A dynamic pattern of mechanical stimulation promotes ossification in avian embryonic long bones.

We have performed a set of finite element analyses of embryonic chick hindlimb skeletal rudiments at several time points during development, around the time of initial bone formation. Using optical projection tomography, we created anatomically accurate rudiment and muscle morphologies for each stage. The changes in pattern and magnitude of biophysical stimuli (such as stress, strain, hydrostatic pressure and fluid flow) were computed, and were found to transform as bone formation proceeded in the rudiment. For each biophysical stimulus, a single concentration of the stimulus was predicted at the mid-diaphysis some time before initial bone formation begins. Then, several hours before ossification, two concentrations of the stimuli were predicted distal and proximal to the prospective bone collar. Once bone formation had begun, high concentrations of the stimuli were maintained proximal and distal to the bone collar. We propose the hypothesis that patterns of biophysical stimuli resulting from mechanical loading due to muscle contractions initiate and propagate ossification in avian embryonic long bones, whereby a region of the perichondrium experiences a period of time under high cyclic stimulus levels, promoting chondrocyte hypertrophy at the core and prompting bone collar formation some hours later.