Figure 4. BrdU labeling in 56-day-old rats showed the differentiation of osteoblasts. When BrdU was administered 1 hr before death (A), the labeled cells were accumulated in the center of the blood vessels. The disc-shaped osteoblasts lining the subchondral bone surface were not labeled (A, arrowheads). When BrdU was administered 3 days before death (B), almost all the labeled cells in the marrow cavity were skeletal progenitor cells migrating toward the cavity wall (B, arrows). The differentiated osteoblasts on the bone surface were labeled (B, arrowheads). The labeled cells in the erosive front were multinuclear chondroclasts (C) with strong TRAP activities (D). Three-day BrdU labeling showed more osteoblasts and chondroclasts in the erosive zone during mandibular advancement (F: 14 days after the experiment) than during natural growth (E: 49 days old).