Proteoglycans in articular cartilage revealed with a quick freezing and deep etching method

Extraskeletal myxoid chondrosarcoma: a study using a quick-freezing and deep-etching method

A case of extraskeletal myxoid chondrosarcoma (ESMC), which developed in the right thigh of a middle-aged Japanese woman, was studied using immunohistochemistry, conventional electron microscopy, and the quick-freezing and deep-etching (QF-DE) method. In addition to typical light microscopic findings of ESMC, conventional electron microscopy indicated that the tumor cells had features of chondrocytes. Immunohistochemically, the tumor cells showed a positive immunoreaction for S100 protein. A diagnosis of ESMC was made. An interesting observation was the ultrastructural features of collagen fibrils in the myxoid matrix highlighted by the QF-DE method. These collagen fibrils consisted of relatively thin collagen (20-35 nm) with pleated surface structures. The surface striation at 65 nm was obscure. We consider that such a finding of collagen fibrils identified by the QF-DE method is one of the characteristics of the myxoid matrix of ESMC, and this is useful for the differential diagnosis of myxoid soft tissue tumors.

Structure of pericellular matrix around agarose-embedded chondrocytes

OBJECTIVE

Determine whether the structure of the type VI collagen component of the chondrocyte pericellular matrix (PCM) generated by agarose-embedded chondrocytes in culture is similar to that found in native articular cartilage.

METHODS

Confocal microscopy, quick-freeze deep-etch electron microscopy, and real-time polymerase chain reaction (PCR) were used to investigate temporal and spatial patterns of type VI collagen protein deposition and gene expression by bovine chondrocytes during 4 weeks of culture within a 2% agarose hydrogel. Similar analyses were performed on chondrocytes within samples of intact cartilage obtained from the same joint surfaces as those used for cell isolation for comparison.

RESULTS

Type VI collagen accumulated uniformly around cells embedded in agarose, with the rate of deposition slowing after the second week. After 1 week, PCM fibrils were observed to be oriented perpendicular to the cell surface, in contrast with the primarily tangential fibrillar arrangement observed in native articular cartilage. Expression of col6 in agarose-embedded cells was initially much higher ( approximately 400%) than that in chondrocytes within cartilage. Expression of col6 in the cultured chondrocytes declined by approximately 60% after 1 week, and remained stable thereafter.

CONCLUSIONS

PCM structure and composition around cells in a hydrogel scaffold may be different than that in native cartilage, with potential implications for mass transport, mechanotransduction, and ultimately, the success of tissue engineering approaches.

Chondrosarcoma with myxoid change: a study using a quick-freezing and deep-etching method

A middle-aged Japanese woman visited the Orthopedics Department of Nihon University Nerima Hikarigaoka Hospital complaining of pain in the left hip joint that had started approximately 8 months earlier. Following several examinations, including imaging diagnoses, an incisional biopsy demonstrated a malignant acetabular bone tumor, which was removed and examined by a quick-freezing and deep-etching (QF-DE) method, conventional electron microscopy, and light microscopy. Histologically, the tumor was a chondrosarcoma with marked myxoid changes. An interesting extracellular matrix was observed by the QF-DE method. The myxoid area consisted of a fine meshwork of proteoglycans (PG) without obvious aggrecans, which resembled that of PG usually present in the pericellular matrix of normal cartilage. Thin collagen fibrils with pleated surface structures of regular periodicity were also seen, which were sparsely distributed in wide areas except for the pericellular matrix. These collagen fibrils were of the type that are mainly located in the pericellular side of the territorial matrix in normal cartilage. A myxoid matrix consisting of thin collagen fibrils on the background of pericellular type PG suggested that the myxoid matrix in the chondrosarcoma resembled those of the pericellular and pericellular sides of the territorial matrices in normal cartilage.

Anionic sites in articular cartilage revealed by polyethyleneimine staining

Articular cartilage is a unique tissue that contains neither blood vessels nor nerves, and that performs mechanical loading during joint movement. These properties are endowed by abundant glycosaminoglycans (GAGs), which are capable of retaining water-soluble substances. The GAGs attach to core proteins and form proteoglycans. Although many studies have focused on proteoglycans and collagen fibrils in cartilage, little is known about the nature of the negative charge of GAGs. Recently, we investigated this subject using a cationic dye, polyethyleneimine (PEI), with several different techniques such as pre-embedding, post-embedding, and quick-freezing and deep-etching methods. In addition, we investigated whether the anionic charge is altered at low pH, using PEI and cationic colloidal gold (CCG) labeling. The shapes of PEI-positive structures revealed by the pre-embedding method varied at different pHs. Three-dimensional analysis using the quick-freezing and deep-etching method demonstrated that meshwork structures composed of fine filaments were decorated with tiny PEI granules. Additionally, the meshwork structure was broken down after chondroitinase ABC digestion. These data indicate that the large PEI deposits observed in pre-embedding preparations are, at least in part, artificial images, and that the meshwork structure consists of chondroitin sulfate-retaining anionic sites. Low pH conditions changed PEI or CCG labeling patterns, showing that negative charges of GAGs in articular cartilage are altered under environmental pH conditions. These findings demonstrate that binding capacities of anionic sites to water-soluble or ionic substances are greatly affected by pH alterations without actually decreasing the number of anionic sites. Therefore, to understand cartilage dynamics and the pathogenesis of joint diseases in greater detail, alterations of anionic charge during mechanical loading or under pathological conditions should be examined in future studies.

The ultrastructure of anionic sites in rat articular cartilage as revealed by different preparation methods and polyethyleneimine staining

The ultrastructure of anionic sites in the middle layer of rat articular cartilages was studied by two methods, the quick-freezing and deep-etching method, and the quick-freezing and freeze-substitution method. The anionic sites were visualized with a cationic tracer, polyethyleneimine. They were also compared with those revealed in tissues subjected to conventional fixation, such as pre-embedding or post-embedding. With the deep-etching method, three-dimensional meshwork structures were observed more clearly in the extracellular matrix compared with those seen in conventional ultrathin sections. In combination with polyethyleneimine staining, in which no chemical contrast was needed for visualization of anionic sites, numerous stained particles were detected around filaments in the extracellular matrix, indicating that they were anionic sites consisting mainly of proteoglycans. With the pre-embedding method and polyethyleneimine staining, the shapes of aggregated stained particles varied with different preparation procedures, including chemical fixation and contrasting. The fine meshworks were also observed with the post-embedding method and polyethyleneimine staining. It is suggested that such images of anionic sites, as revealed by the deep-etching method and the post-embedding polyethyleneimine-staining method with low-temperature dehydration, are probably closer to native states than those revealed by the conventional pre-embedding polyethyleneimine-staining method.

A histochemical study of anionic sites in the intermediate layer of rat femoral cartilage using polyethyleneimine at different pH levels

Anionic sites in the intermediate layer of young rat hyaline cartilages were examined using a cationic dye, polyethyleneimine (PEI), at different pH levels. Femoral heads were resected and fixed in 2.5% glutaraldehyde and treated with 0.5% PEI at pH 7.4, pH 2.5 or pH 1.0. Some cartilage samples were first digested with chondroitinase ABC or hyaluronidase. The PEI deposits at pH 7.4 appeared to be irregular shapes. Their sizes seemed to be larger than those at pH 2.5 or pH 1.0. The PEI deposits were also found on the surface of collagen fibrils at both pH 7.4 and pH 2.5 even after the chondroitinase ABC digestion, but were not found at pH 1.0. Moreover, they disappeared after hyaluronidase digestion. Accordingly, it is suggested that PEI-positive structures varied depending on pH levels. In addition, hyaluronan may be localized near collagen fibrils, but most sulphated proteoglycans may not.