Light and electron microscopic morphology of the temporomandibular joint in growing and mature crab-eating monkeys (Macaca fascicularis): the condylar articular layer

Mandibular Cartilage Collagen Network Nanostructure: Insights for Regeneration

BACKGROUND

Mandibular condyle cartilage (MCC) has a unique structure among articular cartilages; however, little is known about its nanoscale collagen network architecture, hampering design of regeneration therapies and rigorous evaluation of regeneration experiment outcomes in preclinical research. Helium ion microscopy is a novel technology with a long depth of field that is uniquely suited to imaging open 3D collagen networks at multiple scales without obscuring conductive coatings.

OBJECTIVE

The objective of this research was to image, at the micro- and nanoscales, the depth-dependent MCC collagen network architecture.

DESIGN

MCC was collected from New Zealand white rabbits. Images of MCC zones were acquired using helium ion, transmission electron, and light microscopy. Network fibril and canal diameters were measured.

RESULTS

For the first time, the MCC was visualized as a 3D collagen fibril structure at the nanoscale, the length scale of network assembly. Fibril diameters ranged from 7 to 110 nm and varied by zone. The articular surface was composed of a fine mesh that was woven through thin layers of larger fibrils. The fibrous zone was composed of approximately orthogonal lamellae of aligned fibrils. Fibrocyte processes surrounded collagen bundles forming extracellular compartments. The proliferative, mature, and hypertrophic zones were composed of a branched network that was progressively remodeled to accommodate chondrocyte hypertrophy. Osteoid fibrils were woven around osteoblast cytoplasmic processes to create numerous canals similar in size to canaliculi of mature bone.

CONCLUSION

This multiscale investigation advances our foundational understanding of the complex, layered 3D architecture of the MCC collagen network.

Biomechanical properties of the mandibular condylar cartilage and their relevance to the TMJ disc

Mandibular condylar cartilage plays a crucial role in temporomandibular joint (TMJ) function, which includes facilitating articulation with the TMJ disc, reducing loads on the underlying bone, and contributing to bone remodeling. To improve our understanding of the TMJ function in normal and pathological situations, accurate and validated three-dimensional (3-D) finite element models (FEMs) of the human TMJ may serve as valuable diagnostic tools as well as predictors of thresholds for tissue damage resulting from parafunctional activities and trauma. In this context, development of reliable biomechanical standards for condylar cartilage is crucial. Moreover, biomechanical characteristics of the native tissue are important design parameters for creating functional tissue-engineered replacements. Towards these goals, biomechanical characteristics of the condylar cartilage have been reviewed here, highlighting the structure-function correlations. Structurally, condylar cartilage, like the TMJ disc, exhibits zonal and topographical heterogeneity. Early structural investigations of the condylar cartilage have suggested that the tissue possesses a somewhat transversely isotropic orientation of collagen fibers in the fibrous zone. However, recent tensile and shear evaluations have reported a higher stiffness of the tissue in the anteroposterior direction than in the mediolateral direction, corresponding to an anisotropic fiber orientation comparable to the TMJ disc. In a few investigations, condylar cartilage under compression was found to be stiffer anteriorly than posteriorly. As with the TMJ disc, further compressive characterization is warranted. To draw inferences for human tissue using animal models, establishing stiffness-thickness correlations and regional evaluation of proteoglycan/glycosaminoglycan content may be essential. Efforts directed from the biomechanics community for the characterization of TMJ tissues will facilitate the development of reliable and accurate 3-D FEMs of the human TMJ.

Tensile properties of the mandibular condylar cartilage

Mandibular condylar cartilage plays a crucial role in temporomandibular joint (TMJ) function, which includes facilitating articulation with the temporomandibular joint disc and reducing loads on the underlying bone. The cartilage experiences considerable tensile forces due to direct compression and shear. However, only scarce information is available about its tensile properties. The present study aims to quantify the biomechanical characteristics of the mandibular condylar cartilage to aid future three-dimensional finite element modeling and tissue engineering studies. Porcine condylar cartilage was tested under uniaxial tension in two directions, anteroposterior and mediolateral, with three regions per direction. Stress relaxation behavior was modeled using the Kelvin model and a second-order generalized Kelvin model, and collagen fiber orientation was determined by polarized light microscopy. The stress relaxation behavior of the tissue was biexponential in nature. The tissue exhibited greater stiffness in the anteroposterior direction than in the mediolateral direction as reflected by higher Young's (2.4 times), instantaneous (1.9 times), and relaxed (1.9 times) moduli. No significant differences were observed among the regional properties in either direction. The predominantly anteroposterior macroscopic fiber orientation in the fibrous zone of condylar cartilage correlated well with the biomechanical findings. The condylar cartilage appears to be less stiff and less anisotropic under tension than the anatomically and functionally related TMJ disc. The anisotropy of the condylar cartilage, as evidenced by tensile behavior and collagen fiber orientation, suggests that the shear environment of the TMJ exposes the condylar cartilage to predominantly but not exclusively anteroposterior loading.

Tissue Engineering the Mandibular Condyle

Tissue engineering provides the revolutionary possibility for curing temporomandibular joint (TMJ) disorders. Although characterization of the mandibular condyle has been extensively studied, tissue engineering of the mandibular condyle is still in an inchoate stage. The purpose of this review is to provide a summary of advances relevant to tissue engineering of mandibular cartilage and bone, and to serve as a reference for future research in this field. A concise anatomical overview of the mandibular condyle is provided, and the structure and function of the mandibular condyle are reviewed, including the cell types, extracellular matrix (ECM) composition, and biomechanical properties. Collagens and proteoglycans are distributed heterogeneously (topographically and zonally). The complexity of collagen types (including types I, II, III, and X) and cell types (including fibroblast-like cells, mesenchymal cells, and differentiated chondrocytes) indicates that mandibular cartilage is an intermediate between fibrocartilage and hyaline cartilage. The fibrocartilaginous fibrous zone at the surface is separated from hyaline-like mature and hypertrophic zones below by a thin and highly cellular proliferative zone. Mechanically, the mandibular condylar cartilage is anisotropic under tension (stiffer anteroposteriorly) and heterogeneous under compression (anterior region stiffer than posterior). Tissue engineering of mandibular condylar cartilage and bone is reviewed, consisting of cell culture, growth factors, scaffolds, and bioreactors. Ideal engineered constructs for mandibular condyle regeneration must involve two distinct yet integrated stratified layers in a single osteochondral construct to meet the different demands for the regeneration of cartilage and bone tissues. We conclude this review with a brief discussion of tissue engineering strategies, along with future directions for tissue engineering the mandibular condyle.

Microanatomy of the moose temporomandibular joint (Alces alces; Linnaeus, 1758)

Temporomandibular joint (TMJ) histology was investigated in 8 female Scandinavian moose. 5 were 1-year-old with a carcass weight (cw) of 125-140 kg, and 3 were 2-years-old (160-175 kg cw). The condylar articular surface consisted of a connective tissue lining with parallel collagen fibres. Numerous blood vessels were observed adjacent to the joint chamber. Below the fibrous layer, a proliferative cellular zone of undifferentiated mesenchymal cells was situated. These cells differentiated into chondroblasts and hypertrophied chondrocytes. Further down, an endochondral ossification process was initiated. Vertically directed invaginations were observed. A similar cellular organization was identified in the temporal component. However, the undifferentiated mesenchymal cell layer was discontinuous. The disc showed dense collagen bundles without main alignment. Vessels were identified throughout the entire disc. The results indicate that the cellular organization of the moose TMJ is similar to the TMJ histology found in other mammals but differences do occur.

Cartilage matrix production and chondrocyte enlargement as contributors to mandibular condylar growth in monkeys (Macaca fascicularis)

In an attempt to determine the contributions to condylar growth of cartilaginous matrix production and chondrocyte enlargement, four prepubertal male monkeys (Macaca fascicularis) were used. Each animal received 1 mCi 3H-proline per kilogram of body weight 24 hours before death. From the left condyles, the anterior and central segments of the growth cartilage in the lateral and medial joint region were examined with the light microscope. Volumes of cells and extracellular matrix, as well as the intensity of radioautographic labeling, were estimated stereologically in four layers from the articular surface to the zone of endochondral ossification. On the assumption of a steady-state system, these data led to the conclusion that an average chondrocyte in the anterior segment of the growth cartilage contributed to condylar growth by about 3000 microns 3 of newly formed matrix and about 1000 microns 3 of increased cell size. The respective estimates in the central segment were about 3700 microns 3 and 2000 microns 3. Thus, unlike findings in the condylar cartilage of 20-day-old rats, matrix production in the simian condyle exceeded chondrocyte enlargement, although in varying ratios of about 3:1 anteriorly and 1.85:1 centrally. On the other hand, chondrocyte enlargement from the anterior segment to the central segment increased by about 100%, while the increase of about 25% in matrix production was outweighed by variation between individual animals. These findings suggest that the relative contribution of the two growth components varies considerably, on the one hand, with the species or possibly with the stage of condylar development examined and, on the other hand, between various anteroposterior locations within the condylar cartilage.(ABSTRACT TRUNCATED AT 250 WORDS)