This experiment was to examine space flight effects in the non-weightbearing
bone of the rat periodontal ligament. The objective is to determine the relative
influence of weightlessness on cell census of an osteogenic tissue. Previous
flight data have documented a compromise in preosteoblasts, but the present
was the first opportunity to evaluate postflight recovery of osteoblast differentiation.
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 per- fused in situ while bones and teeth of the smalll animals were
fixed by immersion in neutral buffered formalin. Maxillary halves were demineralized;
specimens were divided with a razor blade along the midsegittal plane of the
mesial root of the first molar; and the medial surface was embedded. First molars
and surround- ing periodontium were serially sectioned at 3 µm 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 (³170 µm3).
Since the hitogenesis sequence is A'A'C'D'osteoblast, the relative incidence
of A+A' to C+D cells is an osteogenic index. An insignifcant difference in A+A'
or C+D cells may reflect partial recovery of preosteoblast formulation (AÞC)
during the twelve-hour postflight period. Large flight rats, however, demonstrated
increased numbers of A+A', indicating an inhibition of preosteoblast formation
(AÞC). Hence, at least with older rats, a seven-day flight is adequate to reduce
PDL osteogenic potential, suggesting an inhibition in PDL osteoblast differen-
tiation 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.
W.E. et al.: Census of Osteoblast Precursor Cells in Periodontal Ligament
(PDL) of Spacelab-3 Rats. Abstract 83.3. 36th Annual Fall Meeting, American
Physiological Society, Buffalo, N.Y., October 13-18, 1985, Physiologist, vol.
28, no. 4, 1985, p. 376.
Roberts, W.E. et al.: Nuclear Morphometric Analysis of Osteoblast Precursor
Cells in Periodontal Ligament, SL-3 Rats. American Journal of Physiology, vol.
252, 1987, pp. R247-R251.
¥ = publication of related ground-based study