Cortical bone remodeling in Amniota

Abstract

In this dissertation, contributions to understanding the formation, function and evolution of secondary osteons are presented. By using comparative methods and including fossil taxa, new insights are developed. I analyze and describe several aspects of secondary osteons to uncover functional, ecological and phylogenetic implications that can be applied across taxa. Unlike most studies that focus on a particular species, the inclusion of a diverse number of taxa provides insight and allows for assertions regarding preconceived ideas of the form and function of secondary osteons.<br /> The secondary osteon can arguably form from either a response to damage or from a need to mobilize mineral. However, it seems that secondary osteons may be mostly influenced by the former, with changes in loading affecting many aspects including orientation, size and lamellae organization, as well as distribution of Haversian tissue. Only in cases of high demand for mineral, such as during the antler growing season, does resorption of the cortex (and subsequent infilling - formation of secondary osteon) take place. Even so, each element can be affected differently and the even distribution patterns of resorption suggest that these resorptions spaces are regulated to maintain mechanical competence of the bone. Since the need of mineral in the body can be obtained from other means, such as diet, efflux of calcium on quiescent bone surfaces and osteocytic osteolysis or via bone remodeling of cancellous bone, all of which may have the added benefit of more efficient transport to circulatory system, secondary osteons do not necessarily need to be implicated directly in mineral mobilization. Additionally, secondary osteons first appeared early in vertebrate evolution among the placoderms in the early Silurian seas. It is likely that calcium and phosphorus could easily be acquired from the surrounding water, thus secondary osteons were likely forming for repairing bone or maintaining bone quality.<br /> Mass is a strong indicator of the overall presence of secondary osteons, but also osteon size. Secondary osteons may be regulated by mechanical stimuli with a tendency to form along the main loading direction. However, in dierent environments, from terrestrial to aquatic, this stimulus is reduced or altered such that osteons are less regulated and irregular shapes are more likely to form. Even though most squamates and crocodiles do not form secondary osteons; the largest species do tend to form Haversian tissue. Distribution patterns are not all the same in the large species (e.g., sauropod dinosaurs) and relative growth rates and resorption of the medullary region during growth may influence the extent of Haversian tissue development. Haversian tissue increases with age and this is likely related to increased microcrack density, osteocyte apoptosis and increase brittleness of the bone. Overall, bone quality deteriorates with time and in some cases needs to be repaired or maintained by providing new bone tissue in the form of secondary osteons. The lack of secondary osteons in most ectotherms may be due to high safety factors and overdesigned bones; with relatively low activity levels, crocodiles and lepidosaurs may not require repair as readily as many birds and mammals. The increase in metabolic rate may provide a means to maintain higher remodeling rates. In hibernating animals, a decrease in metabolic rate also coincides with a decrease in bone remodeling. The overall contribution of load and damage may be relatively reduced in small animals as both endotherms and ectotherms rarely show secondary osteons. However, in the furcula, a bone that undergoes a large amount of deformation, even very small bird species develop Haversian bone.<br /> To understand the observations of secondary osteons, consideration of the environmental context, bone quality and type and magnitude of loading are needed. With a deeper understanding of the function and development of secondary osteons in modern taxa, paleobiological inferences can be made within extinct taxa, providing additional tools to uncover the life history of fossil organisms, and in turn, fossil taxa can provide novel insights into the evolution and function by extending the taxonomic diversity.