Differential cell replication within the periosteum of the pig mandibular ramus

The periosteum is anatomically and functionally divided into two layers. The inner osteoblastic layer contributes to local appositional bone growth. The outer fibroblastic layer receives the attachments of muscles and responds to bone growth by a more global enlargement. Coordinated growth of the layers could theoretically be produced by parallel replication rates in the layers followed by migration of fibroblastic layer cells. Alternatively, replication rates in the layers could differ, those in the osteoblastic layer reflecting local apposition and those in the fibroblastic layer responding to total bone growth. To test these alternatives, we compared two regions of the pig mandible, one appositional and one resorptive, equidistant from the major growth sites of the bone. Four 2-week-old pigs were injected i.p. with 5-bromo-2'-deoxyuridine (BrdU) to label replicating cells. Three hours subsequent to BrdU injection, animals were sacrificed. The mandibles were sectioned and processed immunocytochemically for BrdU. Periosteal cell mitotic activity was analyzed selectively at the level of the mandibular foramen on the medial and lateral ramal surfaces. The proportion of labeled cells was determined by grid-point analysis. Individual differences were minor but regional differences were striking. As expected, the osteoblastic layer of the lateral surface exhibited a greater proportion of mitotic cells than did the medial surface (p = 0.037). However, no such difference was seen in the fibroblastic layer, where medial and lateral sides exhibited identical replication activity. These results strongly support the second alternative, that cell division is differentially controlled in the two periosteal layers.