Purpose

The osteochondral unit is the pivotal element of the mammalian joint, with a fundamental clinical relevance. It is known that in terrestrial mammals, that the biochemical components of the tissues of the osteochondral unit are strongly preserved across a wide range of species. There is, however, very limited knowledge of specific adaptations of the architecture of the cartilage extracellular matrix in aquatic mammals.

The aim of this study was, therefore, to investigate if and how the structure of both the cartilage and the bone component of the osteochondral unit differs between mammals living on land or in water.

Methods and Materials

To investigate this we analyzed the microstructural composition and architectural features of the osteochondral units from the humeral head of six aquatic and nine terrestrial mammalian species. Osteochondral tissue samples were harvested post-mortem from the weight bearing central area of the humeral head. Histological assesment was performed, and included saf-O staining and polarized light microscopy to visualize the orientation of the collagen fibers. The mechanical properties were assessed by uniaxial unconfined compression and Micro Computed Tomography (micro-CT) was performed.

Results

Aquatic mammals feature cartilage with essentially random collagen fiber configuration, lacking the depth-dependent, arcade-like organization characteristic of terrestrial mammalian species. They have a less stiff articular cartilage at equilibrium with a significantly lower peak modulus, and at the osteochondral interface do not have a calcified cartilage layer, displaying only a thin, highly porous subchondral bone plate. Moreover, patches of cartilage tissue are present throughout the subchondral bone.

Conclusion

This totally different constitution of the osteochondral unit in aquatic mammals reflects that accommodation of loading is the primordial function of the osteochondral unit. Recognizing the crucial importance of this microarchitecture-function relationship is pivotal for the development of durable functional regenerative approaches for treatment of joint damage.