Online Program Schedule

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Session Schedule & Abstracts




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Thursday 30th June, 2016

BON1
Symposium: New insights into skeletal microstructure of vertebrates, extant and extinct 1

Room: Salon A   9:30 am–11:00 am

Moderator(s): E. Rega, M. Dean, & T. Owerkowicz
BON1-1  9:30 am  Scaling of Haversian systems in a phylogenetically diverse sample of mammals is consistent with physical and physiological constraints. Middleton KM*, University of Missouri; Hurtado AC, Brown University; Swartz SM, Brown University   middletonk@missouri.edu
Abstract: Haversian systems (HS) or secondary osteons are the hallmarks of bone remodeling, resulting from the resorption of older bone via osteoclast activity and deposition of new bone by osteoblasts. The outer edge of each HS is marked by a cement line, where the HS intersects with either primary bone or another, older, HS that it is replacing. Although some canaliculi, the cytoplasm-containing channels connecting osteocytes, cross the cement line, the vast majority of osteocytes within a single HS are supplied via the HS’s central canal. The requirement for nutrients and oxygen to diffuse from the central canal out to the cement line means that the size of a single HS might be constrained either by the laws of physics or physiology. We tested the hypothesis that the size of a single HS might scale either isometrically, suggesting a diffusion-limited process, or following basal metabolic rate (BMR), in a phylogenetically diverse range of mammals across over four orders of magnitude in body mass. Using an historical sample of femoral mid-diaphyseal cross-sections, we estimated cortical area, counted total number of HS and calculated mean HS diameter and percent remodeled bone. Contrary to the conventional view that remodeling is absent or rare in small mammals, we found strong phylogenetic signal in HS count, with evidence for both HS in small mammals and large mammals with relatively few HS. Phylogenetically informed reduced major axis regression of mean HS area on body mass revealed a scaling exponent of 0.68 (95% CI 0.59-0.76). The 95% CI included both isometry and the often-cited mass-specific scaling relationship for BMR, 0.75. We conclude that HS size in mammals is likely limited by either diffusion or scaling, with evidence for each currently equivocal. Based on the finding of significant phylogenetic signal in HS counts, we propose that this trait may track phylogenetic patterns in basal metabolic rate, however this hypothesis remains to be tested.

BON1-2  10:00 am  Osteocyte mechanobiology: influence on bone modeling and remodeling and its bearing on functional interpretation of skeletal morphology. Main R.P.*, Purdue University   rmain@purdue.edu
Abstract: The bony skeleton displays a high degree of phenotypic plasticity during development and adulthood in response to environmental, dietary, and mechanical stimuli. The influence of mechanical stimuli on bone morphology is hypothesized to be mediated by matrix-bound osteocytes regulating bone formation and resorption. The skeletons of all major amniote groups respond to differential physical stimuli, suggesting a plesiomorphic function for osteocytes in mechanosensitive anabolic pathways. In this talk, I review our current understanding of the fundamental cellular mechanisms by which the skeleton senses and responds to mechanical stimuli, current in vivo and in vitro models for studying adaptive plasticity in the skeleton, and the extent to which we can use skeletal histomorphology to infer functional skeletal adaptation. While much has been learned in the past thirty years about vertebrate skeletal mechanobiology using model taxa, fundamental questions remain regarding how taxonomic variation in the osteocyte lacunar-canalicular network might affect the potential for different groups to adapt to physical stimuli, the role metabolic rate may play in skeletal plasticity, and finally, how the skeletons of vertebrate taxa lacking osteocytes (e.g. many teleosts, chondrichthyans) respond to changes in physical demands on the skeleton.

BON1-3  10:15 am  Bone microstructure in hibernating mammals with implications for mechanical performance. Donahue SW*, Colorado State University; Wojda SJ, Colorado State University; Hinrichs J, Colorado State University; McGee-Lawrence ME, Georgia Regents University   seth.donahue@colostate.edu
Abstract: Physical inactivity leads to increased bone resorption, elevated serum and urinary calcium concentrations, bone loss, bone mechanical property loss, and increased fracture risk in humans and the experimental menagerie (e.g., mice, rats, turkey, dogs, sheep). Grizzly and black bears, yellow-bellied marmots, arctic ground squirrels, and 13-lined ground squirrels do not lose bone mass or mechanical properties during prolonged (4-8 months) hibernation. Bone remodeling (i.e., bone resorption and formation) continues during hibernation, although at significantly reduced levels compared to summer levels in bears and marmots. These changes in bone remodeling during hibernation leave histological signatures in bone such as the density of secondary osteons and lines of arrested growth (LAG). Hibernating bears are anuric, yet serum calcium concentration remains at homeostatic levels throughout the entire year. These findings suggest that hibernating bears and rodents have biological mechanisms to preserve bone tissue integrity when challenged with prolonged physical inactivity. Reduced bone remodeling in hibernators likely contributes to the conservation of metabolic energy. Neural signals and circulating factors (e.g., calcium regulatory hormones) likely contribute to the changes at the bone cell level that are involved in bone tissue preservation. Normal balance between bone resorbing osteoclasts and bone forming osteoblasts is likely maintained to preserve normal serum calcium concentrations during anuria. Identification of the molecular mechanisms that regulate bone cell function during hibernation may contribute to the development of new therapies for osteoporosis and inform our understanding of how hibernators have adapted to survive extreme environmental conditions. Funding from NIH (NIAMS AR050420).

BON1-4  10:30 am  Developmental mechanisms and evolutionary advantage of metatarsal elongation and fusion in bipedal jerboas (Dipodidae). Saxena A, University of California San Diego; Gutierrez HL, University of California San Diego; Cooper KL*, University of California San Diego   kcooper@ucsd.edu
Abstract: Throughout the radiation of vertebrates, alterations to the size and shape of skeletal elements have transformed the ancestral body plan and allowed species to expand into niches in all three-dimensions. For example, extreme hindlimb modifications in the bipedal jerboas enable ricochetal locomotion at high speeds over long distances in an open desert environment. The disproportionately elongated metatarsals shift the hindfeet rostral to the center of mass, and fused metatarsals are thought to resist the increased bending forces associated with bipedal take off and landing. Here, we explore the developmental mechanisms that establish growth rate and proportion in the limb bones and that promote lateral fusion of the metatarsals. We have shown that the size of terminally differentiated hypertrophic chondrocytes, and thus endochondral growth rate, is influenced by the amount of cellular dry mass produced after a phase of cytoplasmic swelling. Current work is focused on identifying the genetic mechanism(s) that regulate mass production in growth plates that elongate at different rates. Together with metatarsal elongation, the most derived of the jerboas have co-evolved fusion of the three central metatarsals into a single element. We find this occurs gradually from the second to fourth week of postnatal development and involves precise pattern and activity of osteoblasts that deposit bone around the circumference of the three elements while osteoclasts metabolize bone at the interfaces. These phenotypes together provide an opportunity to identify the cellular and genetic mechanisms that shape the skeleton during evolution and that contribute to bone development.

BON1-5  10:45 am  Effects of growth rate and flight on wing bone laminarity in bats and birds. Lee AH*, Midwestern University   alee712@gmail.com
Abstract: During flapping flight, vertebrate wing bones presumably experience high torsional loads. To resist these flight-induced loads, the wing bones of volant vertebrates have independently evolved similar anatomical form. Whether or not the histological form of wing bones is also constrained to resist torsion is less clear. Previous work suggests that laminar bone, which is a vascularized tissue with abundant circumferentially oriented vascular canals, may be a flight adaptation in adult birds. I have shown, however, that laminar bone is not necessary for flight; bats do not have any laminar bone in humeri despite them being as rigid to torsion as those in comparably-sized birds. Phylogenetically-informed scaling analyses reveal that birds simply grow faster than bats. These results suggest that laminar bone is partly influenced by growth rate. To test this hypothesis further, my lab has tracked the proportion of laminar bone in the humerus, ulna, and radius of developing pigeons. Contrary to biomechanical expectations, wing bone laminarity is lower in post-fledge individuals than in pre-fledge ones. Logistic regression reveals a direct and significant correlation between wing bone laminarity and growth rate. Put together, my work suggests that wing bone laminarity may an expression of ontogenetic allometry. Evolutionary shifts in ontogenetic allometry may explain interspecific variation of wing bone laminarity across birds.



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