Deep time evolution of bone cells and the implications for bone metabolism in vertebrate history

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Most vertebrates have cellular bone in which bone cells (osteocytes) are enclosed in cave-like lacunae within the bone matrix and are connected to each other via dendritic cell processes that extend in a highly complex network of tubules (canaliculi). Recent studies have shown that this so called lacunocanalicular network plays a pivotal role in bone metabolism, like orchestrating modeling and reshaping of bone surfaces according to strain changes, detecting micro-cracks in bone and inducing their repair, and playing a significant role in mineral homoeostasis. Furthermore, it has been shown that the number of osteocytes per unit of bone volume is proportional to bone growth rate, and their density and shape varies depending on the local strain environment that acts on a certain bone during its formation. Given the multitude of functions in bone metabolism of extant vertebrates, it is inferred that patterns of osteocyte distribution and morphology allow new insights into the biology of fossil vertebrates. However, despite the fact that the lacunocanalicular network is usually well preserved in fossil bone, the evolutionary history of osteocytes is poorly known. In the proposed project, the lacunocanalicular network will be investigated quantitatively and qualitatively throughout vertebrate history to gain a deep time perspective on the evolution of osteocytes, spanning the range from the first vertebrates with cellular bone, the jawless osteostracans, to basal jawed vertebrates (placoderms and acanthodians), osteichthyan fishes, stem-tetrapods, amphibians, and amniotes. Fossil bone will be investigated by light microscopy, scanning electron microscopy (SEM), and synchrotron X-ray phase contrast tomography. The focus will lie on lacunar density and arrangement, lacunar shape and size, morphology of canaliculi, as well as the type of bone matrix in which the lacunocanalicular system is embedded. The results obtained from fossil taxa will be compared with osteological thin sections derived from extant vertebrates for which the metabolic rate is known, and with data from modern bone cell biology. This will allow to draw inferences about metabolic rate, bone modeling and remodeling, and local strain regimes that acted on the skeleton in long-extinct animals and will be regarded in the context of body size, ontogenetic age, and habitat (e.g., aquatic versus terrestrial). A test of phylogenetic signal of lacunocanalicular characters in different lineages of vertebrates will be conducted using different metrics. The results of this project will be important for our understanding of the development of bone metabolism throughout vertebrate history and especially against the background of major evolutionary transitions.