RESUMO
Because rat incisors continuously erupt they provide an opportunity for the study of dental plaque at all stages of its development. The youngest plaque would be visible at the gingival margin of the tooth as it erupts, and the older plaque higher on the tooth. The ultrastructural features of these plaques were studied by transmission electron microscopy. Cocci and short rods colonized the cementum surface, forming a monolayer. The plaque had a maximum thickness of about 40 microns, with the inner third rich in fibrillar matrix and the organisms forming microcolonies perpendicular to the tooth surface. Cells were haphazardly distributed in a loose matrix on the surface of the plaque. In the area of plaque disorganization the cementum was covered by isolated groups of bacteria and the matrix had holes in it. The rat mandibular incisor may provide a unique model for study of how plaque on cementum is initially formed, matures and finally is degraded.