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 osteoclas t phenotype, Although the role of osteogenic cells in these changes has bee n well documented in vitro, their contribution in vivo has not been establi shed, In this study, a synchronized wave of resorption was activated along the mandibular periosteum, The periosteum adjacent to the bone surface stud ied was separated by a computer-assisted technique into an osteogenic alkal ine phosphatase-positive compartment and an outer nonosteogenic compartment . Specific markers (nonspecific esterase [NSE], tartrate-resistant acid pho sphatase [TRAP], and EDI antibody, a marker of the monocyte-macrophage line age) were used to follow osteoclast differentiation quantitatively as a fun ction of time after activation of resorption, from day 0 to day 4 (peak of resorption in this model). Local cell proliferation was assessed in paralle l. Between day 0 and day 3, the thickness of the osteogenic compartment dec reased by 50% (p < 0.0002), In the osteogenic compartment, proliferating fe ll numbers fell by 80% at 1/2 day, NSE+ cells (located farthest from the bo ne 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 ini tial 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 I there were 20.5 osteoclast nuclei per millimeter of bone surface. The cel l 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 osteoc last nuclei on day 4, In the nonosteogenic compartment, an entry of ED1(+)/ NSE- was observed on 1/2 day (+23 cells, p < 0.02 vs. day 0). This was foll owed 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), TRA P(+) 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 w as gradually replenished by cells migrating from the nonosteogenic compartm ent, which was supplemented by ED1(+) cells recruited from the circulation early after activation. Moreover, osteogenic cells appeared to be as crucia l in vivo for the acquisition of the TRAP phenotype as previously shown in vitro. (C) 2000 by Elsevier Science Inc. All rights reserved.