Histomorphometric assessment of double-zonal osteons in human cortical bone

The process of bone remodeling produces four different types of osteons: type I (the most common), type II, drifting and zonal osteons (DZ). The latter, characterized by a hyper-mineralized ring within osteon lamellae, have not been studied extensively in the anthropological literature. While they have been recognized since the 1970s, DZ osteons have given rise to more hypotheses than answers about the possible causes of their formation, probably because of misidentifications and a lack of reproducible protocols. Specifically, it has been proposed that these osteons may be the result of systemic physiological stress, but others have argued that they would be a natural product of aging.  In this context, the purpose of this project is to investigate the possible causes of the formation of DZ osteons and consists of three parts: 1) the development of a protocol in polarized light microscopy that is tested against a gold standard method; 2) the evaluation of metabolic influences linked to sex, age and pathologies, the latter being based on a comparison between individuals diagnosed with metabolic diseases and those without evidence of pathological condition on the formation of DZ osteons and 3) the evaluation of mechanical loads on the formation of DZ osteons. The sample used in this study consists of Eurocanadian settlers from St. Matthew Historical Cemetery, Quebec City (1771-1860).  The results presented show that 1) the use of polarized light microscopy to identify DZ osteons is a reliable and accurate alternative to more expensive technologies. In addition, new insights regarding the formation of DZ osteons has been provided by revealing an abrupt alteration or change in the orientation of collagen fibers at the same location as the hyper-mineralized ring. 2) No difference was observed between the different groups, either by pathological status, age or sex, suggesting that DZ osteons are not associated with metabolic changes. 3) According to the characterization of DZ osteons by a change/alteration in the orientation of collagen fibers, the hypothesis according to which higher biomechanical loads would induce the formation of DZ osteons was explored on paired femurs and humerus. The results do not allow to unequivocally support this hypothesis: whereas the femur has a greater density of DZ than the humerus, the interindividual mechanical loading variation was negatively correlated with DZ osteon frequency. Additionally, DZ osteons were found to be significantly smaller than type I osteons, suggesting that the size of the resorptive bay is a determining factor in DZ formation. Thus, the overall results presented in this dissertation suggest that DZ osteons are not the result of metabolic or mechanic instabilities. Instead, they are formed like type I osteons, but in the context of a smaller resorptive bay. Their smaller size implies fewer cells responsible for bone formation, which could increase the risk of impairment in the matrix collagen synthesis, leading to the subsequent hyper-mineralization. Consequently, their formation would be the result of osteoclastic activity and future research should focus on that aspect.

Keyphrases: Double-zonal osteons., Histomorphometry, Bone remodeling, Biomechanics, Cross-sectional geometry, Paleopathology, Polarized-light Microscopy, Type I osteons, Scanning electron microscopy