70 - The Onset and Development of Air Space in the Humerus of the Domestic Turkey

Poster Board Number: 70
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Co-authors:

Kristin Stover - Assistant Professor, Ohio State University; Joseph Groenke - Ohio University; Nicholas Sze - Wright State University; Patrick O'Connor - Ohio University

Abstract Body :Postcranial skeletal pneumaticity occurs when diverticula of the respiratory system infiltrate and expand within individual marrow-filled bones. Unique to birds among living tetrapods, the fossil record indicates this trait first evolved ~ 200 million years ago in non-avian dinosaurs and pterosaurs. Whereas previous research has characterized variation in pneumaticity expression across different species, the development of pneumaticity within individual bones or the skeletal system more generally remains poorly understood. Identifying the developmental mechanisms that facilitate pneumatization is key to understanding the variable expression of this trait both within individual skeletons and across species. The goals of the current study, using an in-development turkey model, include: (1) establishing the temporal onset of pneumatization in the humerus and (2) quantifying expansion of air space within the element throughout a post-hatch ontogenetic series.Using micro-CT data from turkeys spanning an ontogenetic series, we tracked the first appearance and progression of pneumatic air spaces within the humerus. We measured humeral whole bone and internal air space volume in post-hatch individuals between Week 0 and Week 22. From these data, we (1) identified when pneumaticity first appears (radiographically), (2) characterized pneumatic expansion through the interior of the bone, and (3) quantified the relative proportion of air in the humerus.Pneumaticity first appears no later than Week 4 post-hatch. Air space proportion varies between 0 and 38% over the growth series. Air space initially expands from proximal to distal within the humerus, with peak internal air space proportion occurring at Week 11. Subsequent to this time point, relative internal air proportion decreases as bone growth continues. We also identified significant differences in air space volume between left and right humeri of the same individual. The variable relationship between air space expansion and bone growth, not to mention the observed bilateral asymmetry, was unexpected but suggests that pneumaticity expression is influenced by factors related to end-stage skeletal growth and/or wing loading (e.g., increased forelimb activity with display, etc.).This study is a prerequisite for future projects aimed at characterizing tissue-level interactions driving epithelial expansion into the skeleton at specific ontogenetic time points (e.g., pre-, and post-invasion). The dynamic nature of air space expansion within the humerus also indicates a need to assess factors related to skeletal growth and bone loading that potentially impact the pneumatization process, particularly as it pertains to driving phenotypic variation in skeletal pneumaticity in birds more generally.