Cell replication in craniofacial periosteum: appositional vs. resorptive sites

The size and the shape of craniofacial bones results from periosteal activity, which can be either appositional or resorptive. The periosteum is often used as a source of graft material for osteogenesis, but differences in cellular makeup and proliferative capacity may render resorptive regions unsuitable for transplant. This study was undertaken to characterize the cells in appositional and resorptive periosteum, and to assess variation in proliferative activity. Young pigs (n = 9) were injected with bromodeoxyuridine to label replicating cells and killed 3 h later. The mandibular ramus, hard palate and zygomatic arch were examined for patterns of periosteal activity, and replicating cells were quantified in 16 appositional and eight resorptive regions. Sections were also reacted for markers of osteogenic (Runx2) and osteoclastic [CTR (calcitonin receptor), RANK, TRAP, CD14] lineage, and for an endothelial label (lectin). Replicating cells were often associated with the vasculature; most were unreactive for markers of differentiation. Although the fibrous layers of periosteum had fewer replicating cells per unit area than inner layers (P < 0.005), this was in part due to lower cellularity. Appositional periostea differed from resorptive periostea in having thicker fibrous layers (197 vs. 89 mu m, P = 0.02) and higher replication density in the inner layers (606 vs. 329 labeled cells mm-2, P = 0.02). Osteoprogenitors were numerous in the inner layers of appositional but very scarce in resorptive periostea. Multinucleated osteoclasts were never seen in appositional regions, but mononuclear cells positive for osteoclastic lineage markers were plentiful, especially in the most rapidly growing areas. These cells appeared to be macrophages accompanying a growth rate so rapid as to resemble a response to trauma. In conclusion, appositional and resorptive periostea differ strikingly in morphology and cell content. Resorptive periosteum is a poor choice for osteogenic grafting.