AbstractsBiology & Animal Science

Ambient temperature impacts limb and body growth in mammals. Animals chronically exposed to cold have stouter bodies and shorter extremities than littermates housed at warmer temperatures. These changes resemble the ecological-thermoregulatory predictions of Allen’s Rule, but the mechanisms underlying this response are unknown. This study tested the hypothesis that cold temperature reduces bone blood supply and thereby elongation, while warm temperature enhances bone perfusion and growth. METHODS: Male CF-1 mice (N=95) were housed at 7, 21, and 27C from weaning (3.5 weeks) through maturity (12 weeks) and diet and activity levels were recorded. Prior to euthanasia at 4.5, 6.5 and 12 week age points, mice received an intracardiac injection of fluorescent microspheres to measure regional blood flow. RESULTS: Ears, limbs, and tails of warm-reared mice were significantly longer than in the cold with no change in body mass. Cold-reared mice had the shortest extremities, consumed the most food, were most active, and had enlarged hearts and kidneys. Hindlimb bone blood flow was significantly decreased in the cold mice suggesting that vascular factors might underlie the growth differences, however, bone blood flow was not consistently increased at the warmer temperatures suggesting a potential threshold effect. In vitro experiments demonstrated that cold temperature inhibits, and warm temperature enhances, growth of cultured mouse metatarsals absent intact vasculature indicating a direct effect of temperature on the growing tissue. Analyses of cartilaginous growth plates revealed significant temperature-associated changes in cell size, cell proliferation, and extracellular matrix. These results closely match in vivo data in which ear and tail temperature were strongly correlated with their overall growth. CONCLUSION: Hindlimb vascular supply was significantly reduced in the cold, but the variation at warmer temperatures suggests that this may not be the primary mechanism causing temperature-induced alterations in skeletal growth. When viewed together with the in vivo tissue temperature data, results from in vitro metatarsal cultures strongly support a direct role of temperature in modulating bone and cartilage growth. These results have important implications for interpreting skeletal morphology of mammalian species living at climatic extremes.