The right mandible, ulna/radius, both sides of the maxilla, and last two thoractic and first lumbar vertebrae were recovered from large (350 g at launch) and small (200 g at launch) space-flown rats and comparable controls. Large rats were perfused in situ while bones and teeth of the small animals were fixed by immersion in neutral buffered formalin. Maxillary halves were demineralized; specimens were divided with a razor blade along the midsagittal plane of the mesial root of the first molar; and the medial surface was embedded. First molars and surrounding periodontium were serially sectioned at 3 micrometers and stained. Nuclear length and width were measured at 1,000x with an ocular micrometer. Nuclear volume was calculated and fibroblastlike cells were classified: A+A' (40-79); B (80-119); C (120-169); and D ( 170 µm3).

Results:

Since the histogenesis sequence is A'A'¹C'D' osteoblast, the relative incidence of A+A' to C+D cells is an osteogenic index. An insignificant difference in A+A' or C+D cells may reflect partial recovery of preosteoblast formation (A¹C) during the twelve-hour postflight period. Large flight rats, however, demonstrated increased numbers of A+A', indicating an inhibition of preosteoblast formation (A¹C). Hence, at least with older rats, a seven-day flight is adequate to reduce PDL osteogenic potential, suggesting an inhibition in PDL osteoblast differentiation and/or specific attrition of C+D cells, that does not recover by twelve hours postflight. As circadian rhythmicity is important to the mechanism of osteoblast histogenesis, disruption of the circadian timekeeping system in SL-3 rats may have also interfered with normal osteoblast production. The effect of short-duration space flight on relative numbers of osteogenic cells may be a composite response to multiple factors, including unloading, fluid shifts and altered circadian rhythm.