The Effect of Hypergravity (2g) on Osteoblast Precursor Cells in the Periodontal Ligament of the Rat

Abstract

The effect of weightlessness on bone and osteoblast precursor cells has previously been studied. A marked decrease in bone formation, an increase in less differentiated committed osteogenic cells (A+A'), and a decrease in preosteoblast cells (C+D) was noted. To date, the effect of hypergravity (2g) on osteoblast histogenesis has not been studied in vivo. In vitro studies using nonphysiologic high levels of gravity (20,40g) have shown an increased proliferation of cloned osteoblast-like cells. The purpose of the present study was to evaluate the effect of hypergravity (2g) on osteoblast histogenesis in the rat periodontal ligament (PDL), on the width of the mesial PDL, and on the percentage of forming bone surface on the mesial side of the tooth. Twenty male Wistar rats (SPF: Harlan Sprague Dawley) were randomly assigned to the centrifuge (experimental) or to the stationary (control) group. The experimental group was centrifuged for 14 days at 2g and the stationary group was housed in identical cages in the centrifuge room. The PDL of the mesial and distal surface of the mesial root of the first maxillary molar was analyzed microscopically 100 μm above and below the midroot area. Nuclear volume morphometry was used to classify periodontal ligament cells as: L cells (<40 μm3), A+A' cells (40-79 μm3), B cells (80-119 μm3), C cells (120-169 μm3), and D cells (>170 μm3). The percent of forming bone surface on the mesial side and the width of the PDL were also measured. A 2x2 factorial ANOVA with repeat measures revealed a significant (p < 0.05) decrease in the C cell population and a nearly significant (p < 0.06) increase in the A+A' cell population in the centrifuge group. Comparing bone surfaces, the forming surface had a significant (p < 0.01) increase in the C and D cell populations, a significant (p < 0.01) decrease in the L and A+A' cell populations, and a significant (p < 0.05) decrease in the B cell population. The stationary group weighed significantly (p < 0.01) more than the centrifuge group post-experiment. And an unpaired t-test revealed a nearly significant (p < 0.06) increase in the percent forming bone surface on the mesial side of the maxillary first molar and no significant difference in mesial PDL width. The results showed that the centrifuge group had a trend toward a block in preosteoblast formation. This is similar to that seen with hypogravity. However, it cannot be concluded at this time if this is a direct gravitational effect or related to other factors such as physiological response to stress. Physical stress has been suggested as a potential mechanism for the observed decrease in weight seen in centrifuged animal, while the PDL width does not seem to be affected by gravitational forces, and thus may not be a sensitive marker to osteoblast differentiation inhibition. Finally, the reason for the increase in forming bone surface in the centrifuged group is unclear.