The proximal tibial and humeral metaphysis from groups of animals sacrificed at recovery (R-0) and six (R+6) and twenty-nine days (R+29) postflight was measured using quantitative light microscopic techniques. One section of the proximal humerus of each animal was randomly selected. The following parameters were calculated for each band analyzed in the metaphysis: fractional bone and fractional calcified cartilage volumes; osteoblast, osteoprogenitor cell, and osteoclast nucleus numbers; ratios of osteoblast, osteoprogenitor cell, and osteoclast nuclei to surface area of bone; and fractional fatty marrow volume.

Results:

The study demonstrated a reduction in bone and calcified cartilage volume in flight and synchronous animals, in a region of the metaphysis where a maximum of calcified tissues was seen in vivarium controls. This was associated with a decreased number of functional bone cells (osteoblasts and osteoclasts) in both flight and (probably) synchronous groups. It was also clear that the metaphysis had returned to normal by the end of the 29-day recovery period. The fatty marrow volume was increased only in flight groups R-0 and R+6, but was normal in R+29 animals. The decreased amount of bone and calcified cartilage is believed to be the result of a temporarily slowed or arrested production of calcified cartilage as a substrate for bone formation. This would have resulted from slowed bone elongation during flight and synchronous control conditions. Since the synchronous group seemed to show significant changes quite similar to the flight animals, this data indicate that the general stress as well as the flight itself had an effect on the rate of bone elongation.