Published data demonstrate that mammals from the same litter reared at different temperatures have different adult limb and body proportions in parallel with those of natural populations living at climatic extremes. Animals raised in cold conditions have stout bodies with shortened limbs and extremities, while those housed at warmer temperatures exhibit peripheral appendage elongation. These changes resemble those predicted by "Allen's rule," the ecogeographical principle long recognized to be an adaptive mechanism for thermoregulation. Although typically thought to be genetic in nature, some of these phenotypic changes appear to be ontogenetic responses to temperature stress; however, only descriptive studies have been reported to date and the mechanisms underlying such changes are unknown and have not yet been investigated. Temperature may directly influence cellular kinetics such that growth rate is increased at warm temperatures but retarded in the cold. If correct, then these changes should be manifested in cartilaginous growth plates, sites of rapid longitudinal bone growth during postnatal development. Impaired vasculature could also limit growth rate by reducing the amount of essential oxygen, nutrients, and hormones available to developing bones via cold-induced vasoconstriction. There are currently no empirical data to support these ideas, and growth plate morphology in animals chronically exposed to extreme temperatures throughout ontogeny remains unexplored.
The objective of this study is to test the hypothesis that cold temperature limits bone growth by reducing growth plate kinetics and/or vascular supply, and that warm temperature enhances these processes. A model for investigating climate-induced skeletal changes will be established by housing mice at different temperatures (7, 20, and 27 degrees C) during their active growth period as previously described in the literature. Basic growth data will be recorded throughout a nine week experimental period and an ontogenetic series will be created in order to examine the effects of temperature on bone histology and blood flow at different stages of development. Cell proliferation and apoptosis (programmed death) in long bone growth plates will be quantified using immunohistochemistry, and detailed morphology will be studied under light microscopy. Vascular supply will be analyzed using a combination of immunohistochemical methods to detect new vessel formation and fluorescent microsphere techniques to measure regional bone blood flow.
SCIENTIFIC MERIT: This study will provide information essential to understanding how species modify skeletal proportions in response to environmental pressures and could thereby profoundly impact the manner in which intra- and inter-specific variation in geographical clines is perceived. By discriminating the relative influence of temperature on bone growth and blood supply, this work can further elucidate the morphological variation observed in the fossil record and might also help resolve major phylogenetic controversies across disciplines.
BROADER IMPACTS: These results will enhance understanding of the complex factors involved in regulating normal longitudinal bone growth. The parameters to be examined here are relevant to a variety of biomedical research models of skeletal injury and disease, especially those involving impaired bone growth and vasculature. By integrating ideas from anthropology, ecology, molecular biology, and physiology, this project encourages collaboration between students and senior researchers from diverse backgrounds and specialties.
