Long bone growth and evolution revealed by three-dimensional imaging

Sammanfattning: Propagation phase-contrast synchrotron radiation microtomography is a non-destructive method used for studying histology in three dimensions (3D). Using it, the 3D organization of the diaphyseal cortical vascularization in the humerus of two seymouriamorphs was analyzed in this thesis. Their vascularization suggests a combination of active growth and a long pre-reproductive period, an intermediate condition between that of Devonian tetrapods and early amniotes, reflecting a gradual change in evolution. The focus of the thesis then shifts to the metaphysis of long bones. The latter possesses complex 3D structures difficult to capture in 2D images. Observations in extant tetrapods have shown that hematopoiesis in long-bones requires the presence of tubular marrow processes opening onto an open medullary cavity with a centralized vascular system. A network of tubular marrow processes was found in connection with interconnected small cavities in the metaphyses of seymouriamorphs which may have acted as open spaces containing a centralized vascular mesh. Based on this interpretation, the long-bone marrow cavity of the Permian stem-amniotes studied here could have been the oldest evidence of possible hematopoiesis among tetrapods. As a third focus, both computer simulations (Finite Element Analysis) and empirical experiments were conducted to investigate the role of Secondary Ossification Centers (SOCs) within the epiphyses of mammals. The results indicate that the presence of a SOC protects the growth plate from mechanical stresses, allowing the cells there to withstand six times more stress. Finally, the 3D microanatomy of the metaphyses and epiphyses in the humeri of monotreme, marsupial and placental extant mammals were investigated at different developmental stages. The data were used to produce a nomenclature based on the degree of epiphyseal ossification encompassing the entire development of all the condyles within a single epiphysis. This nomenclature was used to describe the epiphyseal development in a large group of mammals and highlight differences in ossification timing between groups. These results offer a unique glimpse into the development and evolution of long-bones. They highlight the value of visualizing long-bone microstructure in both 2D and 3D, and the need to develop new nomenclatures that reflect the 3D nature of the data.