The cellular changes occurring with allografts of marrow-containing cortical bone.

The cell population within and around allografts of fresh, marrow-containing cortical bone has been studied. The grafts were implanted into intramuscular sites in rats and removed at intervals over a 4 week period for examination by light and electron microscopy to establish the sequence of changes at both the tissue and cellular levels. The tissue organization and the ultrastructural appearances of the cells found at different times after grafting are described; the possible relationships of the different cell types to each other and their likely roles in both the immunological and histogenic events are postulated and discussed. A comparison of the observations in the allograft and in the previously studied autograft suggests that the cells from the granulationtissue that become osteoblasts in the autograft differentiate into fibrocytes and giant cells in the allograft. Furthermore, the increased period of initial degeneration observed in the allograft (2 weeks) compared with the autograft (less than 1 week) may represent the destruction of early, graft-derived attempts at osteogenesis. This, together with the relative paucity of new bone in the allograft, suggests that some cells of graft origin may contribute to the early osteogenesis observed in the autograft.