Ultrastructure of adult human articular cartilage matrix after cryotechnical processing

Collagen XI nucleates self-assembly and limits lateral growth of cartilage fibrils

Fibrils of embryonic cartilage are heterotypic alloys formed by collagens II, IX, and XI and have a uniform diameter of approximately 20 nm. The molecular basis of this lateral growth control is poorly understood. Collagen II subjected to fibril formation in vitro produced short and tapered tactoids with strong D-periodic banding. The maximal width of these tactoids varied over a broad range. By contrast, authentic mixtures of collagens II, IX, and XI yielded long and weakly banded fibrils, which, strikingly, had a uniform width of about 20 nm. The same was true for mixtures of collagens II and XI lacking collagen IX as long as the molar excess of collagen II was less than 8-fold. At higher ratios, the proteins assembled into tactoids coexisting with cartilage-like fibrils. Therefore, diameter control is an inherent property of appropriate mixtures of collagens II and XI. Collagen IX is not essential for this feature but strongly increases the efficiency of fibril formation. Therefore, this protein may be an important stabilizing factor of cartilage fibrils.

Ultrastructure of the echinoderm cuticle after fast-freezing/freeze substitution and conventional chemical fixations

The cuticles of the pedicellaria primordia in the sea urchin Paracentrotus lividus and of the tube foot disk in the sea star Asterias rubens were preserved by different methods, viz., glutaraldehyde fixation followed by osmium tetroxide postfixation, glutaraldehyde-ruthenium red fixation followed by osmium tetroxide-ruthenium red postfixation, and two fast freezing / freeze substitution methods (FF/FS). The gross ultrastructure of the cuticle as well as the influence of the preservation method on this ultrastructure were identical for the two tissues studied. The cuticle ultrastructure was poorly preserved after glutaraldehyde fixation / osmium tetroxide postfixation. Its preservation was improved after ruthenium red was added in the fixative and postfixative, but the best preservation was consistently achieved using FF/FS. Both low-pressure freezing (plunge freezing) and high-pressure freezing were tested, the latter giving seemingly better results. With these methods, the cuticle appeared to be composed of a proximal lower cuticle, an intermediate upper cuticle, and a distal fuzzy coat. In particular, cryoimmobilization methods emphasized or revealed the occurrence of a well-developed fibrillar lower cuticle in the pedicellaria, the complexity of the upper cuticle which consisted of several zones, and the importance of the usually poorly preserved fuzzy coat that is actually the thickest layer of the cuticle. These observations bring new insights on the functions of the cuticle, and particularly of the fuzzy coat. According to its preservation characteristics, the fuzzy coat presumably consists mostly of proteoglycans. This composition could give it shock absorption and antifouling properties. Furthermore, its important thickness also implies that molecules detected by the short sensory cilia must diffuse through and could be selected by the fuzzy coat.

Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage

The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close ( approximately 1 microm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface. The topography of the samples was determined using Ru(CN)(6)(4-), a solute to which the cartilage matrix was impermeable. This revealed a number of pit-like depressions corresponding to the distribution of chondrocytes, which were also observed by atomic force and light microscopy. Subsequent imaging of the same area of the cartilage sample for the diffusion-limited reduction of oxygen indicated enhanced, but heterogeneous, permeability of oxygen across the cartilage surface. In particular, areas of high permeability were observed in the cellular and pericellular regions. This is the first time that inhomogeneities in the permeability of cartilage toward simple solutes, such as oxygen, have been observed on a micrometer scale.

Immunolocalization of collagen types II and III in single fibrils of human articular cartilage

Type II and III fibrillar collagens were localized by immunogold electron microscopy in resin sections of human femoral articular cartilage taken from the upper radial zone in specimens from patients with osteoarthritis. Tissue samples stabilized by high-pressure cryofixation were processed by freeze-substitution, either in acetone containing osmium or in methanol without chemical fixatives, before embedding in epoxy or Lowicryl resin, respectively. Ultrastructural preservation was superior with osmium-acetone, although it was not possible to localize collagens by this method. In contrast, in tissue prepared by low-temperature methods without chemical fixation, collagens were successfully localized with mono- or polyclonal antibodies to the helical (Types II and III) and amino-propeptide (Type III procollagen) domains of the molecule. Dual localization using secondary antibodies labeled with 5- or 10-nm gold particles demonstrated the presence of Types II and III collagen associated within single periodic banded fibrils. Collagen fibrils in articular cartilage are understood to be heteropolymers mainly of Types II, IX, and XI collagen. Our observations provide further evidence for the complexity of these assemblies, with the potential for interactions between at least 11 distinct collagen types as well as several noncollagenous components of the extracellular matrix.