Identifying Mechanisms and Evolutionary Implications of Mechanosensitive Growth in the Embryonic Limb Skeleton

The developing skeleton experiences loading as a result of embryo movement. This is necessary for normal longitudinal growth of limb skeletal elements. Herein, we address the hypothesis that mechanical input can alter limb proportions by modifying growth. We aimed to identify mechanisms which underpin altered growth in response to mechanical input and demonstrate that environmental factors such as incubation temperature can alter motility in oviparous species, altering limb proportions. Embryonic chicks were immobilised with a neuromuscular blocker between day 10‐18 of development and growth monitored by repeat MRI scans. Control embryos were treated with vehicle only. Crocodile eggs were incubated at 28°C and 32°C and movement monitored throughout development. Embryos were euthanized at near‐hatching age and body size and limb proportions measured. Proliferation, hypertrophy and apoptosis, components of endochondral ossification, were evaluated in limb growth cartilage of all groups. Growth was reduced by immobilisation predominantly in the metatarsals and femur in chicks. The distribution of proliferation was altered, volume of hypertrophic chondrocytes decreased (29% P<0.05) and apoptosis increased in these elements, suggesting that mechanical input is necessary to direct and regulate these processes. Temperature influences motility of embryonic crocodiles and alters limb proportion, with greater temperatures producing longer tibias and shorter digits. This may explain the variation in limb proportionality observed within some reptile species and our data suggest that plasticity in limb form during development may be evolutionarily advantageous. This work is funded by the Anatomical Society.