Osteoclasts differentiate from resident precursors in an in vivo model of synchronized resorption: a temporal and spatial study in rats.

Osteoclasts differentiate from mononucleated precursors expressing monocyte markers, which gradually evolve to preosteoclasts expressing the osteoclast phenotype. Although the role of osteogenic cells in these changes has been well documented in vitro, their contribution in vivo has not been established. In this study, a synchronized wave of resorption was activated along the mandibular periosteum. The periosteum adjacent to the bone surface studied was separated by a computer-assisted technique into an osteogenic alkaline phosphatase-positive compartment and an outer nonosteogenic compartment. Specific markers (nonspecific esterase [NSE], tartrate-resistant acid phosphatase [TRAP], and ED1 antibody, a marker of the monocyte-macrophage lineage) were used to follow osteoclast differentiation quantitatively as a function of time after activation of resorption, from day 0 to day 4 (peak of resorption in this model). Local cell proliferation was assessed in parallel. Between day 0 and day 3, the thickness of the osteogenic compartment decreased by 50% (p < 0.0002). In the osteogenic compartment, proliferating cell numbers fell by 80% at 12 day, NSE(+) cells (located farthest from the bone surface) increased 3. 9-fold on day 4 vs. day 0 (p < 0.005), ED1(+) cells decreased between day 0 and day 2 (p < 0.02) before returning to their initial value, and TRAP(+) cells increased 2.7-fold between day 1 and day 3 (p < 0.0005). Resorption was absent in the site studied on day 0, but on day 4 there were 20.5 osteoclast nuclei per millimeter of bone surface. The cell ratio changed from 30.3 NSE(+) and ED1(+) (some of which were also TRAP(+)) cells per millimeter on day 0 to 37.6 mononucleated cells plus 20.5 osteoclast nuclei on day 4. In the nonosteogenic compartment, an entry of ED1(+)/NSE(-) was observed on 12 day (+23 cells, p < 0.02 vs. day 0). This was followed by a return of ED1(+) cell numbers to the control level on day 1, and a transient increase in NSE(+) cells (+47% on day 2 vs. day 1, p < 0.02). TRAP(+) cells were never seen in this compartment. Proliferating cell numbers did not change throughout the study. Our results strongly suggest that the osteoclasts present on day 4 differentiated from the pool of TRAP(+), ED1(+), and NSE(+) cells present at the site on day 0. The osteogenic compartment was gradually replenished by cells migrating from the nonosteogenic compartment, which was supplemented by ED1(+) cells recruited from the circulation early after activation. Moreover, osteogenic cells appeared to be as crucial in vivo for the acquisition of the TRAP phenotype as previously shown in vitro.

H1 and H2 histamine receptors modulate osteoclastic resorption by different pathways: evidence obtained by using receptor antagonists in a rat synchronized resorption model.

We have previously postulated that mast cells (MC) may act as accessory cells in bone resorption. In this study we obtained evidence that histamine, the most abundant mediator released upon MC degranulation, is one of many factors modulating resorption. As the effect of histamine is mediated through different receptors, we tested the effects of mepyramine (1.5 mg/kg/day) and cimetidine (125 mg/kg/day), that antagonize H1 and H2 receptors, respectively. These effects were assessed morphometrically in a well-defined rat model of synchronized resorption at different stages of the process. On day 4 after induction (i.e., at the peak of resorption in this model), both agents reduced resorption significantly. Mepyramine acted by disturbing osteoclast activation and by reducing osteoclast activity (P < 0.01), while cimetidine principally reduced the size of the osteoclast population (P < 0.01). On day 6 (stage of declining resorption), the same resorption score as on day 4 was maintained in the mepyramine group, mainly through a marked increase in osteoclast activity (P < 0.01). In contrast, cimetidine continued to strongly reduce resorption (P < 0.01) and led to a further drop in the osteoclast population (P < 0.01). One day after induction, nonspecific esterase (NSE)-positive cells (putative osteoclast precursors) were significantly less numerous after treatment with the two agents. Significant changes in the MC population in the vicinity of the zone undergoing resorption occurred on days 4 and 6. The periosteal microvasculature adjacent to the reference bone zone was also markedly modified, especially in the cimetidine group. These results show that histamine intervenes in resorption through both H1 and H2 receptors. However, the mechanisms triggered by these receptors were quite different: H2 receptors appeared to be more strategic, as no replenishment of the osteoclast population occurred after the initial depletion in precursors. Histamine also appears to influence other neighbouring compartments, in which disturbances are probably linked to defective resorption. These findings support our hypothesis by which MC are accessory cells of resorption in this model.

Partial inhibition of bone resorption by disodium cromoglycate in a synchronized model of bone remodeling.

In a previous study we observed that mast cell degranulation might be associated to bone resorption. To verify this assumption, the efficiency of cromoglycate was assessed on a synchronized sequence of bone remodeling induced in male Wistar rats along the mandibular cortex. After 4 days (time of osteoclastic peak in this model), cromoglycate (15/mg/kg and 30 mg/kg/d per os) decreased the number of degranulating mast cells, especially in the population adjacent to the bone surface. Concomitantly the extent of resorption surface was decreased vs untreated animals. Active osteoclasts were lower whereas the total number of osteoclasts (both active and inactive) was not statistically modified. The mean osteoclast-bone interface was not modified. No dose effect was found. These data indicate that mast cell degranulation is involved in the events leading to osteoclast resorption, presumably in the mechanisms providing osteoclast access to bone surface.