Cell Kinetic Analysis of Osteoblast Histogenesis in the Rat Mandibular Condyle Following Exposure to Hard and Soft Diets

Abstract

Microgravity of spaceflight has been associated with inhibition of bone formation, decreased number of osteoblasts (Ob), and suppression of Ob histogenesis. This study was designed to study Ob histogenesis, i.e., the proliferation and differentiation of cells producing Ob. Inhibition of Ob formation seems to be a principal factor in osteopenia induced by spaceflight. Osteoblast histogenesis is a stress/strain mediated mechanism and appears to be gravity dependent under certain conditions. On Spacelab-3 (SL-3), the rat mandibular condyle was evaluated as a model for space study. Data indicated an increase in A+ A' cells (noncommitted and committed self-perpetuating precursor cells), and a decrease in C + D type cells (preosteoblasts that are in the G1 or G2 stage of the cell cycle), accompanied by a decrease in Obs. A stress/strain mediated increase in nuclear volume (A'=> C) is an important rate-limiting step in Ob differentiation. It is hypothesized that this step is inhibited in microgravity. Additional ground studies are needed to confirm these data and define the mechanism of action. Studies have shown that a soft diet decreases overall mechanical loading of the masticatory apparatus. A soft diet was used in this study to simulate the potential microgravity-like effects of decreased mechanical loading on the mandibular condyle, i.e., a decreased masticatory force was used as a model for microgravity. Eighteen male, Sprague-Dawley rats (six to eight weeks of age) were divided into three groups: 1) the experimental group maintained on a soft paste diet; 2) a control group, pair fed equal amounts of food as the experimental group; and 3) a control group fed ad libitum. Both control groups were fed standard Purina TM Rat Chow (sPRC) pellets. The experimental diet consisted of sPRC, crushed and mixed with sufficient water to form a paste. After two weeks on their respective diets, each animal was injected i.p. with xylenol orange, a bone label. One week later calcein green was administered similarly. Two days after the last label, animals were injected i.p. with 3H-thymidine at 8 a.m. One hour later each animal was anesthesized with ketamine and decapitated. Mandibles were fixed for 72 hours. Nuclear morphometry and labeling index data were collected from autoradiographs prepared from demineralized sections. Due to technique problems, only three paste group condyles and five condyles from each of the control groups could be examined. Results showed no significant differences at p < 0.05 in labeling index. Nuclear morphometry data indicated a significant difference in L, A+ A', C, and C + D cells. Rats fed a paste diet had on the average 12 percent more type A+ A' cells, and 17-25 percent less C + D combined cells than the two control groups. These data indicate a similar shift in Ob histogenesis to the left (more A + A' cells) as that found in the POL and the condyle of rats flown in space. These findings support the hypothesis that osteopenia induced in spaceflight may be the result of a reduction in a local mechanical component, or a lack of anti-gravity posturing. Therefore, it may be concluded that a soft paste diet mimics microgravity by decreasing mechanical loading and inhibiting the stress/strain mediated increase in nuclear volume (A'=> C). The condyle from rats fed a soft diet may be a valuable model for studying osteopenia.