. As observed by transmission electron microscopy of reconstituted fibrils, the heterotrimeric invertebrate collagen formed fibrils of no apparent periodicities as opposed to the regular 64-nm banding pattern of milk shark (Rhizoprionodon acutus) cartilage collagen. This is also the first report on the molecular species of collagen in an invertebrate cornea. Our results strongly support the functioning of minor vertebrate collagens as major collagens in some invertebrates, close similarity of collagens in two tissues with different functions and would hold significance to our understanding of collagen polymorphism and the evolution of the extracellular matrix.

Cartilage in pulmonary valves of Syrian hamsters

The presence of cartilage in the pulmonary valve has been reported in birds, but not in mammals. We describe here the occurrence of cartilaginous tissue in the pulmonary valves of 40 (11.4%) of 351 Syrian hamsters examined using histological, histochemical and/or immunohistochemical techniques. The cartilaginous deposits were located along the fibrous attachments of the valve leaflets to the wall of the pulmonary artery trunk. Our findings indicate that the proximal attachments of the leaflets to their respective sinuses, and especially that of the ventral leaflet, are the most prone valvular regions to develop cartilaginous foci. Nonetheless, the possible function of these foci remains an open question. Formation of cartilage in the pulmonary valve starts within the first month of life, that is during the period in which the valve reaches histological maturation. The earliest evidence of chondrogenesis is the presence of small groups of cells embedded in a type II collagen-positive extracellular matrix. These groups of cells, which can appear as early as one day after birth, increase moderately in size and differentiate into hyaline cartilaginous tissue. The precursors of the cartilaginous cells are presumed to be neural crest-derived elements. However, the factor or factors involved in the differentiation of these precursors into chondrocytes are still unknown. In this regard, our observations cast doubt on the hypothesis that the formation of cardiac cartilages is primarily due to locally intense mechanical stimulation.

Age-associated changes in collagen content and its subtypes within rat corpora cavernosa with computerized histomorphometric analysis

OBJECTIVES

To study the age-associated changes in the percentage of collagen and subtypes I, III, and IV within the corpora cavernosa in a rat model.

METHODS

The corpora cavernosa tissues were obtained from 30 male Wistar rats at three different ages. Processed with Masson's trichrome staining for collagen and with immunohistochemical staining for the collagen subtypes, the values of the collagen percentage, the percentage of area, and relative proportion of each collagen subtype within the rat corpora cavernosa were measured using an automatic image analysis system. The relationships between an increase in age and these parameters were analyzed.

RESULTS

The percentage of collagen within the corpora cavernosa was higher in the old rats (80 weeks) than in the young (20 weeks) and intermediate-age (40 weeks) rats (P = 0.02 and P = 0.25, respectively) and significantly increased with age (P = 0.021). The values of the percentage of area of collagen subtypes III and IV also increased significantly with age (P = 0.039 and P = 0.019, respectively). The value of the percentage of area of collagen subtype I was not significantly increased (P = 0.159). Also, no significant differences were found in the relative proportions of all three collagen subtypes with age among the three age groups.

CONCLUSIONS

The percentages of collagen within rat corpora cavernosa significantly increased, but not strongly, with age, especially collagen subtypes III and IV. However, the relative proportion of each subtype did not change with age. Therefore, we conclude that the amount of collagen may only partly contribute to erectile dysfunction in the aging process of the rat.

Measurement of the axial periodicity of collagen fibrils using an image processing method

In this paper, a method is described based on a computer-aided analysis of electron-optical images of collagen fibrils from various tissues, in order to determine the axial periodicity of such fibrils. The method gives information at a level of 2-3nm.

Analysis of collagenase-cleavage of type II collagen using a neoepitope ELISA

We have developed monoclonal antibody 5109 against a unique highly acidic sequence in type II collagen. When paired with previously reported monoclonal antibody 9A4, 5109 can be used as the capture antibody in an ELISA assay for the neoepitope generated by collagenase-cleavage of type II collagen. The assay detects the sequence ZGlyGluX(759)GlyAspAspGlyProSerGlyAlaGluGlyProX(771)GlyProGlnGly(775) where Z is a variable length polypeptide, X is proline or hydroxyproline, and Gly(775) corresponds to C-terminal amino acid of the 3/4 piece after collagenase cleavage. Antibody 5109 detects the first and 9A4 the second underlined sequence. Antibody 5109 recognizes its epitope with a K=1.2x10(-8) M independently of hydroxylation of X(759). When X(771) is proline, the sequence is 90x more sensitively detected by this ELISA than when it is hydroxyproline. Type II collagen of human articular cartilage was fragmented by cyanogen bromide (CNBr) and trypsin. The immunoreactive fragment was captured with 5109 and sequenced. Proline(771) averaged 81% hydroxylated. Other 3rd position prolines were >97% hydroxylated. In urine of control individuals of 50-70 years of age, we failed to detect the presence of the collagen fragment in a majority (8/10) of specimens. The two controls with measurable levels averaged 123 pM. In a similar age cohort of osteoarthritic patients, the majority (9/10) showed measurable values of urinary collagen fragments averaging 312 pM. This assay can be used for monitoring type II collagen metabolism in patients with osteoarthritis.

Formation of cartilage in cardiac semilunar valves of chick and quail

The development of cartilage in the aortic and pulmonary valves of chick and quail was studied using histological, histochemical and immunohistochemical techniques. In both species, the earliest evidence of chondrogenesis is the formation of smooth muscle alpha-actin-negative prechondrogenic (type II collagen-negative) cellular condensations in the tunica media of the proximal aorta and pulmonary trunk, in front of or slightly distal to the valvular commissures. Such condensations are present as early as stage 37 of Hamburger-Hamilton in the aortic and pulmonary valves of the chick. In quail embryos, they form somewhat later, namely, at stage 38 in the aortic valves and stage 39 in the pulmonary valves. In the chick, synthesis of type II collagen starts in the central core of the aortic cellular condensations at stage 38. In the pulmonary valves of chick and aortic and pulmonary valves of quail, production of type II collagen does not begin until stage 40. This production then gradually increases toward the periphery of the condensations, which remain devoid of perichondrium prior to hatching. After birth, the condensations become transformed into hyaline cartilaginous foci. In the aortic valves of some chickens and quails, more or less extensive deposits of hyaline cartilage or fibrocartilage form along the attachments of the leaflets to their supporting sinuses. They develop later than the commissural cartilages. The present findings, together with previous data from the literature, suggest that the aortic and pulmonary valve cartilages differentiate from neural crest-derived nonmuscular cells.

Occurrence of a novel collagen with three distinct chains in the cranial cartilage of the squid Sepia officinalis: comparison with shark cartilage collagen

A unique collagen with three distinct chains, was purified from the cranial cartilage of the squid Sepia officinalis, by pepsinisation and salt precipitation and compared with shark cartilage collagen. These chains, which were different from the known cartilage collagen chains, were referred as C1, C2 and C3, had approximate molecular weights of 105 kDa, 115 kDa and 130 kDa, respectively, and were present in a ratio of 3:2:1, suggestive of two molecules of composition, [(C1)2C2] and [C1C2C3]. These collagens were purified by fractionation at acid and neutral pH, and by ammonium sulfate precipitation. Solubility data indicated that this collagen was more crosslinked than the type I collagen isolated from cartilage of shark, Carcharius acutus. In vitro fibrillogenesis revealed that the sepia collagen formed denser aggregates, as compared to shark collagen, and was stabilised by a higher degree of carbohydrate association. Polyclonal antisera raised against shark collagen was also reactive against the sepia collagens, while the converse was not true, indicating the high immunospecificity of the latter. These results demonstrate collagen polymorphism in an invertebrate cartilage and may hold significance in understanding tissue calcification and molecular evolution. Further, these collagens may represent ancestral forms of vertebrate minor collagens like typeV/XI.

The effect of lithium treatment on collagenous tissues: an electron microscope study

Structural alterations of mouse skin collagen fibrils brought about by treatment with lithium chloride at various doses, as studied by electron microscopy, are discussed. The effect of the duration of treatment on such fibrils is also investigated. An analysis of positive staining patterns from fibrils from mice treated with lithium is included. In addition, this review describes how computer analyses of electron-optical images from collagen fibrils can lead to information about the changes produced by lithium on the molecular architecture of the fibril.

General principles of wound healing

Wound healing is a complex process involving different biologic and immunologic systems. Despite improvements in diagnostics and therapy, wound failures remain a clinical problem. The approach to a nonhealed wound is an interdisciplinary challenge that should not be underestimated. Better understanding of the complex wound-healing cascade helps our approach to wound healing and its possible failure. Manipulations of the involved immunologic features offer future therapeutic strategies.

Denatured thiolated collagen. I. Synthesis and characterization

A new thiolating reagent is used to introduce sulphur groups into denatured atelocollagen. The procedure is easy to control and applicable on a large scale. The reagent is a reactive dicarboxylic acid compound containing sulphur in the form of a disulphide functionality. It is prepared by reacting N,N'-disuccinoylcystamine with 1,1'-carbonyldiimidazole. When this reagent is added to a solution of denatured atelocollagen in dimethylsulphoxide, amide bonds are formed between the carbonyl functions of the reagent and epsilon-NH2 of lysine and hydroxylysine residues from the protein. The disulphide groups introduced can then be reduced by reaction with 1,4-dithiothreitol to give the-SH form of the modified protein. Control of the stoichiometry between the reagent and the protein can lead to varying modification levels. A maximum level of 0.33 mmol SH per gram of protein can be attained, which corresponds to complete thiolation of the lysine and hydroxylysine residues. Thiolated denatured atelocollagen exhibits gelatin-like behaviour, by being highly soluble in water at all pH values and by forming heat-reversible gels.

Inhibitors of collagen synthesis and hypophysectomy: effects on androgen induced growth of ventral prostate in rats

To elucidate the effect of decreased amounts of collagen on prostatic growth, beta-aminopropionitrile, one of the agents that selectively inhibit collagen formation, was administered, with or without exogenous androgen and/or hypophysectomy, to rats of varying ages. The weight of the ventral prostate and the collagen content and the ratio of type III to type I collagen in the ventral prostate were examined. Administration of beta-aminopropionitrile, with or without exogenous androgen, reduced the collagen content in the ventral prostate in any age groups, but increased the weight of the ventral prostate only in young rats. Administration of beta-aminopropionitrile to hypophysectomized rats did not change the weight and collagen content. Administration of beta-aminopropionitrile prevented androgen-induced growth of prostatic collagen, resulting in increased prostatic weight in young rat. The ratio of type III to type I collagen in the rat ventral prostate remained unchanged by administration of beta-aminopropionitrile, exogenous androgen, hypophysectomy, or advancing age. It would appear that in young rats prostatic collagen plays an important role in suppressing prostatic growth. The ratio of type III to type I collagen is stable under various experimental conditions in the ventral prostate.

Analysis of connective tissue macromolecular components in Ishibashi rat skin: role of collagen and elastin in cutaneous aging

The Ishibashi (IS) rat, established from cross-breeding between Wistar and wild rats, has a unique skin appearance, with wrinklings and furrows indicative of skin aging appearing at the age of 12 weeks. To understand the underlying mechanism of the formation of wrinkles, macromolecular components of connective tissue, collagen and elastin, in the young (5-6-week-old) and the aged (23-30-week-old) IS rat skins were biochemically analyzed. Hydroxyproline and isodesmosine contents in the aged IS rats were reduced 22% (P < 0.05) and 37% (P < 0.05) compared to the young rats, whereas no significant differences in the contents of both macromolecules in control Sprague-Dawley (SD) rats were seen. The relative content of type III collagen was unaltered between the young and aged skins of both IS and SD rats. A relative decrease in the intact elastin molecule (65 kDa) and a relative increase in the elastin fragments with lower molecular weights were observed in the aged IS rat skin by immunoblotting method. These results indicate that the reduction in collagen and elastin contents and increased degradation of elastin molecules in the aged IS rat skin could be related to the formation of wrinkles. Thus, the IS rat may provide a useful model for the study of skin aging.

Phenotypic characterization of the 3/A/1D-1M osteogenic cell line derived from in vivo transplantation of 3/A/1D-1 chondroprogenitor murine teratocarcinoma cells

Bone cells involved in the replacement of cartilage by bone in the endochondral ossification process are known to enter via the medullar pathway. A hypothesis for the development of osteoblasts from chondroblasts was investigated by analyzing the phenotypic characteristics of the 3/A/1D-1M cell line derived from endochondral bone ossicle which was formed after in vivo transplantation of 3/A/1D-1 chondroprogenitor mouse teratocarcinoma cells. The 3/A/1D-1M cell cultures exhibited a triphasic evolution: after reaching confluence (day 3), cultures developed well-delimited cell clusters (days 6-8), which ultimately were organized into multilayered nodules (days 12-15). Electron-microscopic examination of such nodules at day 18 showed the presence of needle-shaped crystals associated with collagen fibrils in the extracellular space. The kinetics of collagen expression, investigated by an immunofluorescence staining procedure showed that, while confluent cultures mainly expressed type III collagen (70% of cells) with some type I (30-40% of cells) and V (30-40% of cells), the type I collagen became the major isoform beginning with day 6. From day 6 onwards, NP40-extracted alkaline phosphatase (AP) activity appeared concomitantly to cell cluster formation, and reached 160 nmol/min/mg of protein at the stage of nodule maturation (day 15). The strong inhibition of enzymatic activity by levamisole and L-homoarginine (IC50 = 0.9 microM and 5 mM, respectively) and its rapid heat inactivation at 56 degrees C (IT50 = 90 s), revealed the bone specificity of AP expressed by 3/A/1D-1M cells. In confluent cultures, brief exposure to parathyroid hormone (10 nM), known to be a bone-resorbing agent, showed a 60% increase in the intracellular cAMP level. In addition, while producing mRNA for the bone-specific protein osteocalcin, 3/A/1D-1M cells also produced type II procollagen mRNA, known to be the major cartilage-related characteristic. This in vitro study demonstrates that the 3/A/1D-1M clonal cell line, originating from 3/A/1D-1 chondroprogenitor cells after in vivo passage, was able to develop differentiated osteoblastic properties as well as the residual expression of the major chondrocytic RNA messenger.

Alteration of collagen biosynthesis and analysis of type I and type III collagens of prostate in young rats following sex hormone treatments

Two series of experiments were conducted to clarify the collagen biosynthesis and isolation of type I and type III collagens of young rats that received various sex hormone treatments. Evidence has been presented that (1) estradiol-17 beta treatment induces prostatic atrophy and suppresses the incorporation of [3H]proline into collagen in prostate; (2) administration of estradiol-17 beta increases the collagen content in the prostate; (3) localization of type I and III collagens in the interstitium of the prostate is detected by indirect immunofluorescence staining; (4) the ratio of type III collagen to type I collagen in the gland decreases following estradiol-17 beta treatment; and (5) neither treatment with testosterone nor administration of testosterone plus estradiol-17 beta in precastrated rats shows discernible effects on these valuables. These findings suggest that estradiol-17 beta increases the accumulation of collagen into the prostate with different extents of influence on the synthesis of type I and III collagens.

Perspectives: a proposed general model of the "mechanostat" (suggestions from a new skeletal-biologic paradigm)

This provisional general model for the skeleton's mechanostat spans the biologic "distance" between the organ and macromolecule. It could apply to bone, cartilage and fibrous tissue, and to bones, joints, ligaments and other organs made wholly or in part from the basic tissues. It suggests where small things such as a cytokine effect on some cell should fit in the overall scheme of skeletal physiology. It proposes that interlocking negative feedback loops provide mechanical-usage-dedicated message traffic routes on which nonmechanical agents could act to optimize or impair postnatal skeletal adaptations to varied mechanical and nonmechanical challenges, and treatments of disease too. It suggests that future research must try to understand the mechanostat's cell- and molecular-biologic roots.

Differentiation and mineralization in chick chondrocytes maintained in a high cell density culture: a model for endochondral ossification

Chondrocytes isolated from the proliferative and differentiating zones of 3-wk-old chick growth plates were cultured in the presence of 10% fetal bovine serum (FBS) and ascorbic acid for up to 21 d in a high cell density culture within Eppendorf tubes. The proliferative, differentiating, and calcification properties of the chondrocytes were examined by immunolocalization and by enzyme histochemical and biochemical methods. The cells maintained a chondrocyte phenotype throughout culture: they were round in shape and synthesized both collagen type II and proteoglycans. The expression of a hypertrophic phenotype was evident by Day 3 of culture and from this time onwards characteristics of terminal differentiation were observed. The cells were positive for both alkaline phosphatase (ALP) activity and c-myc protein and the surrounding matrix stained strongly for collagen type X. Small foci of mineralization associated with individual chondrocytes were first evident by Day 6 and more widespread areas of mineralization occupying large areas of matrix were present by Day 15. Mineralization occurred without the addition of exogenous phosphate to the medium. This culture system displays characteristics that are similar in both morphological and developmental terms to that of chick chondrocyte differentiation and calcification in vivo and therefore offers an excellent in vitro model for endochondral ossification.