Mandibular periosteum moves in the direction of new apposition. This displacement, usually termed “migration,” is thought to involve the fibrous layer of periosteum, with the deeper osteogenic layer remaining at its original location except for its blood vessels. To assess whether periosteal displacement includes cells as well as matrix and whether the osteogenic layer has a role, a longitudinal study was undertaken. Young pigs (n = 10) were injected with a replication marker and killed 3 hr, 2 weeks, or 4 weeks later. Sections of the mandibular ramus were scored for labeled cell density. Some sections were double-labeled with lectin to identify blood vessels. Statistical differences were seen between but not within age groups. Three hours after labeling, the fibroblastic layer had sparse, evenly distributed replicating cells, whereas the osteogenic layer had numerous replicating cells, especially at the caudal border. At 2 and 4 weeks later, a decrease in labeled osteogenic layer cells was accompanied by an increase in labeled osteocytes. Zones of labeled osteocytes in these late-sacrifice groups were used to approximate the position of the ramal borders at the time of injection. Beyond these zones, in active growing sites, labeled cells were found not only in the fibrous layer but also in the osteogenic layer and in bone. Therefore, periosteal displacement does involve cells and is not restricted to the fibrous layer. Anat Rec 290:1366-1376, 2007. © 2007 Wiley-Liss, Inc.