Spaceflight and age affect tibial epiphyseal growth plate histomorphometry.

Growth plate histomorphometry of rats flown aboard the Soviet biosatellite COSMOS 2044, a 14-day spaceflight, was compared with that of control groups. In growth plates of flight animals, there was a significant increase in cell number per column and height of the proliferative zone and a reduction in height and cell number in the hypertrophy/calcification zone. No significant differences were found in matrix organization at the ultrastructural level of flight animals, indicating that although spaceflight continues to affect bone growth of 15-wk-old rats, extracellular matrix is not altered in the same manner as seen previously in younger animals. All groups showed growth plate characteristics attributed to aging: lack of calcification zone, reduced hypertrophy zone, and unraveling of collagen fibrils. Tail-suspended controls did not differ from other controls in any of the parameters measured. Our results suggest that growth plates of older rats are less responsive to unloading by spaceflight or suspension than those of younger rats and provide new evidence about the modifying effect of spaceflight on the growth plate.