PREOSTEOBLAST PRODUCTION 55 HOURS AFTER A 12.5-DAY SPACEFLIGHT ON COSMOS-1887

The influence of 12.5 days of spaceflight and a 55 h stressful recovery period (at 1 g) on fibroblastlike osteoblast precursor cells was assessed in the periodontal ligament (PDL) of rats that were 91 days old at launch. Nuclear morphometry was used as a marker for precursor cell differentiation in 3 μm sections cut in the midsagittal plane from the maxillary first molar. According to nuclear volume, cells were classified as preosteoblasts (C+D cells, ≥ 120 μm3) and less differentiated progenitor cells (A+A' cells, 40-79 μm3). Compared with synchronous controls (stimulated flight conditions), the 55 h postlight recovery period at 1 g resulted in a 40% decrease in the A+A' cell population, a 42% increase in the C+D cells, and a 39% increase in the number of PDL fibroblastlike cells near the bone surface. These results are consistent with a postflight osteogenic response in PDL. This recovery response occurred despite physiological stress in the flight animals that resulted in a highly significant (P ≤ 0.001) increase in adrenal weight. The data suggest that after spaceflight there is a strong and rapid recovery mechanism for osteoblast differentiation that is not suppressed by physiological stress.